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rfc:rfc1587

Network Working Group R. Coltun Request for Comments: 1587 RainbowBridge Communications Category: Standards Track V. Fuller

                                                   Stanford University
                                                            March 1994
                        The OSPF 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.

Table Of Contents

 1.0 Abstract .................................................  1
 2.0 Overview .................................................  2
 2.1 Motivation ...............................................  2
 2.2 Proposed Solution ........................................  3
 3.0 Implementation Details ...................................  5
 3.1 The N-bit ................................................  5
 3.2 Type-7 Address Ranges ....................................  5
 3.3 Type-7 LSAs ..............................................  5
 3.4 Originating Type-7 LSAs ..................................  7
 3.5 Calculating Type-7 AS External Routes ....................  8
 3.6 Incremental Updates ...................................... 10
 4.0 Originating Type-5 LSAs .................................. 10
 4.1 Translating Type-7 LSAs .................................. 10
 4.2 Flushing Translated Type-7 LSAs .......................... 13
 5.0 Acknowledgements ......................................... 13
 6.0 References ............................................... 13
 7.0 Security Considerations .................................. 13
 8.0 Authors' Addresses ....................................... 14
 Appendix A: Type-7 LSA Packet Format ......................... 15
 Appendix B: The Options Field ................................ 16
 Appendix C: Configuration Parameters ......................... 17

1.0 Abstract

 This document describes a new optional type of OSPF area, 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.

Coltun & Fuller [Page 1] RFC 1587 OSPF NSSA Option March 1994

2.0 Overview

2.1 Motivation

 Wide-area transit networks (such as the NSFNET regionals) 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, BARRNet has one site composed of a class-B network,
 130.57.0.0, and a class-C network, 192.31.114.0.  From BARRNet's
 perspective, this configuration looks something like this:
                  192.31.114
                      |
                    (cloud)
                -------------- 130.57.4
                      |
                      |
                   ------ 131.119.13 ------
                   |BR18|------------|BR10|
                   ------            ------
                                        |
                                        V
                                to BARRNet "core" OSPF system
 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.
 Topologically, this cloud looks very much like an OSPF stub area.
 The advantages of running the cloud as an OSPF stub area are:
           1. Type-5 routes (OSPF external link-state advertisements
              (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.

Coltun & Fuller [Page 2] RFC 1587 OSPF NSSA Option March 1994

           4. The cloud can become, logically, a part of the transit
              network's OSPF routing system.
           5. Translated type-5 LSAs that are sent into the
              backbone from the cloud (which is a separate
              stub area) may be considered "leaf" nodes
              when performing the Dijkstra calculation.
 However, the current definition of the OSPF protocol [1] imposes
 topological limitations which restrict simple cloud topologies from
 becoming OSPF stub areas.  In particular, it is illegal for a stub
 area to import routes external to OSPF; it is not possible for
 routers BR18 and BR10 to both be members of the stub area and to
 import the routes learned from RIP or other IP routing protocols as
 type-5 (OSPF external LSAs) into the OSPF system.  In order to run
 OSPF out to BR18, BR18 must be a member of a non-stub area or the
 OSPF backbone to import routes other than its directly-connected
 network(s).  Since it is not acceptable for BR18 to maintain all of
 BARRNet's external (type-5) routes, BARRNet is forced by OSPF's
 topological limitations to run OSPF out to BR10 and to run RIP
 between BR18 and BR10.

2.2 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 spec), 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,
 allows no 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) area 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 AS External LSAs have virtually the same syntax as the

