rfc2519.txt

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   In summary, the aggregation strategy for a provider shall be:

   -    In announcing customer routes:

        For routes originated by itself/dedicated-AS/private-AS:
           tag with "no-export" when appropriate, and advertise the
           large block and suppress the more-specifics

        For routes originated by other customer ASs:
           advertise

        For any other routes:
           do not advertise

   -    In announcing full routes:

        For routes originated by itself/dedicated-AS/private-AS:
           tag with "no-export" when appropriate, and advertise the
           large block and suppress the more-specifics

        For any other routes:
           advertise





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RFC 2519             Inter-Domain Route Aggregation        February 1999


5. An Example

   Consider the example shown in Figure 3 where AS 1000 is a "Tier 1"
   provider with two large aggregates 208.128.0.0/12 and 166.55.0.0/16,
   and AS 2000 is a customer of AS 1000 with a "portable address"
   160.75.0.0/16 and an address 208.128.0.0/19 allocated from AS 1000.
   Assume that 208.128.0.0/19 does not need to be propagated beyond AS
   1000.

                             +----------------+
                             |    AS 1000     |
                             | 208.128.0.0/12 |
                             | 166.55.0.0/16  |
                             +----------------+
                                     |
                                     | BGP
                                     |
                                     |
                             +----------------+
                             |     AS 2000    |
                             | 208.128.0.0/19 |
                             | 160.75.0.0/16  |
                             +----------------+

                                  Figure 3

   Then, based on the framework presented, AS 1000 would

      - originate and advertise the BGP routes 208.128.0.0/12 and
        166.55.0.0/16, and suppress more-specifics originated by
        itself/private-ASs/dedicated-ASs

      - advertise the routes received from the customer AS 2000

   and AS 2000 would

      - originate BGP route 208.128.0.0/19 and 160.75.0.0/16

      - advertise both 160.75.0.0/16 and 208.128.0.0/19 to its provider
        AS 1000 and suppress the more specifics originated by
        itself/private-AS/dedicated-AS, tagging the route 208.128.0.0/19
        with "no-export"

      - advertise both 160.75.0.0/16 and 208.128.0.0/19 to its BGP
        customers (if any) and suppress the more-specifics originated by
        itself/private-AS/dedicated-AS, plus any other routes the
        customers may desire to receive




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   The sample configuration which implement these policies (in Cisco
   syntax) is given in Appendix A.

6. Acknowledgments

   The authors would like to thank Roy Alcala of MCI for a number of
   interesting hallway discussions related to this work.  The IETF's IDR
   Working Group also provided many helpful comments and suggestions.

7. References

   [1] Rekhter, Y. and T. Li, "An Architecture for IP Address Allocation
       with CIDR", RFC 1518, September 1993.

   [2] Fuller, V., Li, T., Yu, J. and K. Varadhan, "Classless Inter-
       Domain Routing (CIDR): an Address Assignment and Aggregation
       Strategy", RFC 1519, September 1993.

   [3] Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
       RFC 1771, March 1995.

   [4] Rekhter, Y. and P., Gross, "Application of the Border Gateway
       Protocol in the Internet", RFC 1772, March 1995.

   [5] Rekhter, Y., "Routing in a Multi-provider Internet", RFC 1787,
       April 1995.

   [6] Chandra, R., Traina, P. and T. Li, "BGP Communities Attribute",
       RFC 1997, August 1996.

   [7] Chen, E. and T. Bates, "An Application of the BGP Community
       Attribute in Multi-home Routing", RFC 1998, August 1996.

   [8] Ferguson, P. and H. Berkowitz, "Network Renumbering Overview: Why
       would I want it and what is it anyway?", RFC 2071, January 1997.

   [9] Berkowitz, H., "Router Renumbering Guide", RFC 2072, January
       1997.

   [10] Stewart, J., Bates, T., Chandra, R., and Chen, E., "Using a
        Dedicated AS for Sites Homed to a Single Provider", RFC 2270,
        January 1998.

   [11] Carpenter, B., Crowcroft, J. and Y. Rekhter, "IPv4 Address
        Behaviour Today", RFC 2101, February 1997.

   [12] Carpenter, B. and Y. Rekhter, "Renumbering Needs Work", RFC
        1900, February 1996.



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   [13] Cisco systems, Cisco IOS Software Version 10.3 Router Products
        Configuration Guide (Addendum), May 1995.

8.  Authors' Addresses

   Enke Chen
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134-1706

   Phone: [phone]
   EMail: [e-mail]


   John W. Stewart, III
   Juniper Networks, Inc.
   385 Ravendale Drive
   Mountain View, CA  94043

   Phone: [phone]
   EMail: [e-mail]






























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RFC 2519             Inter-Domain Route Aggregation        February 1999


A. Appendix A:  Example Cisco Configuration

   This appendix lists the Cisco configurations for AS 2000 of the
   examples presented in Section 5.  The configuration here uses the
   AS-path for outbound filtering although it can also be based on BGP
   community.  Several route-maps are defined that can be used for
   peering with the upstream provider, and for peering with customers
   (announcing full routes or customer routes).

!!# inject aggregates
ip route 160.75.0.0 255.255.0.0 Null0 254
ip route 208.128.0.0 255.255.224.0 Null0 254
!
router bgp 2000
network 160.75.0.0 mask 255.255.0.0
network 208.128.0.0 mask 255.255.224.0
neighbor x.x.x.x remote-as 1000
neighbor x.x.x.x route-map export-routes-to-provider out
neighbor x.x.x.x send-community
!
!!# match all
ip as-path access-list 1 permit .*
!
!!# List of internal AS and private ASs that are safe to aggregate
ip as-path access-list 10 permit ^$
ip as-path access-list 10 permit ^64999_
ip as-path access-list 10 deny .*
!
!!# list of other customer ASs
ip as-path access-list 20 permit ^3000_

!!# List of prefixes to be tagged with "no-export"
access-list 101 permit ip 208.128.0.0 0.0.0.0 255.255.224.0 0.0.0.0
!!# Filter out the more specifics of large aggregates, and permit the rest
access-list 102 permit ip 160.75.0.0 0.0.0.0 255.255.0.0 0.0.0.0
access-list 102 deny ip 160.75.0.0 0.0.255.255 255.255.128.0 0.0.127.255
access-list 102 permit ip 208.128.0.0 0.0.0.0 255.255.224.0 0.0.0.0
access-list 102 deny ip 208.128.0.0 0.0.31.255 255.255.240.0 0.0.16.255
access-list 102 permit ip any any
!

!!# route-map with the upstream provider
route-map export-routes-to-provider permit 10
match ip address 101
set community no-export
route-map export-routes-to-provider permit 20
match as-path 10
match ip address 102



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RFC 2519             Inter-Domain Route Aggregation        February 1999


route-map export-routes-to-provider permit 30
match as-path 20
!
!!# route-map with BGP customers that desire only customer routes
route-map export-customer-routes permit 10
match as-path 10
match ip address 102
route-map export-customer-routes permit 20
match as-path 20
!
!!# route-map with BGP customers that desire full routes
route-map export-full-routes permit 10
match as-path 10
match ip address 102
route-map export-full-routes permit 20
match as-path 1
!


































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