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rfc:rfc3101

Network Working Group P. Murphy Request for Comments: 3101 US Geological Survey Obsoletes: 1587 January 2003 Category: Standards Track

             The OSPF Not-So-Stubby Area (NSSA) Option

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This memo documents an optional type of Open Shortest Path First
 (OSPF) area that is somewhat humorously referred to as a "not-so-
 stubby" area (or NSSA).  NSSAs are similar to the existing OSPF stub
 area configuration option but have the additional capability of
 importing AS external routes in a limited fashion.
 The OSPF NSSA Option was originally defined in RFC 1587.  The
 functional differences between this memo and RFC 1587 are explained
 in Appendix F.  All differences, while expanding capability, are
 backward-compatible in nature.  Implementations of this memo and of
 RFC 1587 will interoperate.

Murphy Standards Track [Page 1] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Table Of Contents

 1.0 Overview .................................................  2
    1.1 Motivation - Transit Networks .........................  2
    1.2 Motivation - Corporate Networks .......................  4
    1.3 Proposed Solution .....................................  5
 2.0 NSSA Intra-Area Implementation Details ...................  7
    2.1 The N-bit .............................................  7
    2.2 Type-7 Address Ranges .................................  7
    2.3 Type-7 LSAs ...........................................  8
    2.4 Originating Type-7 LSAs ...............................  9
    2.5 Calculating Type-7 AS External Routes ................. 10
    2.6 Incremental Updates ................................... 14
    2.7 Optionally Importing Summary Routes ................... 14
 3.0 Intra-AS Implementation Details .......................... 15
    3.1 Type-7 Translator Election ............................ 15
    3.2 Translating Type-7 LSAs into Type-5 LSAs .............. 16
    3.3 Flushing Translated Type-7 LSAs ....................... 19
 4.0 Security Considerations .................................. 20
 5.0 Acknowledgements ......................................... 21
 6.0 Contributors ............................................. 22
 7.0 References ............................................... 22
 Appendix A: The Options Field ................................ 23
 Appendix B: Router-LSAs ...................................... 24
 Appendix C: Type-7 LSA Packet Format ......................... 26
 Appendix D: Configuration Parameters ......................... 27
 Appendix E: The P-bit Policy Paradox ......................... 28
 Appendix F: Differences from RFC 1587 ........................ 30
 Author's Addresses ........................................... 32
 Full Copyright Statement ..................................... 33

1.0 Overview

1.1 Motivation - Transit Networks

 Wide-area transit networks often have connections to moderately
 complex "leaf" sites.  A leaf site may have multiple IP network
 numbers assigned to it.  Typically, one of the leaf site's networks
 is directly connected to a router provided and administered by the
 transit network while the others are distributed throughout and
 administered by the site.  From the transit network's perspective,
 all of the network numbers associated with the site make up a single
 "stub" entity.  For example, BBN Planet has one site composed of a
 class-B network, 130.57.0.0, and a class-C network, 192.31.114.0.
 From BBN Planet's perspective, this configuration looks something
 like the diagram on the next page, where the "cloud" consists of the
 subnets of 130.57 and network 192.31.114, all of which are learned by
 RIP on router BR18.

Murphy Standards Track [Page 2] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

                192.31.114
                    |
                  (cloud)
              -------------- 130.57.4
                    |
                    |
                 ------ 131.119.13 ------
                 |BR18|------------|BR10|
                 ------            ------
                                      |
                                      V
                              to BBN Planet "core" OSPF system
 Topologically, this cloud looks very much like an OSPF stub area.
 The advantages of running the cloud as an OSPF stub area are:
    1. External routes learned from OSPF's Type-5 AS-external-LSAs are
       not advertised beyond the router labeled "BR10".  This is
       advantageous because the link between BR10 and BR18 may be a
       low-speed link or the router BR18 may have limited resources.
    2. The transit network is abstracted to the "leaf" router BR18 by
       advertising only a default route across the link between BR10
       and BR18.
    3. The cloud becomes a single, manageable "leaf" with respect to
       the transit network.
    4. The cloud can become, logically, a part of the transit
       network's OSPF routing system.
 However, the original definition of the OSPF protocol (See [OSPF])
 imposes topological limitations that restrict simple cloud topologies
 from becoming OSPF stub areas.  In particular, it is illegal for a
 stub area to import routes external to OSPF; thus it is not possible
 for routers BR18 and BR10 to both be members of the stub area and to
 import into OSPF as Type-5 AS-external-LSAs routes learned from RIP
 or other IP routing protocols.  In order to run OSPF out to BR18,
 BR18 must be a member of a non-stub area or the OSPF backbone before
 it can import routes other than its directly connected network(s).
 Since it is not acceptable for BR18 to maintain all of BBN Planet's
 Type-5 AS external routes, BBN Planet is forced by OSPF's topological
 limitations to only run OSPF out to BR10 and to run RIP between BR18
 and BR10.

Murphy Standards Track [Page 3] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

1.2 Motivation - Corporate Networks

 In a corporate network that supports a large corporate infrastructure
 it is not uncommon for its OSPF domain to have a complex non-zero
 area infrastructure that injects large routing tables into its Area 0
 backbone.  Organizations within the corporate infrastructure may
 routinely multi-home their non-zero OSPF areas to strategically
 located Area 0 backbone routers, either to provide backbone
 redundancy or to increase backbone connectivity or both.  Because of
 these large routing tables, OSPF aggregation via summarization is
 routinely used and recommended.  Stub areas are also recommended to
 keep the size of the routing tables of non-backbone routers small.
 Organizations within the corporation are administratively autonomous
 and compete for corporate backbone resources.  They also want
 isolation from each other in order to protect their own network
 resources within the organization.
 Consider the typical example configuration shown below where routers
 A1, B1 and A2, B2 are connected to Area 1 and Area 2 respectively,
 and where routers A0 and B0 are Area 0 border routers that connect to
 both Area 1 and Area 2.  Assume the 192.168.192/20 and 192.168.208/22
 clouds are subnetted with a protocol different from the corporate
 OSPF instance.  These other protocols could be RIP, IGRP, or second
 and third OSPF instances separate from the corporate OSPF backbone
 instance.
 Area 1 and Area 2 would like to be stub areas to minimize the size of
 their link state databases.  It is also desirable to originate two
 aggregated external advertisements for the subnets of 192.168.192/20
 and 192.168.208/22 in such a way that the preferred path to
 192.168.192/20 is through A0 and the preferred path to 192.168.208/22
 is through B0.
                +---A0------Area 0 cloud------B0---+
                |   |                          |   |
                |   |                          |   |
                |   |T1                   56kbs|   |
           56kbs|   |                          |   |T1
                |   |                          |   |
                |   |       Area 1 cloud       |   |
                |   A1-----192.168.192/20-----B1   |
                |                                  |
                +---A2                        B2---+
                     |                         |
                     |      Area 2 cloud       |
                     +-----192.168.208/22------+

Murphy Standards Track [Page 4] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 The current standard OSPF stub area has no mechanism to support the
 redistribution of routes for the subnets of 192.168.192/20 and
 192.168.208/22 within a stub area or the ability to aggregate a range
 of external routes in any OSPF area.  Any solution to this dilemma
 must also honor Area 1's path of choice to 192.168.192/20 through A0
 with redundancy through B0 while at the same time honoring Area 2's
 path of choice to 192.168.208/20 through B0 with redundancy through
 A0.