Coltun & Fuller [Page 3] RFC 1587 OSPF NSSA Option March 1994

 Type-5 AS External LSAs with the obvious exception of the link-state
 type (see section 3.2 for more details). There are two major semantic
 differences between type-5 and type-7 LSAs.
        o  Type-7 LSAs may be originated by and advertised
           throughout an NSSA; as with stub areas, NSSA's do not
           receive or originate type-5 LSAs.
        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
           which they contain can be propagated into the backbone
           area (see section 3.6).
 In order to allow limited exchange of external information across an
 NSSA area 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 area border routers may
 be configured with address ranges so that several type-7 LSAs may be
 represented by a single type-5 LSA.
 In addition, an NSSA area border router can originate a default
 type-7 LSA (IP address of 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 to route to AS-external destinations.  Like
 stub areas, NSSAs may be connected to the backbone at more than one
 area border router, but may not be used as a transit area.  Note that
 the default route originated by an NSSA area border router is never
 translated into a type-5 LSA, however, a default route originated by
 an NSSA internal AS boundary router (one that is not also an area
 border router) may be translated into a type-5 LSA.
 It should also be noted that unlike stub areas, all OSPF summary
 routes (type-3 LSAs) must be imported into NSSAs.  This is to ensure
 that OSPF internal routes are always chosen over OSPF external
 (type-7) routes.
 In our example topology the subnets of 130.57 and network 192.31.114,
 will still be learned by RIP on router BR18 but now both BR10 and
 BR18 can be in an NSSA and all of BARRNets external routes are hidden
 from BR18; BR10 becomes an NSSA area 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 BARRNet's backbone.

Coltun & Fuller [Page 4] RFC 1587 OSPF NSSA Option March 1994

3.0 Implementation Details

3.1 The N-bit

 The N-bit ensures that all members of a 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's) external LSA
 flooding capability.  As explained in section 10.5 of the OSPF
 specification, 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 cleared.
 To support the NSSA option an additional check must be made in the
 function that handles receiving Hello packet to verify that both the
 N-bit and the E-bit found in the Hello packet's option field match
 the value of the options that have been configured in the receiving
 interface.  A mismatch in the options causes processing of the
 received Hello packet to stop and the packet to be dropped.

3.2 Type-7 Address Ranges

 NSSA area border routers may be configured with type-7 address
 ranges.  Each address range is defined as an [address,mask] pair.
 Many separate type-7 networks may then be represented by in a single
 address range (as advertised by a type-5 LSA), just as a subnetted
 network is composed of many separate subnets.  Area border routers
 may then summarize type-7 routes by advertising a single type-5 route
 for each type-7 address range.  The type-5 route, resulting from a
 type-7 address range match will be distributed to all type-5 capable
 areas.  Section 4.1 gives the details of generating type-5 routes
 from type-7 address ranges.
 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 External route tag.

3.3 Type-7 LSAs: NSSA External Link-State Advertisements

 External routes are imported into NSSAs as type-7 LSAs by the NSSA's
 AS boundary routers.  An NSSA AS boundary routers is a router which

Coltun & Fuller [Page 5] RFC 1587 OSPF NSSA Option March 1994

 has an interface associated with the NSSA and is exchanging routing
 information with routers belonging to another AS.  As with type-5
 LSAs a separate type-7 LSA is originated for each destination
 network.  To support NSSA areas, the link-state database must
 therefore be expanded to contain a type-7 LSA.
 Type 7-LSAs are identical to type-5 LSAs except for the following
 (see  section  12.3.4  "AS external links" in the OSPF
 specification).
    1. The type field in the LSA header is 7.
    2. Type-7 LSAs are only flooded within the NSSA.
       The flooding of type-7 LSAs follow the same rules
       as the flooding of type 1-4 LSAs.
    3. Type-7 LSAs are kept within the NSSA's LSDB (are
       area specific) whereas because type-5 LSAs are
       flooded to all type-5 capable areas, type-5 LSAs
       global scope in the router's LSDB.
    4. At the area border router, selected type-7 LSAs are
       translated into type 5-LSAs and flooded into the
       backbone.
    5. Type 7 LSAs have a  propagate (P) bit which is
       used to flag the area border router to translate the
       type-7 LSA into a type-5 LSA. Examples of how the P-bit
       is used for loop avoidance are in the following sections.
    6. Those type-7 LSAs that are to be translated into type-5
       LSAs must have their forwarding address set.
       Type-5 LSAs that have been translated from type-7 LSAs
       for the most part must contain a forwarding address.
       The execption to this is if the translation to a type-5
       LSA is the result of an address range match, in which
       case the type-5 LSA will not contain a forwarding address
       (see section 4.1 for details).
       The forwarding address contained in type-5 LSAs will
       result in more efficient routing to the AS external
       networks when there are multiple NSSA area
       border routers. Having the forwarding address in the
       type-7 LSAs will ease the translation of type-7 into
       type-5 LSAs as the NSSA area border router will
       not be required to compute the forwarding address.
       If the network between the NSSA AS boundary router and the
       adjacent AS is advertised into OSPF as an internal OSPF

