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Network Working Group Mark Knopper Request for Comments: 1482 Steven J. Richardson

                                                         June 1993
  Aggregation Support in the NSFNET Policy-Based Routing Database

Status of this memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard.  Distribution of this memo is


 This document describes plans for support of route aggregation, as
 specified in the descriptions of Classless Inter-Domain Routing
 (CIDR) [1] and the BGP-4 protocol [2], by the NSFNET Backbone Network
 Service.  Mechanisms for exchange of route aggregates between the
 backbone service and regional/midlevel networks are specified.
 Additionally, the memo proposes the implementation of an Aggregate
 Registry which can be used by network service providers to share
 information about the use of aggregation.  Finally, the operational
 impact of incorporating CIDR and aggregation is considered, including
 an analysis of how routing table size will be affected.  This impact
 analysis will be used to modify the deployment plan, if necessary, to
 maximize operational stability.

1. Introduction

 The Internet network service provider community and router vendors
 (as well as the IESG and various IETF working groups) have agreed
 that the time for deployment of route aggregation is upon us. This
 topic has been discussed in the BGP-D, NJM and ORAD working groups at
 several IETF meetings; it was a discussion topic of the NSFNET
 Regional Techs' Meetings in January and June, 1993; and it was also a
 topic of several meetings of the Federal Engineering Planning Group
 and Engineering and Operations Working Group of the Federal Network
 All have generally agreed that Summer, 1993 is the time to enable
 BGP-4 and CIDR aggregation.  Each of the parties is responsible for
 its own aspect of CIDR implementation and practice. This memo
 describes Merit's plans for support of route aggregation on the
 NSFNET, and a proposal for implementing a database of aggregation
 information for use by network providers.

Knopper & Richardson [Page 1] RFC 1482 Routing Aggregation Support July 1993

2. Aggregation Support by the Backbone Service

 The NSFNET backbone service includes a Policy-Based Routing Database
 system which currently holds the set of network numbers that are
 accepted by the backbone service with a list of Autonomous System
 numbers from which announcements of these network numbers are
 expected.  In order to implement CIDR, the database system will be
 modified to allow aggregation of routing information to be
 The NSFNET will (initially) not support de-aggregation on its
 outbound announcements. See section 2.3.

2.1 Current Configuration Capabilities

2.1.1 Inbound Announcements

 An example of the way a network number is currently configured is as
       35      1:237   2:233   3:183   4:266   5:267  6:1225
 This shows that network number 35 (ie., a class A net
 number) is configured on the T3 backbone such that routing
 announcements are expected from up to 6 autonomous systems. The
 primary path is via AS 237, secondary is via AS 233, etc.

2.1.2 Outbound Announcements

 Currently the NSFNET database has a list of AS's or network numbers
 for each neighbor AS that are announced by the backbone to that AS.
 These announcements are specified currently by "announcetoAS"
 statements--which implement policies submitted by midlevels to
 Merit--and then included in the ANSnet router configuration files.
 There are two forms of these statements.  The first form uses the
 "norestrict" clause and indicates that all of the network numbers
 within each AS in the list should be announced to the neighbor
 midlevel AS. For example:
       announcetoAS 42 norestrict ASlist 22 26 38 60 68
 In this example, the NSFNET is configured to announce to neighboring
 midlevel AS 42, all networks in the routing table that were announced
 from AS's 22, 26, 38, 60 and 68.
 If the "norestrict" keyword is changed to "restrict", this indicates
 that an explicit announce list of network numbers for the AS is
 specified in the configuration file. The NSFNET will only announce

Knopper & Richardson [Page 2] RFC 1482 Routing Aggregation Support July 1993

 network numbers that were announced by the AS's in the list, *AND*
 which appear in the "restrict list" of network numbers submitted
 separately by the midlevel.
    For example,
       announcetoAS 42 restrict ASlist 22
       announce 192.135.237 <other info>
 These statements mean that AS 42 only wishes to hear announcements
 from the backbone about the nets in AS 22 which are explicitly listed
 here (i.e., net 192.135.237).
 It is also possible, when using the "restrict" keyword, to list
 specific "noannounce" lines. Those indicate that all of the networks
 listed in the routing table for the AS should be announced except
 those listed on the noannounce clauses.  (There is also a
 "noannouncetoAS" statement[4].)