1.3 Proposed Solution

 This document describes a new optional type of OSPF area, somewhat
 humorously referred to as a "not-so-stubby" area (or NSSA), which has
 the capability of importing external routes in a limited fashion.
 The OSPF specification defines two general classes of area
 configuration.  The first allows Type-5 LSAs to be flooded throughout
 the area.  In this configuration, Type-5 LSAs may be originated by
 routers internal to the area or flooded into the area by area border
 routers.  These areas, referred to herein as Type-5 capable areas (or
 just plain areas in the OSPF specification), are distinguished by the
 fact that they can carry transit traffic.  The backbone is always a
 Type-5 capable area.  The second type of area configuration, called
 stub, does not allow Type-5 LSAs to be propagated into/throughout the
 area and instead depends on default routing to external destinations.
 NSSAs are defined in much the same manner as existing stub areas.  To
 support NSSAs, a new option bit (the "N" bit) and a new type of LSA
 (Type-7) are defined.  The "N" bit ensures that routers belonging to
 an NSSA agree on its configuration.  Similar to the stub area's use
 of the "E" bit, both NSSA neighbors must agree on the setting of the
 "N" bit or the OSPF neighbor adjacency will not form.
 Type-7 LSAs provide for carrying external route information within an
 NSSA. Type-7 LSAs have virtually the same syntax as Type-5 LSAs with
 the obvious exception of the link-state type.  (See section 2.3 for
 more details.)   Both LSAs are considered a type of OSPF AS-
 external-LSA.  There are two major semantic differences between
 Type-5 LSAs and Type-7 LSAs.
    o  Type-7 LSAs may be originated by and advertised throughout an
       NSSA; as with stub areas, Type-5 LSAs are not flooded into
       NSSAs and do not originate there.
    o  Type-7 LSAs are advertised only within a single NSSA; they are
       not flooded into the backbone area or any other area by border
       routers, though the information that they contain may be
       propagated into the backbone area.  (See Section 3.2.)

Murphy Standards Track [Page 5] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 In order to allow limited exchange of external information across an
 NSSA border, NSSA border routers will translate selected Type-7 LSAs
 received from the NSSA into Type-5 LSAs.  These Type-5 LSAs will be
 flooded to all Type-5 capable areas.  NSSA border routers may be
 configured with address ranges so that multiple Type-7 LSAs may be
 aggregated into a single Type-5 LSA.  The NSSA border routers that
 perform translation are configurable.  In the absence of a configured
 translator one is elected.
 In addition, an NSSA border router should originate a default LSA (IP
 network is 0.0.0.0/0) into the NSSA.  Default routes are necessary
 because NSSAs do not receive full routing information and must have a
 default route in order to route to AS-external destinations.  Like
 stub areas, NSSAs may be connected to the Area 0 backbone at more
 than one NSSA border router, but may not be used as a transit area.
 Note that a Type-7 default LSA originated by an NSSA border router is
 never translated into a Type-5 LSA, however, a Type-7 default LSA
 originated by an NSSA internal AS boundary router (one that is not an
 NSSA border router) may be translated into a Type-5 LSA.
 Like stub areas, NSSA border routers optionally import summary routes
 into their NSSAs as Type-3 summary-LSAs.  If the import is disabled,
 particular care should be taken to ensure that summary routing is not
 obscured by an NSSA's Type-7 AS-external-LSAs.  This may happen when
 the AS's other IGPs, like RIP and ISIS, leak routing information into
 the NSSA.  In these cases all summary routes should be imported into
 the NSSA.  The recommended default behavior is to import summary
 routes into NSSAs.  Since Type-5 AS-external-LSAs are not flooded
 into NSSAs, NSSA border routers should not originate Type-4 summary-
 LSAs into their NSSAs.  Also an NSSA's border routers never originate
 Type-4 summary-LSAs for the NSSA's AS boundary routers, since Type-7
 AS-external-LSAs are never flooded beyond the NSSA's border.
 When summary routes are not imported into an NSSA, the default LSA
 originated into it by its border routers must be a Type-3 summary-
 LSA.  This default summary-LSA insures intra-AS connectivity to the
 rest of the OSPF domain, as its default summary route is preferred
 over the default route of a Type-7 default LSA.  Without a default
 summary route the OSPF domain's inter-area traffic, which is normally
 forwarded by summary routes, might exit the AS via the default route
 of a Type-7 default LSA originated by an NSSA internal router.  The
 Type-7 default LSAs originated by NSSA internal routers and the no-
 summary option are mutually exclusive features. When summary routes
 are imported into the NSSA, the default LSA originated by a NSSA
 border router into the NSSA should be a Type-7 LSA.
 In our transit topology example the subnets of 130.57 and network
 192.31.114 will still be learned by RIP on router BR18, but now both

Murphy Standards Track [Page 6] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 BR10 and BR18 can be in an NSSA and all of BBN Planet's external
 routes are hidden from BR18; BR10 becomes an NSSA border router and
 BR18 becomes an AS boundary router internal to the NSSA.  BR18 will
 import the subnets of 130.57 and network 192.31.114 as Type-7 LSAs
 into the NSSA.  BR10 then translates these routes into Type-5 LSAs
 and floods them into BBN Planet's backbone.
 In our corporate topology example if Area 1 and Area 2 are NSSAs the
 external paths to the subnets of the address ranges 192.168.192/20
 and 192.168.208/22 can be redistributed as Type-7 LSAs throughout
 Area 1 and Area 2 respectively, and then aggregated during the
 translation process into separate Type-5 LSAs that are flooded into
 Area 0.  A0 may be configured as Area 1's translator even though B0
 is the translator of Area 2.

2.0 NSSA Intra-Area Implementation Details

2.1 The N-bit

 The N-bit ensures that all members of an NSSA agree on the area's
 configuration.  Together, the N-bit and E-bit reflect an interface's
 (and consequently the interface's associated area) external LSA
 flooding capability.  As explained in [OSPF] Section 10.5, if Type-5
 LSAs are not flooded into/throughout the area, the E-bit must be
 clear in the option field of the received Hello packets.  Interfaces
 associated with an NSSA will not send or receive Type-5 LSAs on that
 interface but may send and receive Type-7 LSAs.  Therefore, if the
 N-bit is set in the options field, the E-bit must be clear.
 To support the NSSA option an additional check must be made in the
 function that handles the receiving of the Hello packet to verify
 that both the N-bit and the E-bit found in the Hello packet's option
 field match the area type and ExternalRoutingCapability of the area
 of the receiving interface.  A mismatch in the options causes
 processing of the received Hello packet to stop and the packet to be
 dropped.

2.2 Type-7 Address Ranges

 NSSA border routers may be configured with Type-7 address ranges.
 Each Type-7 address range is defined as an [address,mask] pair.  Many
 separate Type-7 networks may fall into a single Type-7 address range,
 just as a subnetted network is composed of many separate subnets.
 NSSA border routers may aggregate Type-7 routes by advertising a
 single Type-5 LSA for each Type-7 address range.  The Type-5 LSA
 resulting from a Type-7 address range match will be distributed to
 all Type-5 capable areas.  Section 3.2 details how Type-5 LSAs are
 generated from Type-7 address ranges.

Murphy Standards Track [Page 7] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 A Type-7 address range includes the following configurable items.
    o  An [address,mask] pair.
    o  A status indication of either Advertise or DoNotAdvertise.
    o  An external route tag.