Coltun & Fuller [Page 6] RFC 1587 OSPF NSSA Option March 1994

       route, the forwarding address should be the next hop
       address as is currently done in type-5 LSAs, but unlike
       type-5 LSAs if the intervening network is not advertised
       into OSPF as an internal OSPF route, the forwarding
       address should be any one of the router's active OSPF
       interface addresses.
 Type-5 and type-7 metrics and path types are directly comparable.

3.4 Originating Type-7 LSAs

 NSSA AS boundary routers may originate type-7 LSAs.  All NSSA area
 border routers must also be AS boundary routers since they all must
 have the capability of translating a type-7 LSAs into a type-5 LSAs
 (see section 3.6 routes for the translation algorithm).  NSSA area
 border routers must set the E-bit (external bit) as well as the B-bit
 (border bit) in its router (type-1) LSAs (both in the backbone and in
 the NSSA area).
 When an NSSA internal AS boundary router originates a type-7 LSA that
 it wants to be translated into a type-5 LSA by the NSSA area border
 router (and subsequently flooded into the backbone), it must set the
 P-bit in the LS header's option field and add a valid forwarding
 address in the type-7 LSA.
 If a router is attached to another AS and is also an NSSA area border
 router, it may originate a both a type-5 and a type-7 LSA for the
 same network.  The type-5 LSA will be flooded to the backbone (and
 all attached type-5 capable areas) and the type-7 will be flooded
 into the NSSA.  If this is the case, the P-bit must be reset in the
 type-7 NSSA so the type-7 LSA isn't again translated into a type-5
 LSA by another NSSA area border router.
 A type-7 default route (network 0.0.0.0) may be originated into the
 NSSA by an NSSA area border router or by an NSSA AS boundary router
 which is internal to the NSSA.  The type-7 default route originated
 by the NSSA area border router must have the P-bit reset so that the
 default route originated by the NSSA area border router will not find
 its way out of the NSSA into the rest of the AS system via another
 NSSA area border router.  The type-7 default route originated by an
 NSSA AS boundary router which is not an NSSA area border router may
 have the P-bit set.  Type-7 routes which are originated by the NSSA
 area border router will not get added to other NSSA area border
 router's routing table.
 A default route must not be injected into the NSSA as a summary
 (type-3) LSA as in the stub area case.  The reason for this is that
 the preferred summary default route would be chosen over all more

Coltun & Fuller [Page 7] RFC 1587 OSPF NSSA Option March 1994

 specific type-7 routes.  Because summary routes are preferred to
 external routes and to ensure that summary routes are chosen over
 external within the NSSA, all summary routes (unlike stub areas in
 which this is optional) must be imported into an NSSA.