2.2 New Configuration Features for Aggregation

 There will be three new capabilities for which the backbone service
 can be configured to support aggregation. The first two allow
 aggregates to be accepted and stored in the backbone routing tables
 based on announcements by the regional network (autonomous system or
 AS) peers.  The third allows the announcement of aggregates to the AS
 neighbor peers. The following sections give examples of the three
 We use the notation <net-IP prefix-length> to describe an aggregate.
 This refers to the IP prefix "net-IP", with a mask which has
 "prefix-length" 1's as counted from the high-order end. For example,
 <192.64.128 17> is equivalent to <192.64.128,> [5].
 (The form using prefix-length rather than the mask is more compact.)

2.2.1 NSFNET accepts aggregates

 In this case the regional peer router is CIDR-capable (i.e., runs
 BGP-4) and the announcement comes into the backbone as an IP address
 To illustrate this in the spirit of sec. 2.1.1:
       <192.64.128 17>         1:189 2:24 3:267
 In this example, independent of the "class" of IP network number, an
 aggregate containing network addresses matching a pattern in which

Knopper & Richardson [Page 3] RFC 1482 Routing Aggregation Support July 1993

 the first 17 bits match the prefix 192.64.128 will be accepted in
 announcements to the NSFNET service.  The primary path to
 destinations covered by the prefix is expected via AS 189, the
 secondary, via AS 24, etc.

2.2.2 NSFNET aggregates by proxy

 The other method of incorporating CIDR aggregate announcements into
 the backbone routing tables is that of aggregation by proxy.  In this
 case, the backbone is configured to perform aggregation on behalf of
 a peer AS which is not configured to announce the aggregate to the
 backbone (i.e., an AS which does not connect to the backbone via a
 CIDR-capable peer).
 An example of this aggregation technique is:
       proxy <192.64.128 17>     1:189  2:24  3:267
               if  <192.64.192 24>
               or  <192.64.129 24>
               or  <192.64.167 24>
 (Note: the syntax used in this document is arbitrary and is only used
 to illustrate the method. The syntax to be used in actual routing
 requests is to be determined.)
 In this example, the aggregate <192.64.128 17> will be stored and
 propagated within the backbone as an aggregate under a set of
 conditions.  Initially, the GateD support will allow an "OR" list of
 conditions such that if one of the aggregates in the list matches the
 proxy aggregate will be stored[6].  For the case above, this means
 that, if any of the CIDR aggregates:
       <192.64.192 24>
       <192.64.129 24>
       <192.64.167 24>
 (which--under the current, class-based IP address system--are
 equivalent to the class C net numbers 192.64.192, 192.64.129, or
 192.64.167, respectively) is heard, the backbone router will act as
 though it heard the announcement of the single CIDR aggregate
 <192.64.128 17>.

2.2.3 NSFNET announces aggregates

 The functionality of the current system, as outlined in sec. 2.1.2,
 above, will continue to exist once CIDR is implemented. The
 "norestrict" function (or its equivalent in the new software) will
 specify that all network reachability information received from a set

Knopper & Richardson [Page 4] RFC 1482 Routing Aggregation Support July 1993

 of Autonomous Systems, including any aggregates, will be announced.
 It should also be possible to use to the equivalents of the
 "restrict" keyword and the "announce" (or "noannounce") statement in
 order to limit the announcements of the aggregations within an AS to
 any desired subset.

2.3 Specifically Unsupported Capabilities, Limits of Initial Deployment

 There are some aspects of aggregation which will specifically not be
 supported in the initial deployment of CIDR capabilities on the
 NSFNET backbone.  In particular, when the NSFNET service announces
 routes to midlevel peers, de-aggregation will not be performed [3].
 Therefore, a peer which needs to receive full routing information
 should run a protocol which supports CIDR (initially, BGP-4; later,
 IDRP). Peer networks using default routing will be able to reach
 networks that are part of aggregated routing information across the
 backbone (as in section 6.4 of [3]).