2.3 Type-7 LSAs

 External routes are imported into NSSAs as Type-7 LSAs by NSSA AS
 boundary routers.  An NSSA AS boundary router (ASBR) is a router that
 has an interface associated with the NSSA and is exchanging routing
 information with routers belonging to another AS.  Like OSPF ASBRs,
 an NSSA router indicates it is an NSSA ASBR by setting the E-bit in
 its router-LSA.  As with Type-5 LSAs a separate Type-7 LSA is
 originated for each destination network.  To support NSSAs the link-
 state database must therefore be expanded to contain Type-7 LSAs.
 Type-7 LSAs are identical to Type-5 LSAs except for the following
 (see [OSPF] Section 12.4.4, "AS external links").
    1. The Type field in the LSA header is 7.
    2. Type-7 LSAs are only flooded within the originating NSSA.  The
       flooding of Type-7 LSAs follows the same rules as the flooding
       of Type-1 and Type-2 LSAs.
    3. Type-7 LSAs are only listed within the OSPF area data
       structures of their respective NSSAs, making them area
       specific.  Type-5 LSAs, which are flooded to all Type-5 capable
       areas, have global scope and are listed in the OSPF protocol
       data structure.
    4. NSSA border routers select which Type-7 LSAs are translated
       into Type-5 LSAs and flooded into the OSPF domain's transit
       topology.
    5. Type-7 LSAs have a propagate (P) bit that, when set, tells an
       NSSA border router to translate a Type-7 LSA into a Type-5 LSA.
    6. Those Type-7 LSAs that are to be translated into Type-5 LSAs
       must have their forwarding address set.

Murphy Standards Track [Page 8] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 Type-5 LSAs that are translations of Type-7 LSAs copy the Type-7
 LSAs' non-zero forwarding addresses.  Only those Type-5 LSAs that are
 aggregations of Type-7 LSAs may have 0.0.0.0 as a forwarding address.
 (See Section 3.2 for details.)  Non-zero forwarding addresses produce
 efficient inter-area routing to an NSSA's AS external destinations
 when it has multiple border routers.  Also the non-zero forwarding
 addresses of Type-7 LSAs ease the process of their translation into
 Type-5 LSAs, as NSSA border routers are not required to compute them.
 Normally the next hop address of an installed AS external route
 learned by an NSSA ASBR from an adjacent AS points at one of the
 adjacent AS's gateway routers.  If this address belongs to a network
 connected to the NSSA ASBR via one of its NSSAs' active interfaces,
 then the NSSA ASBR copies this next hop address into the forwarding
 address field of the route's Type-7 LSA that is originated into this
 NSSA, as is currently done with Type-5 LSAs. (See [OSPF] Section
 12.4.4.1.)  For an NSSA with no such network the forwarding address
 field may only be filled with an address from one of the its active
 interfaces or 0.0.0.0.  If the P-bit is set, the forwarding address
 must be non-zero; otherwise it may be 0.0.0.0.  If an NSSA requires
 the P-bit be set and a non-zero forwarding address is unavailable,
 then the route's Type-7 LSA is not originated into this NSSA.
 When a router is forced to pick a forwarding address for a Type-7
 LSA, preference should be given first to the router's internal
 addresses (provided internal addressing is supported).  If internal
 addresses are not available, preference should be given to the
 router's active OSPF stub network addresses.  These choices avoid the
 possible extra hop that may happen when a transit network's address
 is used.  When the interface whose IP address is the LSA's forwarding
 address transitions to a Down state (see [OSPF] Section 9.3), the
 router must select a new forwarding address for the LSA and then re-
 originate it.  If one is not available the LSA should be flushed.
 The metrics and path types of Type-5 LSAs are directly comparable
 with the metrics and path types of Type-7 LSAs.

2.4 Originating Type-7 LSAs

 NSSA AS boundary routers may only originate Type-7 LSAs into NSSAs.
 An NSSA internal AS boundary router must set the P-bit in the LSA
 header's option field of any Type-7 LSA whose network it wants
 advertised into the OSPF domain's full transit topology.  The LSAs of
 these networks must have a valid non-zero forwarding address.  If the
 P-bit is clear the LSA is not translated into a Type-5 LSA by NSSA
 border routers.

Murphy Standards Track [Page 9] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 When an NSSA border router originates both a Type-5 LSA and a Type-7
 LSA for the same network, then the P-bit must be clear in the Type-7
 LSA so that it isn't translated into a Type-5 LSA by another NSSA
 border router.  If the border router only originates a Type-7 LSA, it
 may set the P-bit so that the network may be aggregated/propagated
 during Type-7 translation.  If an NSSA's border router originates a
 Type-5 LSA with a forwarding address from the NSSA, it should also
 originate a Type-7 LSA into the NSSA.  If two NSSA routers, both
 reachable from one another over the NSSA, originate functionally
 equivalent Type-7 LSAs (i.e., same destination, cost and non-zero
 forwarding address), then the router having the least preferred LSA
 should flush its LSA.  (See [OSPF] Section 12.4.4.1.)  Preference
 between two Type-7 LSAs is determined by the following tie breaker
 rules:
    1. An LSA with the P-bit set is preferred over one with the P-bit
       clear.
    2. If the P-bit settings are the same, the LSA with the higher
       router ID is preferred.
 A Type-7 default LSA for the network 0.0.0.0/0 may be originated into
 the NSSA by any NSSA router.  The Type-7 default LSA originated by an
 NSSA border router must have the P-bit clear.  An NSSA ASBR that is
 not an NSSA border router may originate a Type-7 default LSA with the
 P-bit set.  A Type-7 default LSA may be installed by NSSA border
 routers if and only if its P-bit is set.  (See Appendix E.)
 NSSA border routers must originate an LSA for the default destination
 into all their directly attached NSSAs in order to support intra-AS
 routing and inter-AS routing.  This default destination is advertised
 in either a Type-3 LSA or a Type-7 LSA, as described in Section 2.7.
 The default LSA's metric should be configurable. For Type-7 default
 LSAs, the metric type (1 or 2) should also be configurable.

2.5 Calculating Type-7 AS External Routes

 This calculation must be run when Type-7 LSAs are processed during
 the AS external route calculation.  This calculation may process
 Type-5 LSAs, but it is written that way only for comparison sake.
 Non-default Type-7 LSAs with the P-bit clear may be installed in the
 OSPF routing table of NSSA border routers.  Since these LSAs are not
 propagated throughout the OSPF domain, traffic that originates
 external to an NSSA and that passes through one of the NSSA's border
 routers may be unexpectedly diverted into the NSSA.  (See Appendix
 E.)