3.5 Calculating Type-7 AS External Routes

 This section is very similar to section 16.4 (Calculating AS external
 routes) in the OSPF specification.  An NSSA area border router should
 examine both type-5 LSAs and type-7 LSAs if either type-5 or type-7
 routes need to be updated.  Type-7 LSAs should be examined after
 type-5 LSAs.  An NSSA internal router should examine type-7 LSAs when
 type-7 routes need to be recalculated.
 In relation to the steps to calculate the routing table as presented
 in the OSPF specification (chapter 16, "Calculation of the Routing
 Table"), type-7 LSAs should be examined after step 5 where the routes
 to external destinations are calculated.
 Type-7 routes are calculated by examining type-7 LSAs.  Each of LSAs
 are considered in turn. Most type-7 LSAs describe routes to specific
 IP destinations.  A type-7 LSA can also describe a default route for
 the NSSA (destination = DefaultDestination).  For each type-7 LSA:
    1. If the metric specified by the LSA is LSInfinity, the
       age of the LSA equals MaxAge or the advertising router
       field is equal to this router's router ID, examine the
       next advertisement.
    2. Call the destination described by the LSA N. Look up the
       routing table entry for the AS boundary router (ASBR) that
       originated the LSA. If no entry exists for the ASBR
       (i.e., ASBR is unreachable), do nothing with this LSA and
       consider the next in the list.
       If the destination is the default route (destination =
       DefaultDestination) and if the originator of the LSA and
       the calculating router are both NSSA area border routers
       do nothing with this LSA and consider the next in the list.
       Else, this LSA describes an AS external path to destination
       N. If the forwarding address (as specified in the forwarding
       address field of the LSA) is 0.0.0.0, the packets routed
       to the external destination N will be routed to the
       originating ASBR. If the forwarding address is not 0.0.0.0,
       look up the forwarding address in the routing table. Packets
       routed to the external destination N will be routed within
       the NSSA to this forwarding address. An intra-area path

Coltun & Fuller [Page 8] RFC 1587 OSPF NSSA Option March 1994

       must therefore exist to the forwarding address. If no such
       path exists, do nothing with the LSA and consider the next
       in the list.
       Call the routing table distance to the forwarding address
       (or the distance to the originating ASBR if the forwarding
       address is 0.0.0.0) X, and the cost specified in the type-7
       LSA Y. X is in terms of the link-state metric, and Y is a
       Type-1 or Type-2 external metric.
    3. Now, look up the routing table entry for the destination
       N. If no entries exist for N, install the AS external path
       to N, with the next hop equal to the list of next hops to
       the forwarding address/ASBR, and the advertising router equal
       to 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.
    4. Else, if the paths present in the table are not Type-1 or
       Type-2 external paths, do nothing (AS external paths have
       the lowest priority).
    5. Otherwise, compare the cost of this new AS external path
       to the ones present in the table. Note that type-5 and
       type-7 routes are directly comparable. Type-1 external
       paths are always shorter than Type-2 external paths.
       Type-1 external paths are compared by looking at the sum
       of the distance to the forwarding address/ASBR and the
       advertised Type-1 paths (X+Y). Type-2 external paths are
       compared by looking at the advertised Type-2 metrics,
       and then if necessary, the distance to the forwarding
       address/ASBR.
       When a type-5 LSA and a type-7 LSA are found to have the
       same type and an equal distance, the following priorities
       apply (listed from highest to lowest) for breaking the tie.
               a. Any type 5 LSA.
               b. A type-7 LSA with the P-bit set and the forwarding
                  address non-zero.
               c. Any other type-7 LSA.
       If the new path is shorter, it replaces the present paths
       in the routing table entry. If the new path is the same
       cost, it is added to the routing table entry's list of
       paths.

Coltun & Fuller [Page 9] RFC 1587 OSPF NSSA Option March 1994

3.6 Incremental Updates

 Incremental updates for type-7 LSAs should be treated the same as
 incremental updates for type-5 LSAs (see section 16.6 of the OSPF
 specification).  That is, if a new instance of a type-7 LSA is
 received it is not necessary to recalculate the entire routing table.
 If there is already an OSPF internal route to the destination
 represented by the type-7 LSA, no recalculation is necessary.
 Otherwise, the procedure in the proceeding section will have to be
 performed but only for the external routes (type-5 and type-7) whose
 networks describe the same networks as the newly received LSA.

4.0 Originating Type-5 LSAs

4.1 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 not an area border router this translation
 algorithm should be skipped.  All reachable area border routers in
 the NSSA should now be examined noting the one with the highest
 router ID.  If this router has the highest router ID, it will be the
 one translating type-7 LSAs into type-5 LSAs for the NSSA, otherwise
 the translation algorithm should not be performed.
 All type-7 routes that have been added to the routing table should be
 examined.  If the type-7 LSA (associated with the route being
 examined) has the P-bit set and a non-zero forwarding address, the
 following steps should be taken.
    The translation procedure must first check for a configured type-7
    address range.  Recall that an type-7 address range consists of an
    [address,mask] pair and a status indication of either Advertise or
    DoNotAdvertise.  At most a single type-5 LSA is made for each
    range.  If the route being examined falls within the type-7
    address range, (the [address,mask] pair of the route equal to or a
    more specific instance of the [address,mask] pair of the type-7
    address range), one of following three actions may take place.
       1. When the range's status indicates Advertise and the
          route's address and mask are equal to the address
          and mask of the type-7 range a type-5 LSA should be
          originated if:
          o there currently is no type-5 LSA originated from
            this router corresponding to the type-7 LSA,