3. CIDR Aggregate Registry

 In discussions with network service providers, it has become apparent
 that there is a great need for sharing of aggregate information; this
 is necessary to fulfill the coordination referred to in sec. 2.3.
 Beyond the need to implement CIDR aggregation facilities in the
 NSFNET Policy-Based Routing Database (as described in section 2),
 there is a clear need to have a separate database which will allow
 aggregate information from any Autonomous System to be stored and
 made available for easy electronic retrieval. This information can be
 used for routing coordination and policy configuration in the larger,
 non-NSFNET-centric, inter-domain context.
 One of the expected uses of such a database is to help determine, as
 CIDR matures, the granularity of aggregation of network reachability
 information with respect to policy.  The useful scope of aggregation
 is the subject of much discussion[5][7], and will be influenced by
 such considerations as how network number allocation has been
 handled, and whether the network provider has renumbered its client
 networks to conform to CIDR aggregation boundaries. Rules and issues
 regarding network number allocation with CIDR are discussed in [8]
 and [7].
 In order further these goals, Merit proposes to implement a "CIDR
 Aggregate Registry" to provide sharing of aggregate information for
 the Internet inter-domain routing community. Initially, this will be
 a simple database without much structure. It is not intended to hold
 only aggregates which are announced or accepted by the NSFNET
 service; rather, it should be a community registry that all will be
 invited to use and make use of.

Knopper & Richardson [Page 5] RFC 1482 Routing Aggregation Support July 1993

 The Aggregate Registry will consist of a list of aggregate
 announcement statements. Each statement consists of four types of
 information, along with contact information:
    1) CIDR Aggregate: The aggregate identifier, consisting of a
    network number prefix and the prefix length. For example,
    <192.29.128 16>.
    2) Home AS: The source AS number for the aggregate. That is, the
    AS number of the network service provider that initially
    aggregates the network reachability information into the aggregate
    for announcement to its neighbors.
    3a) Announcing AS: An AS number that announces this aggregate to
    its neighbor AS's.
    3b) Neighbor AS list: A list of neighbor AS's to whom the
    aggregate will be announced by the AS named in 3a.
    4) Contact information: eg. e-mail address and name or NIC handle
    of the administrative and technical contacts for the source AS.
 Thus, a given aggregate is listed once as announced by its source AS.
 It may then be listed once again per transit AS which announces the
 aggregate downstream to its neighbors.  For example, the CIDR
 aggregate <199.29.128 16> could be listed as:
        CIDR aggregate  home ann  neighbor
        (prefix-length) AS   AS   AS list         contacts
       <199.29.128 16>  100  100  200 201 690
       <199.29.128 16>  100  690  266 267 1225... <contact info>
       <199.29.128 16>  100  200  297 372         <contact info>
       <199.29.128 16>  100  201  771 1262        <contact info>
       Note: This can be represented using the syntax used for objects
       in the RIPE-81 paper[9].
 Here, AS 100 (the source AS) performs any aggregation and announces
 the CIDR aggregate <199.29.128 16> to neighbor ASs 200, 201, and 690.
 In turn, AS 200 announces this same aggregate to its neighbor ASs 297
 and 372; further lines show announcements of the given aggregate by
 AS 690 and AS 201.
 Note that this registry reflects both the simple list of aggregates
 that are supported by the union of network providers, as well as
 information on inter-domain topology for the Internet.  Merit will
 implement procedures for registering any network provider's

Knopper & Richardson [Page 6] RFC 1482 Routing Aggregation Support July 1993

 aggregates in the Registry; for those CIDR aggregates carried over
 the NSFNET backbone, Merit will implement procedures for integrating
 this Registry with the process of updating the aggregate routing
 announcements.  Requests to update the information will be handled
 via e-mail or on-line registration tools.

4. Effects of CIDR on Operational Aspects of the Internet

 The introduction of CIDR will clearly necessitate various changes
 beyond the introduction of new router software.  In particular, Merit
 and other network service providers will have to adjust tools,
 reports, and procedures as CIDR is implemented and evolved, and these
 changes will have to be coordinated in order to ensure a smooth
 transition to the CIDR-capable Internet.
 While this document is by no means exhaustive, some of the areas
 affected are discussed briefly below; what is intended is to foster
 an awareness of some these changes, so as to initiate thinking about
 and planning for this transition.  While it is obvious that CIDR and
 policy routing imply greater coordination of many operational
 matters, it is not clear how profoundly this will affect the day-to-
 day running of the Internet.
 (Note:  Aspects of the actual phased deployement of CIDR are covered
 in [3] and [10].)