Murphy Standards Track [Page 10] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 An NSSA border router should examine both Type-5 LSAs and Type-7 LSAs
 if either Type-5 or Type-7 routes need to be updated or recalculated.
 This is done as part of the AS external route calculation.  An NSSA
 internal router should examine Type-7 LSAs when Type-7 routes need to
 be recalculated.
 What follows is only a modest modification of [OSPF] Section 16.4.
 Original paragraphs are tagged with [OSPF].  Paragraphs with minor
 changes are tagged with ~[OSPF].  Paragraphs specific to NSSA are
 tagged with [NSSA].
 AS external routes are calculated by examining AS-external-LSAs, be
 they Type-5 or Type-7.  Each of the AS-external-LSAs is considered in
 turn.  Most AS-external-LSAs describe routes to specific IP
 destinations.  An AS-external-LSA can also describe a default route
 for the Autonomous System (Destination ID = DefaultDestination,
 network/subnet mask = 0x00000000).  For each AS-external-LSA:
 ~[OSPF]
    (1) If the metric specified by the LSA is LSInfinity, or if the
        age of the LSA equals MaxAge, then examine the next LSA.
        [OSPF]
    (2) If the LSA was originated by the calculating router itself,
        examine the next LSA.
        [OSPF]
    (3) Call the destination described by the LSA N.  N's address is
        obtained by masking the LSA's Link State ID with the
        network/subnet mask contained in the body of the LSA.  Look up
        the routing table entries that match the LSA's type for the AS
        boundary router (ASBR) that originated the LSA.  For a Type-5
        LSA, routing table entries may only be selected from each
        attached Type-5 capable area.  Since the flooding scope of a
        Type-7 LSA is restricted to the originating NSSA, the routing
        table entry of its ASBR must be found in the originating NSSA.
        If no entries exist for the ASBR (i.e. the ASBR is unreachable
        over the transit topology for a Type-5 LSA, or, for a Type-7
        LSA, it is unreachable over the LSA's originating NSSA), do
        nothing with this LSA and consider the next in the list.
        [NSSA]
        Else if the destination is a Type-7 default route (destination
        ID = DefaultDestination) and one of the following is true,
        then do nothing with this LSA and consider the next in the
        list:

Murphy Standards Track [Page 11] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

          o  The calculating router is a border router and the LSA has
             its P-bit clear.  Appendix E describes a technique
             whereby an NSSA border router installs a Type-7 default
             LSA without propagating it.
          o  The calculating router is a border router and is
             suppressing the import of summary routes as Type-3
             summary-LSAs.
          [NSSA]
        Else, this LSA describes an AS external path to destination N.
        Examine the forwarding address specified in the AS-external-
        LSA.  This indicates the IP address to which packets for the
        destination should be forwarded.
        [OSPF]
        If the forwarding address is set to 0.0.0.0 then packets
        should be sent to the ASBR itself.  If the LSA is Type-5, from
        among the multiple non-NSSA routing table entries for the ASBR
        (both NSSA and non-NSSA ASBR entries might exists on an NSSA
        border router), select the preferred entry as follows:
        ~[OSPF]
          If RFC1583Compatibility is set to "disabled", prune the set
          of routing table entries for the ASBR as described in OSPF
          Section 16.4.1.  In any case, among the remaining routing
          table entries, select the routing table entry with the least
          cost; when there are multiple least cost routing table
          entries the entry whose associated area has the largest OSPF
          Area ID (when considered as an unsigned 32-bit integer) is
          chosen.
          [OSPF]
        Since a Type-7 LSA only has area-wide flooding scope, when its
        forwarding address is set to 0.0.0.0, its ASBR's routing table
        entry must be chosen from the originating NSSA.  Here no
        pruning is necessary since this entry always contains non-
        backbone intra-area paths.
        [NSSA]
        If the forwarding address is non-zero look up the forwarding
        address in the routing table.  For a Type-5 LSA the matching
        routing table entry must specify an intra-area or inter-area
        path through a Type-5 capable area.  For a Type-7 LSA the
        matching routing table entry must specify an intra-area path
        through the LSA's originating NSSA.  If no such path exists

Murphy Standards Track [Page 12] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

        then do nothing with this LSA and consider the next in the
        list.
        [NSSA]
    (4) Let X be the cost specified by the preferred routing table
        entry for the ASBR/forwarding address, and Y the cost
        specified in the LSA.  X is in terms of the link state metric,
        and Y is a type 1 or 2 external metric.
        [OSPF]
    (5) Now, look up the routing table entry for the destination N.
        If no entry exists for N, install the AS external path to N,
        with the next hop equal to the list of next hops to the
        ASBR/forwarding address, and advertising router equal to the
        ASBR.  If the external metric type is 1, then the path-type is
        set to Type-1 external and the cost is equal to X + Y.  If the
        external metric type is 2, the path-type is set to Type-2
        external, the link-state component of the route's cost is X,
        and the type 2 cost is Y.
        [OSPF]
    (6) Otherwise compare the AS external path described by the LSA
        with the existing paths in N's routing table entry.  If the
        new path is preferred, it replaces the present paths in N's
        routing table entry.  If the new path is of equal preference,
        it is added to the present paths in N's routing table entry.
        [OSPF]
        Preference is defined as follows:
        (a) Intra-area and inter-area paths are always preferred over
            AS external paths.
            [OSPF]
        (b) Type 1 external paths are always preferred over type 2
            external paths.  When all paths are type 2 external paths,
            the paths with the smallest advertised type 2 metric are
            always preferred.
            [OSPF]
        (c) If the new AS external path is still indistinguishable
            from the current paths in N's routing table entry, and
            RFC1583Compatibility is set to "disabled", select the
            preferred paths based on the intra-AS paths to the
            ASBR/forwarding addresses, as specified in Section 16.4.1.
            Here intra-NSSA paths are equivalent to the intra-area
            paths of non-backbone regular OSPF areas.
            [NSSA]

Murphy Standards Track [Page 13] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

        (d) If the new AS external path is still indistinguishable
            from the current paths in N's routing table entry, select
            the preferred path based on a least cost comparison.  Type
            1 external paths are compared by looking at the sum of the
            distance to the ASBR/forwarding addresses and the
            advertised type 1 metric (X+Y).  Type 2 external paths
            advertising equal type 2 metrics are compared by looking
            at the distance to the ASBR/forwarding addresses.
            ~[OSPF]
        (e) If the current LSA is functionally the same as an
            installed LSA (i.e., same destination, cost and non-zero
            forwarding address) then apply the following priorities in
            deciding which LSA is preferred:
               1. A Type-7 LSA with the P-bit set.
               2. A Type-5 LSA.
               3. The LSA with the higher router ID.
            [NSSA]

2.6 Incremental Updates

 Incremental updates for Type-7 LSAs should be treated the same as
 incremental updates for Type-5 LSAs (see [OSPF] Section 16.6).  When
 a new instance of a Type-7 LSA is received it is not necessary to
 recalculate the entire routing table.  Call the destination described
 by the Type-7 LSA N.  N's address is obtained by masking the LSA's
 Link State ID with the network/subnet mask contained in the body of
 the LSA.  If there is already an intra-area or inter-area route to
 the destination, no recalculation is necessary (internal routes take
 precedence).
 Otherwise, the procedure in the preceding section will have to be
 performed but only for the external routes (Type-5 and Type-7) whose
 destination is N.  Before this procedure is performed, the present
 routing table entry for N should be invalidated.

2.7 Optionally Importing Summary Routes

 In order for OSPF's summary routing to not be obscured by an NSSA's
 Type-7 AS-external-LSAs, all NSSA border router implementations must
 support the optional import of summary routes into NSSAs as Type-3
 summary-LSAs.  The default behavior is to import summary routes.  A
 new area configuration parameter, ImportSummaries, is defined in
 Appendix D.  When ImportSummaries is set to enabled, summary routes

Murphy Standards Track [Page 14] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 are imported.  When it is set to disabled, summary routes are not
 imported.  The default setting is enabled.
 When OSPF's summary routes are not imported, the default LSA
 originated by an NSSA border router into the NSSA should be a Type-3
 summary-LSA. This protects the NSSA from routing intra-AS traffic out
 the AS via the default route of a Type-7 default LSA originating from
 one of the NSSA's internal routers.  When summary routes are imported
 into the NSSA, the default LSA originated by an NSSA border router
 must not be a Type-3 summary-LSA; otherwise its default route would
 be chosen over the potentially more preferred default routes of
 Type-7 default LSAs.