Coltun & Fuller [Page 10] RFC 1587 OSPF NSSA Option March 1994

          o the path type or the metric in the corresponding
            type-5 LSA is different from the type-7 LSA or
          o The forwarding address in the corresponding
            type-5 LSA is different from the type-7 LSA.
            The newly originated type-5 LSA will describe
            the same network and have the same network mask,
            metrics, forwarding address, external route tag
            and path type as the type-7 LSA, however, the
            advertising router field will be the router ID
            of this area border router.
       2. When the range's status indicates Advertise and the
          route's address or mask indicates a more specific
          route (i.e., the route's address is subsumed by the
          range or the route has a longer mask), a type-5 LSA
          is generated with link-state ID equal to the range's
          address (if necessary, the link-state ID can also have
          one or more of the range's "host" bits set; see
          Appendix F of the OSPF specification for details),
          the network mask, external route tag and
          path type will be set to the configured type-7 range
          values. The advertising router field will be the
          router ID of this area border router.
          The forwarding address will not be set.
          The path type should always be set to the highest
          path type that is subsumed by the net range.
          The metric for the type-5 LSA will be set as follows:
          o if the path type is externl type 2, the type-5
            metric should be set to the largest type-7 metric
            subsumed by this net range + 1.
          o if the path type is external type 1, the type-5
            metric should be set to the largest metric.
          For example, given a net range of [10.0.0.0, 255.0.0.0]
          for an area that has type-7 routes of:
                  10.1.0.0 path type 1, metric 10
                  10.2.0.0 path type 1, metric 11
                  10.3.0.0 path type 2, metric 5
           a type-5 LSA will be generated with a path type of 2
           and a metric of 6.

Coltun & Fuller [Page 11] RFC 1587 OSPF NSSA Option March 1994

           As another example, given a net range of
           [10.0.0.0, 255.0.0.0] for an area that has
           type-7 routes of:
                  10.1.0.0 path type 1, metric 10
                  10.2.0.0 path type 1, metric 11
                  10.3.0.0 path type 1, metric 5
           a type-5 LSA will be generated with a path type of 1
           and a metric of 11.
           These metric and path type rules will avoid routing
           loops in the event that path type 1 and 2 are both
           used within the area.
       3. When the range's status indicates DoNotAdvertise,
          the type-5 LSA is suppressed and the component networks
          remain hidden from the rest of the AS.
          By default (given that the P-bit is set and the LSA has a
          non-zero forwarding address) if a network is not contained
          in any explicitly configured address range, a type-7 to
          type-5 LSA translation will occur.
          A new instance of a type-5 LSA should be originated and
          flooded to all attached type-5 capable areas if one of the
          following is true.
          1. There currently is no type-5 LSA originated from this
             router corresponding to the type-7 LSA.
          2. The path type or the metric in the corresponding
             type-5 LSA is different from the type-7 LSA.
          3. The forwarding address in the corresponding
             type-5 LSA is different from the type-7 LSA.
          The newly originated type-5 LSAs will describe the same
          network and have the same network mask, metrics, forwarding
          address, external route tag and path type as the type-7 LSA.
          The advertising router field will be the router ID of this
          area border router.
          As with all newly originated type-5 LSAs, a type-5 LSA that
          is the result of a type-7 to type-5 translation (type-7 range
          or default case) is flooded to all attached type-5 capable
          areas.