4.1 NSFNET Configuration Files and Reports; Neighbor AS Configurations

 The addition of CIDR capability to the NSFNET Policy-Based Routing
 Database, as outlined in sec. 2, will require the updating of at
 least the following reports which are currently produced by Merit
 (and available via anonymous FTP from
 Any tools which access this information, such as the various clients
 or scripts released by Merit or developed by others, will have to be
 However, the most striking change will be in the transition from
 rcp_routed to GateD; it is very different in important particulars,
 and follows different conceptual principles [11].
 Network providers which develop any part of their configuration files
 from parsing the NSFNET configuration files or reports *MUST* plan
 for these changes in order to help themselves and the Internet
 community achieve a smooth transition to CIDR.

Knopper & Richardson [Page 7] RFC 1482 Routing Aggregation Support July 1993

4.2 Routing and Administrative Policies

 In this document, Merit has stated its commitment to supporting CIDR
 through both changing policies related to administering the NSFNET
 and developing a CIDR Aggregate Registry for the broader Internet
 In addition to these changes, here are some of the other policies,
 administrative and routing, which must to be coodinated in order to
 achieve optimum benefits of CIDR:
  1. policies of the InterNIC and of network service providers in

assigning (CIDR) IP nets and blocks, as mentioned above;

  1. policies of the various ASs in coordination of transit and other

routing policies;

  1. policies of registration of new networks, from the InterNIC or

network provider, through the CIDR Aggregate Registry, etc.;

  1. policies related to coordination of routing changes;
  1. coordination of routing policies, in general, to avoid new

classes of routing problems due to new methods of routing.

4.3 Realtime Issues

 Issues which have not been examined in detail are:
  1. debugging of routing/connectivity problems;
  1. stability and other properties of routing under various

scenarios of CIDR configuration and network topology;

  1. explicit specification of routing decision algorithms to avoid

routing anomalies;

  1. increased network load due to packets traversing an AS, such as

the NSFNET backbone, before being discarded due to addressing a

      "hole" in a CIDR aggregate.

4.4 Estimate of Reductions in Routing Tables

 An argument in favor of the implementation CIDR is the effect which
 it should have upon the NSFNET and other routing tables [1] [5].  The
 burning question is: What is the magnitude of this effect?  In view
 of the various issues to be dealt with, this is an important

Knopper & Richardson [Page 8] RFC 1482 Routing Aggregation Support July 1993

 In terms of the immediate savings in reduction of the NSFNET backbone
 routing tables, if a set of aggregates were done all at once, a
 recent calculation--which might be characterized as an optimistic
 estimate using a pessimistic algorithm (it looks for the longest
 continuous block of addresses announced to the NSFNET backbone)--
 yields [12]:
      861 size  2 saving  861 announcements
      286 size  4 saving  858 announcements
      117 size  8 saving  819 announcements
       67 size 16 saving 1005 announcements
       13 size 32 saving  403 announcements
        3 size 64 saving  189 announcements
     1347 total   saving 4135 announcements of 12348 (33%).
 Here, the first column represents the number of CIDR aggregates of
 the given "size," and shows the corresponding reduction in net
 announcements due to the adoption of this aggregate.  (A CIDR
 aggregate of "size <n>" is one which encompasses <n> class A, B, or C
 networks; the 67 "size 16" CIDR aggregates actually combine
 announcements for 16 separate networks into a single net aggregate.)
 It is unclear, at this time, whether or not the true savings would be
 of this magnitude, but the extended report provides a basis for
 discussion [12].
 The other aspect of impact upon the routing tables, the reduction in
 the rate of growth (and the concomitant slowing of the rate of
 exhaustion of IP address space), is an entirely different matter.
 Simple calculations related to the rate of class B address space
 exhaustion indicate that CIDR-conformant policies of the InterNIC
 with respect to address assignment is helping [1].
 Clearly, more detailed analysis is desirable in order to better
 understand the realistic gains of the CIDR deployment process, both
 initially and in the longer term.