3.0 Intra-AS Implementation Details

3.1 Type-7 Translator Election

 It is not recommended that multiple NSSA border routers perform
 Type-7 to Type-5 translation unless it is required to route packets
 efficiently through Area 0 to an NSSA partitioned by Type-7 address
 ranges.  It is normally sufficient to have only one NSSA border
 router perform the translation.  Excessive numbers of Type-7
 translators unnecessarily increase the size of the OSPF link state
 data base.
 A new area configuration parameter, NSSATranslatorRole, is defined in
 Appendix D.  It specifies whether or not an NSSA router will
 unconditionally translate Type-7 LSAs to Type-5 LSAs when acting as
 an NSSA border router. Configuring the identity of the translator can
 be used to bias the routing to aggregated destinations. When
 NSSATranslatorRole is set to Always, Type-7 LSAs are always
 translated regardless of the translator state of other NSSA border
 routers.  When NSSATranslatorRole is set to Candidate an NSSA border
 router will participate in the translator election process described
 below.
 A new area parameter, NSSATranslatorState, is maintained in an NSSA's
 OSPF area data structure.  By default all NSSA routers initialize
 NSSATranslatorState to disabled.  When an NSSA border router's
 NSSATranslatorState changes from disabled to either enabled or
 elected, it begins translating the NSSA's Type-7 LSAs into Type-5
 LSAs.  When its NSSATranslatorState changes from either enabled or
 elected to disabled, it ceases translating the NSSA's Type-7 LSAs
 into Type-5 LSAs. (See paragraphs below.)
 A new bit, Nt, is defined for the router-LSAs of NSSAs.  (See
 Appendix B.)  By default routers clear bit Nt when originating
 router-LSAs.  However, when an NSSA border router has its

Murphy Standards Track [Page 15] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 NSSATranslatorState enabled, it sets bit Nt in the router-LSA it
 originates into the NSSA.  An NSSA router whose NSSATranslatorRole is
 set to Always should re-originate a router-LSA into the NSSA whenever
 its NSSATranslatorState changes.
 When an NSSA router with the NSSA's NSSATranslatorRole set to Always
 attains border router status, it should change NSSATranslatorState
 from disabled to enabled.  When it loses border router status, it
 should change NSSATranslatorState from enabled to disabled.
 All NSSA border routers must set the E-bit in the Type-1 router-LSAs
 of their directly attached non-stub areas, even when they are not
 translating.  This allows other NSSA border routers to see their ASBR
 status across the AS's transit topology.  Failure to do so may cause
 the election algorithm to elect unnecessary translators.  Every NSSA
 border router is a potential translator.
 An NSSA border router whose NSSA's NSSATranslatorRole is set to
 Candidate must maintain a list of the NSSA's border routers that are
 reachable both over the NSSA and as ASBRs over the AS's transit
 topology.  Any change in this list, or to the Nt bit settings of
 members of this list, causes the NSSA border router to reevaluate its
 NSSATranslatorState.  If there exists another border router in this
 list whose router-LSA has bit Nt set or who has a higher router ID,
 then its NSSATranslatorState is disabled.  Otherwise its
 NSSATranslatorState is elected.
 An elected translator will continue to perform translation duties
 until supplanted by a reachable NSSA border router whose Nt bit is
 set or whose router ID is greater.  Such an event may happen when an
 NSSA router with NSSATranslatorRole set to Always regains border
 router status, or when a partitioned NSSA becomes whole.  If an
 elected translator determines its services are no longer required, it
 continues to perform its translation duties for the additional time
 interval defined by a new area configuration parameter,
 TranslatorStabilityInterval.  This minimizes excessive flushing of
 translated Type-7 LSAs and provides for a more stable translator
 transition.  The default value for the TranslatorStabilityInterval
 parameter has been defined as 40 seconds. (See Appendix D.)

3.2 Translating Type-7 LSAs into Type-5 LSAs

 This step is performed as part of the NSSA's Dijkstra calculation
 after Type-5 and Type-7 routes have been calculated.  If the
 calculating router is currently not an NSSA border router translator,
 then this translation algorithm should be skipped.  Only installed

Murphy Standards Track [Page 16] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 Type-7 LSAs and those non-default Type-7 LSAs originated by the
 router itself should be examined.  Locally sourced Type-7 LSAs should
 be processed first.
 Note that it is possible for a Type-5 LSA generated by translation to
 supplant a Type-5 LSA originating from a local OSPF external source.
 Future reoriginations of the locally sourced Type-5 LSA should be
 suppressed until the Type-5 LSA generated by translation is flushed.
 A Type-7 LSA and a Type-7 address range best match one another if
 there does not exist a more specific Type-7 address range that
 contains the LSA's network.  For each eligible Type-7 LSA perform the
 following:
    (1) If the Type-7 LSA has the P-bit clear, or its forwarding
        address is set to 0.0.0.0, or the most specific Type-7 address
        range that subsumes the LSA's network has DoNotAdvertise
        status, then do nothing with this Type-7 LSA and consider the
        next one in the list.  Otherwise term the LSA as translatable
        and proceed with step (2).
    (2) If the Type-7 LSA is not contained in any explicitly
        configured Type-7 address range and the calculating router has
        the highest router ID amongst NSSA translators that have
        originated a functionally equivalent Type-5 LSA (i.e. same
        destination, cost and non-zero forwarding address) and that
        are reachable over area 0 and the NSSA, then a Type-5 LSA
        should be generated if there is currently no Type-5 LSA
        originating from this router corresponding to the Type-7 LSA's
        network, or there is an existing Type-5 LSA and either it
        corresponds to a local OSPF external source whose path type
        and metric is less preferred (see Section 2.5 step (6), parts
        (b) and (d)), or it doesn't and the Type-5 LSA's path type or
        cost(s) have changed (See Section 2.5 step (5)) or the
        forwarding address no longer maps to a translatable Type-7
        LSA.
        The newly originated Type-5 LSA will describe the same network
        and have the same network mask, path type, metric, forwarding
        address and external route tag as the Type-7 LSA.  The
        advertising router field will be the router ID of this NSSA
        border router.  The link-state ID is equal to the LSA's
        network address (in the case of multiple originations of
        Type-5 LSAs with the same network address but different mask,
        the link-state ID can also have one or more of the network's
        "host" bits set).