Coltun & Fuller [Page 12] RFC 1587 OSPF NSSA Option March 1994

4.2 Flushing Translated Type-7 LSAs

 If an NSSA area border router has translated a type-7 LSA to a type-5
 LSA that should no longer be translated, the type-5 LSA should be
 flushed (set to MaxAge and flooded).  The translated type-5 LSA
 should be flushed whenever the routing table entry that caused the
 translation changes so that either the routing table entry is
 unreachable or the entry's associated LSA is not a type-7 with the
 P-bit set and a non-zero forwarding address.

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:
                Phani Jajjarvarpu  cisco
                Dino Farinacci     cisco
                Jeff Honig         Cornell University
                John Moy           Proteon, Inc.
                Doug Williams      IBM

6.0 References

 [1] Moy, J., "OSPF Version 2", RFC 1583, Proteon, Inc., March 1994.
 [2] Moy, J., "Multicast Extensions to OSPF", RFC 1584, Proteon, Inc.,
     Proteon, Inc., March 1994.

7.0 Security Considerations

 Security issues are not discussed in this memo.

Coltun & Fuller [Page 13] RFC 1587 OSPF NSSA Option March 1994

8.0 Authors' Addresses

 Rob Coltun
 RainbowBridge Communications
 Phone: (301) 340-9416
 EMail: rcoltun@rainbow-bridge.com
 Vince Fuller
 BARRNet
 Stanford University
 Pine Hall 115
 Stanford, CA, 94305-4122
 Phone: (415) 723-6860
 EMail: vaf@Valinor.Stanford.EDU

Coltun & Fuller [Page 14] RFC 1587 OSPF NSSA Option March 1994

Appendix A: Type-7 Packet Format

        0                                32
        -----------------------------------
        |                | OPTS   |   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 the type-5
 LSAs (see A.4.5 in the OSPF specification) except for:
             The Forwarding Address
 If the network between the NSSA AS boundary router and the adjacent
 AS is advertised into OSPF as an internal OSPF route, the forwarding
 address should be the next hop address but if the intervening network
 is not advertised into OSPF as an internal OSPF route, the forwarding
 address should be any one of the router's active OSPF interface
 addresses.

Coltun & Fuller [Page 15] RFC 1587 OSPF NSSA Option March 1994

Appendix B: The Options Field

 The OSPF options field is present in OSPF Hello packets, Database
 Description packets and all link-state advertisements. See appendix
 A.2 in the OSPF specification for a description of option field.
  1. ———————————–

| * | * | * | * | N/P | MC | E | T |

  1. ———————————–
                     The Type-7 LSA options field
           T-bit:  The T-bit describes the router's TOS capability.
           E-bit:  Type-5 AS external link advertisements are not
                   flooded into/through OSPF stub and NSSA areas.
                   The E-bit ensures that all members of a stub area
                   agree on that area configuration. The E-bit is
                   meaningful only in OSPF Hello packets. When the
                   E-bit is reset in the Hello packet sent out a
                   particular interface, it means that the router
                   will neither send nor receive type-5 AS external
                   link state advertisements on that interface (in
                   other words, the interface connects to a stub
                   area). Two routers will not become neighbors
                   unless they agree on the state of the E-bit.
           MC-bit: The MC-bit describes the multicast capability of
                   the various pieces of the OSPF routing domain
                   [2].
           N-bit:  The N-bit describes the 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 reset in the Hello packet sent out a
                   particular interface, it means that the router
                   will neither send nor 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 reset.
           P-bit:  The P-bit is used only in the type-7 LSA header.
                   It flags the NSSA area border router to translate
                   the type-7 LSA into a type-5 LSA.

Coltun & Fuller [Page 16] RFC 1587 OSPF NSSA Option March 1994

Appendix C: Configuration Parameters

 Appendix C.2 in the OSPF specification lists the area parameters.
 The area ID, list of address ranges for type-3 summary routes and
 authentication type remain unchanged.  Section 3.2 of this document
 lists the configuration parameters for type-7 address ranges.
 For NSSAs the external capabilities of the area must be set to accept
 type-7 external routes.  Additionally there must be a way of
 configuring the NSSA area border router to send a default route into
 the NSSA using a specific metric (type-1 or type-2 and the actual
 cost).

Coltun & Fuller [Page 17]

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