5. Conclusions and Next Steps

 Implementation of CIDR is underway, but there is still a fair amount
 of planning and discussion that is needed for a successful
 transition.  Merit is proposing specific functions for CIDR
 aggregation that will be supported by the NSFNET, as well as a CIDR
 Aggregate Registry that can serve as the basis for inter-domain
 routing coordination.
 The Aggregate Registry will allow a set of tools to be developed that
 can facilitate the design of aggregation policy. A query tool to
 allow lookup of aggregation information for a given network or

Knopper & Richardson [Page 9] RFC 1482 Routing Aggregation Support July 1993

 aggregate would be very useful. Additional database functionality
 will also be desired for more powerful queries. It is specifically a
 goal to work with RIPE to make sure that the Merit and RIPE database
 approaches are compatible and allow interworking of tools. An AS
 topology database would be most useful in routing policy
 determination and coordination as well.
 In addition to these areas, many other issues require further work in
 order to develop the operational framework necessary for the
 successful use of CIDR on the Internet. It is critical that the
 deployment of CIDR and related tools to preserve address and routing
 table space must not compromise the operational stability of the
 NSFNET and the wider Internet.

6. Security Considerations

    Security issues are not discussed in this document.

7. Acknowledgements

 The authors would like to acknowledge the following persons, whose
 comments and discussions have helped to shape this document:
       Dennis Ferguson, Advanced Network and Services, Inc.
       Jeffrey Honig, Cornell University
       William Manning, Rice University/SESQUINET
       The Merit Internet Engineering and Network Management
       Systems groups.

8. Authors' Addresses

 Knopper, Mark A.
 Merit Network, Inc.
 1071 Beal Ave.
 Ann Arbor, MI  48109-2103
 phone:  (313) 763-6061
 fax:    (313) 747-3745
 Richardson, Steven J.
 Merit Network, Inc.
 1071 Beal Ave.
 Ann Arbor, MI  48109-2103
 phone:  (313) 747-4813
 fax:    (313) 747-3745

Knopper & Richardson [Page 10] RFC 1482 Routing Aggregation Support July 1993

9. References

 [1]  Fuller, V., Li, T., Yu, J., and Varadhan, K., "Supernetting: an
      Address Assignment and Aggregation Strategy", RFC1338, Update,
      Work in Progress, June 1992.
 [2]  Rekhter, Y., and Li, T., "A Border Gateway Protocol 4", Work In
      Progress, April 1993.
 [3]  Topolcic, C., "Notes of BGP-4/CIDR Coordination Meeting of 11
      March 93", Work in Progress, March 1993.
 [4]  Villamizer, C., in a document describing rcp_routed.conf options
      and syntax, May, 1993.
 [5]  Syntax used in Ford, P., Rekhter, Y., Braun, H-W., "Improving
      the Routing and Addressing of IP", IEEE Network, pp. 10-15, May
 [6]  Ferguson, D., private correspondence, March, 1993.
 [7]  Rekhter, Y., and Li, T., "An Architecture for IP Address
      Allocation with CIDR", Work in Progress, February, 1993.
 [8]  Gerich, E., "Guidelines for Management of IP Address Space",
      RFC1466, May 1993.
 [9]  Bates, T., Jouanigot, J-M., Karrenberg, D., Lothberg, P., and
      Terpstra, M., "Representation of IP Routing Policies in the RIPE
      Database" (ripe-81), Work in Progress, February, 1993.
 [10] Rekhter, Y., and Topolcic, C., "Exchanging Routing Information
      Across Provider/Subscriber Boundaries in the CIDR Environment",
      Work in Progress, April 1993.
 [11] Fedor, M., Honig, J., Coltun, R., Ferguson, D., "gated-
      config(5)" manpage, from the "gated-R3_0Beta_2" distribution, 7
      October 1992.
 [12] Johnson, D., analysis available via anonymous FTP from, June 1993.
 [13] Topolcic, C., "Schedule for IP Address Space Management
      Guidelines", RFC1367, October, 1993.

Knopper & Richardson [Page 11]

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