Murphy Standards Track [Page 17] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

    (3) Else the Type-7 LSA must be aggregated by the most specific
        Type-7 address range that subsumes it.  If this Type-7 address
        range has the same [address,mask] pair as the LSA's network
        and no other translatable Type-7 LSA with a different network
        best matches this range, then flag the LSA as not contained in
        any explicitly configured Type-7 address range and process the
        LSA as described in step (2).  Otherwise compute the path type
        and metric for this Type-7 address range as described below.
        The path type and metric of the Type-7 address range is
        determined from the path types and metrics of those
        translatable Type-7 LSAs that best match the range plus any
        locally sourced Type-5 LSAs whose network has the same
        [address,mask] pair.  If any of these LSAs have a path type of
        2, the range's path type is 2, otherwise it is 1.  If the
        range's path type is 1 its metric is the highest cost amongst
        these LSAs; if the range's path type is 2 its metric is the
        highest Type-2 cost + 1 amongst these LSAs.  (See Section 2.5
        step (5).)  1 is added to the Type-2 cost to ensure that the
        translated Type-5 LSA does not appear closer on the NSSA
        border than a translatable Type-7 LSA whose network has the
        same [address,mask] pair and Type-2 cost.
        A Type-5 LSA is generated from the Type-7 address range when
        there is currently no Type-5 LSA originated by this router
        whose network has the same [address,mask] pair as the range or
        there is but either its path type or metric has changed or its
        forwarding address is non-zero.
        The newly generated Type-5 LSA will have a link-state ID equal
        to the Type-7 address range's address (in the case of multiple
        originations of Type-5 LSAs with the same network address but
        different mask, the link-state ID can also have one or more of
        the range's "host" bits set).  The advertising router field
        will be the router ID of this area border router.  The network
        mask and the external route tag are set to the range's
        configured values.  The forwarding address is set to 0.0.0.0.
        The path type and metric are set to the range's path type and
        metric as defined and computed above.
        The pending processing of other translation eligible Type-7
        LSAs that best match this Type-7 address range is suppressed.
        Thus at most a single Type-5 LSA is originated for each Type-7
        address range.

Murphy Standards Track [Page 18] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 For example, given a Type-7 address range of [10.0.0.0, 255.0.0.0]
 that subsumes the following Type-7 routes:
               10.1.0.0/24 path type 1, cost 10
               10.2.0.0/24 path type 1, cost 11
               10.3.0.0/24 path type 2, type 2 cost 5
 a Type-5 LSA would be generated with a path type of 2 and a metric 6.
 Given a Type-7 address range of [10.0.0.0, 255.0.0.0] that subsumes
 the following Type-7 routes:
               10.1.0.0/24 path type 1, cost 10
               10.2.0.0/24 path type 1, cost 11
               10.3.0.0/24 path type 1, cost 5
 a Type-5 LSA will be generated with a path type of 1 and a metric 11.
 These Type-7 address range metric and path type rules will avoid
 routing loops in the event that path types 1 and 2 are both used
 within the area.
 As with all newly originated Type-5 LSAs, a Type-5 LSA that is the
 result of a Type-7 LSA translation or aggregation is flooded to all
 attached Type-5 capable areas.
 Like Type-3 address ranges, a Type-7 address range performs the dual
 function of setting propagation policy via its
 Advertise/DoNotAdvertise parameter and aggregation via its network
 address and mask pair. Unlike the origination of Type-3 summary-LSAs,
 the translation of a Type-7 LSA into a Type-5 LSA may result in more
 efficient routing when the forwarding address is set, as is done
 during step (2) of the translation procedure.
 Another important feature of this translation process is that it
 allows a Type-7 address range to apply different properties
 (aggregation, forwarding address, and Advertise/DoNotAdvertise
 status) for the Type-7 routes it subsumes, versus those Type-7 routes
 subsumed by other more specific Type-7 address ranges contained in
 the Type-7 address range.

3.3 Flushing Translated Type-7 LSAs

 If an NSSA border router has either translated or aggregated an
 installed Type-7 LSA into a Type-5 LSA that should no longer be
 translated or aggregated, then the Type-5 LSA should either be
 flushed or reoriginated as a translation or aggregation of other
 Type-7 LSAs.

Murphy Standards Track [Page 19] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 If an NSSA border router is translating Type-7 LSA's into Type-5
 LSA's with NSSATranslatorState set to elected and the NSSA border
 router has determined that its translator election status has been
 deposed by another NSSA border router (see Section 3.1), then, as
 soon as the TranslatorStabilityInterval has expired without the
 router reelecting itself as a translator, Type-5 LSAs that are
 generated by aggregating Type-7 LSAs into their best matched Type-7
 address ranges (see Section 3.2, Step (3)) are flushed.  Conversely
 Type-5 LSAs generated by translating Type-7 LSAs are not immediately
 flushed, but are allowed to remain in the OSPF routing domain as if
 the originator is still an elected translator.  This minimizes the
 flushing and flooding impact on the transit topology of an NSSA that
 changes its translators frequently.

4.0 Security Considerations

 There are two types of issues that need be addressed when looking at
 protecting routing protocols from misconfigurations and malicious
 attacks.  The first is authentication and certification of routing
 protocol information.  The second is denial of service attacks
 resulting from repetitive origination of the same router
 advertisement or origination of a large number of distinct
 advertisements resulting in database overflow.  Note that both of
 these concerns exist independently of a router's support for the NSSA
 option.
 The OSPF protocol addresses authentication concerns by authenticating
 OSPF protocol exchanges.  OSPF supports multiple types of
 authentication; the type of authentication in use can be configured
 on a per network segment basis.  One of OSPF's authentication types,
 namely the Cryptographic authentication option, is believed to be
 secure against passive attacks and provides significant protection
 against active attacks.  When using the Cryptographic authentication
 option, each router appends a "message digest" to its transmitted
 OSPF packets.  Receivers then use the shared secret key and the
 received digest to verify that each received OSPF packet is
 authentic.
 The quality of the security provided by the Cryptographic
 authentication option depends completely on the strength of the
 message digest algorithm (MD5 is currently the only message digest
 algorithm specified), the strength of the key being used, and the
 correct implementation of the security mechanism in all communicating
 OSPF implementations.  It also requires that all parties maintain the
 secrecy of the shared secret key.  None of the standard OSPF
 authentication types provide confidentiality, nor do they protect
 against traffic analysis.  For more information on the standard OSPF
 security mechanisms, see Sections 8.1, 8.2, and Appendix D of [OSPF].

Murphy Standards Track [Page 20] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 [DIGI] describes the extensions to OSPF required to add digital
 signature authentication to Link State data and to provide a
 certification mechanism for router data.  [DIGI] also describes the
 added LSA processing and key management as well as a method for
 migration from or co-existence with standard OSPF V2.
 OSPF addresses repetitive origination of advertisements by mandating
 a limit on how frequent new instances of any particular LSA can be
 originated and accepted during the flooding procedure.  The frequency
 at which new LSA instances may be originated is set to once every
 MinLSInterval seconds, whose value is 5 seconds.  (See [OSPF] Section
 12.4.)  The frequency at which new LSA instances are accepted during
 flooding is once every MinLSArrival seconds, whose value is set to 1
 second.  (See [OSPF] Section 13, Appendix B, and G.1.)
 Proper operation of the OSPF protocol requires that all OSPF routers
 maintain an identical copy of the OSPF link state database.  However,
 when the size of the link state database becomes very large, some
 routers may be unable to keep the entire database due to resource
 shortages; this is termed "database overflow".  When database
 overflow is anticipated, the routers with limited resources can be
 accommodated by configuring OSPF stub areas and NSSAs.  [OVERFLOW]
 details a way of gracefully handling unanticipated database
 overflows.

5.0 Acknowledgements

 This document was produced by the OSPF Working Group, chaired by John
 Moy.
 In addition, the comments of the following individuals are also
 acknowledged:
    Antoni Przygienda  Redback Networks, Inc
    Alex Zinin         cisco
 It is also noted that comments from
    Phani Jajjarvarpu  cisco
    Dino Farinacci     cisco
    Jeff Honig         Cornell University
    Doug Williams      IBM
 were acknowledged in the predecessor of this document, RFC 1587.

Murphy Standards Track [Page 21] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

6.0 Contributors

 This document, RFC 3101, adds new sections, features, edits, and
 revisions to its predecessor, RFC 1587, "The OSPF NSSA Option",
 authored by Rob Coltun, Movaz Networks, and Vince Fuller.  Content
 from RFC 1587 is used throughout RFC 3101.  In addition to adding new
 features, this document makes the NSSA specification consistent with
 the OSPFv2 specification, RFC 2328, authored by John Moy, Sycamore
 Networks, Inc.  Section 2.5, Calculating Type-7 AS External Routes,
 and Section 2.6,  Incremental Updates, rely heavily on text in RFC
 2328's Section 16.4 and Section 16.6 respectively.  Section 4.0,
 Security Considerations, is an edit of similar content in Rob
 Coltun's RFC 2370, "The OSPF Opaque LSA option", Section 6.0.
 Acee Lindem, Redback Networks, Inc, is also recognized for the first
 full known implementation of this specification. Acee's
 implementation resulted in substantive content change.

7.0 References

 [DIGI]     Murphy, S., Badger, M. and B. Wellington, "OSPF with
            Digital Signatures", RFC 2154, June 1997.
 [MUEX]     Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
            1994.
 [OSPF]     Moy, J., "OSPF Version 2", RFC 2328, April 1998.
 [OPAQUE]   Coltun, R., "The OSPF Opaque LSA Option", RFC 2370, July
            1998.
 [OVERFLOW] Moy, J., "OSPF Database Overflow", RFC 1765, March 1995.

Murphy Standards Track [Page 22] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Appendix A: The Options Field

 The OSPF options field is present in OSPF Hello packets, Database
 Description packets and all link state advertisements.  See [OSPF]
 Appendix A.2 and [OPAQUE] Appendix A.1 for a description of the
 options field.  Six bits are assigned but only two (the E-bit and the
 N/P bit) are described completely in this section.
  1. ————————————-

| * | O | DC | EA | N/P | MC | E | * |

  1. ————————————-
                    The Type-7 LSA options field
    E-bit:  Type-5 AS-external-LSAs are not flooded into/through OSPF
            stub areas and NSSAs.  The E-bit ensures that all members
            of a stub area or NSSA agree on that area configuration.
            The E-bit is meaningful only in OSPF Hello and Database
            Description packets.  When the E-bit is clear in the Hello
            packet sent out a particular interface, it means that the
            router will neither send nor receive Type-5 AS-external-
            LSAs on that interface (in other words, the interface
            connects to a stub area or NSSA).  Two routers will not
            become neighbors unless they agree on the state of the E-
            bit.
    N-bit:  The N-bit describes the router's NSSA capability.  The N-
            bit is used only in Hello packets and ensures that all
            members of an NSSA agree on that area's configuration.
            When the N-bit is set in the Hello packet that is sent out
            a particular interface, it means that the router will send
            and receive Type-7 LSAs on that interface.  Two routers
            will not form an adjacency unless they agree on the state
            of the N-bit.  If the N-bit is set in the options field,
            the E-bit must be clear.
    P-bit:  The P-bit is used only in the Type-7 LSA header.  It flags
            the NSSA border router to translate the Type-7 LSA into a
            Type-5 LSA.  The default setting for the P-bit is clear.

Murphy Standards Track [Page 23] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Appendix B: Router-LSAs

 Router-LSAs are the Type-1 LSAs.  Each router in an area originates a
 router-LSA.  The LSA describes the state and cost of the router's
 links (i.e., interfaces) to the area.  All of the router's links to
 the area must be described in a single router-LSA.  For details
 concerning the construction of router-LSAs, see [OSPF] Section
 12.4.1.
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            LS age             |     Options   |       1       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Link State ID                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Advertising Router                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     LS sequence number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         LS checksum           |             length            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  0  Nt|W|V|E|B|        0      |            # links            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Link ID                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Link Data                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Type      |     # TOS     |        TOS 0 metric           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      TOS      |        0      |            metric             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              ...                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      TOS      |        0      |            metric             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Link ID                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Link Data                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              ...                              |
 In router-LSAs, the Link State ID field is set to the router's OSPF
 Router ID.  Router-LSAs are flooded throughout a single area only.

Murphy Standards Track [Page 24] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

    bit V
        When set, the router is an endpoint of one or more fully
        adjacent virtual links having the described area as their
        transit area (V is for virtual link endpoint).
    bit E
        When set, the router is an AS boundary router (E is for
        external).  ALL NSSA border routers set bit E in those
        router-LSAs originated into directly attached Type-5 capable
        areas.  An NSSA's AS boundary routers also set bit E in their
        router-LSAs originated into the NSSA.  (See Section 3.1 for
        details.)
    bit B
        When set, the router is an area border router (B is for
        border).
    bit W
        When set, the router is a wild-card multicast receiver (W is
        for wild).
    bit Nt
        When set, the router is an NSSA border router that is
        unconditionally translating Type-7 LSAs into Type-5 LSAs (Nt
        stands for NSSA translation).  Note that such routers have
        their NSSATranslatorRole area configuration parameter set to
        Always.  (See Appendix D and Section 3.1.)
 The remainder of the router-LSAs specification is defined in [OSPF]
 Section A.4.2.

Murphy Standards Track [Page 25] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Appendix C: Type-7 LSA Packet Format

      0                                 32
      ------------------------------------
      |                | Options |   7   |
      |                -------------------
      |        Link-State Header         |
      |                                  |
      ------------------------------------
      | Network Mask                     |
      ------------------------------------  ______
      |E| TOS  |        metric           |  .
      ------------------------------------  .  repeated for each TOS
      | Forwarding Address               |  .
      ------------------------------------  .
      | External Route Tag               |  ______
      ------------------------------------
 The definitions of the link-state ID, network mask, metrics and
 external route tag are the same as the definitions for Type-5 LSAs
 (See [OSPF] Appendix A.4.5), except for the forwarding address and
 the N/P-bit.  The Options field must have the N/P bit set as
 described in Appendix A when the originating router desires that the
 external route be propagated throughout the OSPF domain.
 Forwarding address
    Data traffic for the advertised destination will be forwarded to
    this address.  If the forwarding address is set to 0.0.0.0, data
    traffic will be forwarded to the LSA's originator (i.e., the
    responsible NSSA AS boundary router).  Normally the next hop
    address of an installed AS external route learned by an NSSA ASBR
    from an adjacent AS points at one of the adjacent AS's gateway
    routers.  If this address belongs to a network connected to the
    NSSA ASBR via one of its NSSAs' active interfaces, then it is the
    forwarding address for the route's Type-7 LSA originated into this
    NSSA.  For an NSSA with no such network the forwarding address is
    either an address from one of its active interfaces or 0.0.0.0.
    If the P-bit is set, the forwarding address must be non-zero,
    otherwise it may be 0.0.0.0. (See Section 2.3 for details.)

Murphy Standards Track [Page 26] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Appendix D: Configuration Parameters

 [OSPF] Appendix C.2 lists the area configuration parameters.  The
 area ID and the list of address ranges for Type-3 summary routes
 remain unchanged.  Section 2.2 of this document lists the
 configuration parameters for Type-7 address ranges.  The following
 area configuration parameters have been added:
    NSSATranslatorRole
       Specifies whether or not an NSSA border router will
       unconditionally translate Type-7 LSAs into Type-5 LSAs.  When
       it is set to Always, an NSSA border router always translates
       Type-7 LSAs into Type-5 LSAs regardless of the translator state
       of other NSSA border routers.  When it is set to Candidate, an
       NSSA border router participates in the translator election
       process described in Section 3.1.  The default setting is
       Candidate.
    TranslatorStabilityInterval
       Defines the length of time an elected Type-7 translator will
       continue to perform its translator duties once it has
       determined that its translator status has been deposed by
       another NSSA border router translator as described in Section
       3.1 and 3.3.  The default setting is 40 seconds.
    ImportSummaries
       When set to enabled, OSPF's summary routes are imported into
       the NSSA as Type-3 summary-LSAs.  When set to disabled, summary
       routes are not imported into the NSSA.  The default setting is
       enabled.
 Implementations must provide a vehicle for setting the P-bit when
 external routes are imported into the NSSA as Type-7 LSAs.  Without
 configuration, the default setting of the P-bit is clear.  (See
 Section 2.3 and Appendix B.)
 For NSSAs the ExternalRoutingCapability area configuration parameter
 must be set to accept Type-7 external routes.  Additionally there
 must be a way of configuring the metric of the default LSA that a
 border router advertises into its directly attached NSSAs. If a
 Type-7 default LSA is advertised, its metric type (1 or 2) should
 also be configurable.

Murphy Standards Track [Page 27] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Appendix E: The P-bit Policy Paradox.

 Non-default Type-7 LSAs with the P-bit clear may be installed in the
 OSPF routing table of NSSA border routers.  (See Section 2.5.)  These
 LSAs are not propagated throughout the OSPF domain as translated
 Type-5 LSAs.  (See Section 3.2.)  Thus, traffic that is external to
 an NSSA and that passes through one of the NSSA's border routers may
 be hijacked into the NSSA by a route installed from a Type-7 LSA with
 the P-bit clear.  This may be contrary to the expected path at the
 source of the traffic.  It may also violate the routing policy
 intended by the Type-7 LSA's clear P-bit.  A Type-7 address range
 that is configured with DoNotAdvertise exhibits the same paradox for
 any installed Type-7 LSAs it subsumes, regardless of the P-bit
 setting.
 This paradox is best illustrated by the following example.  Consider
 an OSPF domain (AS 1842) with connections for default Internet
 routing and to external AS 4156.  NSSA 1 and OSPF Area 2 are
 partially defined in the following diagram:
                            AS 4156
                              |
          Area 2              |
                              |
            A2                A0   Area 0      C0-----Internet
            |                 |                |      Default
            |                 |                |
            |                 |                |
            +-----------------B0---------------+
                              /\
                             /  \
                            /    \
       Internet------------A1    B1------AS 4156 (P-bit clear)
       Default (P-bit set)
                               NSSA 1
 Here A0, B0, and C0 are Area 0 routers, A1 and B1 are NSSA 1 routers,
 and A2 is an Area 2 router.  B0 is a border router for both NSSA 1
 and Area 2.
 If the Type-7 external routes imported by B1 for AS 4156 are
 installed on B0 so that the NSSA 1 tree below A1 can take advantage
 of them, then A2's traffic to AS 4156 is hijacked through B0 by B1,
 rather than its computed path through A0.
 An NSSA border router's installed Type-7 default LSAs will exhibit
 this paradox when it possesses a Type-7 address range [0,0]
 configured with DoNotAdvertise, as these LSAs are not propagated even

Murphy Standards Track [Page 28] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

 though their P-bit is set.  In the example above, if A1's default is
 installed on B0, which has a configured Type-7 address range [0,0]
 with DoNotAdvertise set, then A2's Internet traffic is hijacked
 through B0 by A1 rather than the computed path through C0.

Murphy Standards Track [Page 29] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Appendix F: Differences from RFC 1587

 This section documents the differences between this memo and RFC
 1587.  All differences are backward-compatible.  Implementations of
 this memo and of RFC 1587 will interoperate.

F.1 Enhancements to the import of OSPF's summary routes.

 The import of OSPF's summary routes into an NSSA as Type-3 summary-
 LSAs is now optional.  In RFC 1587 the import of summary routes was
 mandated in order to guarantee that inter-area summary routing was
 not obscured by an NSSA's Type-7 AS-external-LSAs. The current
 recommended default behavior is to import summary routes.  When
 summary routes are not imported into an NSSA, the default LSA
 originated by its border routers must be a Type-3 summary-LSA.
 See Sections 1.3 and 2.7 for details.

F.2 Changes to Type-7 LSAs.

 The setting of the forwarding address in Type-7 LSAs has been further
 refined.
 See Section 2.3 for details.

F.3 Changes to the Type-7 AS external routing calculation.

 The Type-7 external route calculation has been revised.  Most
 notably:
    o  The path preference defined in [OSPF] Section 16.4.1 has been
       included.
    o  A Type-7 default route with the P-bit clear will not be
       installed on an NSSA border router.  This protects the default
       routing of other OSPF Areas.  (See Appendix E.)
    o  The LSA type of two AS-external-LSAs plays no role in
       determining path preference except when the LSAs are
       functionally the same (i.e., same destination, cost and non-
       zero forwarding address).
 See Section 2.5 for details.

Murphy Standards Track [Page 30] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

F.4 Changes to translating Type-7 LSAs into Type-5 LSAs

 The translator election algorithm of RFC 1587 has been updated to
 close a bug that results when the translator with the highest router
 ID loses connectivity to the AS's transit topology.  The default
 translator election process occurs only in the absence of an existing
 translator.
 The identity of the translator is optionally configurable, with more
 than one allowed.  This allows the network designer to choose the
 most cost effective intra-AS route for NSSA originated Type-5 LSA
 aggregations of Type-7 LSAs.
 Self-originated non-default Type-7 LSAs are now included in the
 translation process.
 The translation process has been strengthened to close some of the
 weak points of RFC 1587.
 See Sections 3.1 and 3.2 for details.

F.5 Changes to flushing translated Type-7 LSAs

 An NSSA border router, which was elected by the augmented RFC 1587
 translator selection process defined in Section 3.1 and which has
 been deposed from its translation duties by another NSSA border
 router, flushes its self-originated Type-5 LSAs that resulted from
 the aggregation of Type-7 LSAs.  This prevents these obsolete
 aggregations from short circuiting the preferred path through the new
 translator(s).  A deposed translator continues to maintain its self-
 originated Type-5 LSAs resulting from translation until they age out
 normally.
 See Section 3.3 for details.

F.6 P-bit additions

 The P-bit default has been defined as clear.  RFC 1587 had no default
 setting. (See Appendix C.)
 A discussion on the packet forwarding impact of installing Type-7
 LSAs with the P-bit clear on NSSA border routers has been added as
 Appendix E.

Murphy Standards Track [Page 31] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Author's Addresses

 Pat Murphy
 US Geological Survey
 345 Middlefield Road
 Menlo Park, California 94560
 Phone: (650) 329-4044
 EMail: pmurphy@noc.usgs.net

Murphy Standards Track [Page 32] RFC 3101 The OSPF Not-So-Stubby Area (NSSA) Option January 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
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 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
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 followed, or as required to translate it into languages other than
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 The limited permissions granted above are perpetual and will not be
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

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 Internet Society.

Murphy Standards Track [Page 33]

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