rfc1998.txt

RFC 的详细文档！

       o In most cases, the MED attribute set by a customer is
         sufficient for customer backup routes (e.g., T1 backs up T3).
         However, in certain cases configuration of "LOCAL_PREF" will
         still be necessary until the BGP DPA attribute is available.


   To make use of the BGP community attribute, several community values
   (MCI's AS number: 3561 = 0x0DE9) have been defined that can be used
   by customers to tag routes so that the appropriate "LOCAL_PREF"
   values are configured. Table 2 lists the appropriate community
   attribute values (and the mappings of community to LOCAL_PREF):

                    +---------------------+------------+
                    |     community       | LOCAL_PREF |
                    +---------------------+------------+
                    |3561:70 (0x0DE90046) |         70 |
                    |3561:80 (0x0DE90050) |         80 |
                    |3561:90 (0x0DE9005A) |         90 |
                    +---------------------+------------+

                 Table 2: Community to LOCAL_PREF Mapping





Chen & Bates                 Informational                      [Page 5]

RFC 1998                    Use of Community                 August 1996


   A customer requiring MCI to configure BGP "LOCAL_PREF" values other
   than the default can tag their routes with the defined communities.
   The community values can be configured either based on an AS path
   list or an IP address access list. A cisco systems software specific
   configuration example is given in Appendix A to show how this can be
   achieved.

   A uniform BGP configuration (see Appendix B, again cisco systems
   software specific) is applied by MCI to peers with customers that
   configure the appropriate "LOCAL_PREF" values based on the
   communities received.

   This technique has been tested and is in use with several customers,
   and the response has been very positive. We are in the process of
   migrating all other customized BGP "LOCAL_PREF" configurations to
   this uniform community based configuration approach.

5. References

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

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

   [3] Chen, E., and T. Bates, "Current Practice of Implementing
       Symmetric Routing and Load Sharing in the Multi-Provider
       Internet", Work in Progress.

   [4] Chen, E., and T. Bates, "Destination Preference Attribute for
       BGP", Work in Progress.

   [5] Chen, E., and T. Bates, "Application of the BGP Destination
       Preference Attribute in Implementing Symmetric Routing",
       Work in Progress.

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

6. Security Considerations

   Security issues are not discussed in this memo.

7. Acknowledgments

   The authors would specifically like to thank Ravi Chandra, Tony Li
   and Paul Traina of cisco systems for devising and implementing the
   community attribute.



Chen & Bates                 Informational                      [Page 6]

RFC 1998                    Use of Community                 August 1996


8. Authors' Addresses

   Enke Chen
   MCI
   2100 Reston Parkway
   Reston, VA 22091

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


   Tony Bates
   cisco Systems
   170 West Tasman Drive
   San Jose, CA 95134

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

































Chen & Bates                 Informational                      [Page 7]

RFC 1998                    Use of Community                 August 1996


Appendix

   These appendices list cisco systems software specific configuration
   examples for configuring communities, and for uniform route-map
   definition that sets up the appropriate "LOCAL_PREF" values based on
   the corresponding community values. These examples are given purely
   to show a working example of how the desired effect discussed in this
   document can be achieved. Please refer to [6] for more specific
   information on cisco configuration and syntax.

Appendix A. Community Configuration

   The community values can be configured either based upon an AS path
   list or based an IP address access list. Here is an example that
   includes both cases:

   !!
   router bgp xxxx
   neighbor x.x.x.x remote-as 3561
   neighbor x.x.x.x filter-list 20 out
   neighbor x.x.x.x route-map config-community out
   neighbor x.x.x.x send-community
   !
   !!# match all
   ip as-path access-list 1 permit .*
   !
   !!# list of customer ASs
   ip as-path access-list 20 permit ^$
   ip as-path access-list 20 permit ^64700_
   ip as-path access-list 20 deny .*
   !
   !!# AS path based matching, backup for another ISPs customer
   ip as-path access-list 40 permit _64710_
   ip as-path access-list 40 permit _64711_
   ip as-path access-list 40 deny .*
   !
   !!# route-map
   route-map config-community permit 10
   match as-path 40
   set community 0x0DE90046
   route-map config-community permit 20
   match as-path 1
   !








Chen & Bates                 Informational                      [Page 8]

RFC 1998                    Use of Community                 August 1996


   Note: The community can also be configured based on IP prefixes
   instead of AS numbers.  For example,

   !
   access-list 101 permit ip 192.160.154.0 0.0.0.0 255.255.255.0 0.0.0.0
   !
   route-map config-community permit 10
   match ip address 101
   set community 0x0DE90046
   route-map config-community permit 20
   match as-path 1
   !

Appendix B. Uniform Route-map Configuration

   Here is the uniform route-map that can be used for all BGP
   customers:

   !!# routes primary via another ISP
   ip community-list 70 permit 0x0DE90046
   ip community-list 70 deny
   !
   !!# routes also homed to another ISP, but with DPA or
   !!# AS-path length as the tie-breaker
   ip community-list 80 permit 0x0DE90050
   ip community-list 80 deny
   !
   !!# customer backup routes
   ip community-list 90 permit 0x0DE9005A
   ip community-list 90 deny
   !
   !!# the route-map applied to BGP customers
   route-map set-customer-local-pref permit 10
   match community 70
   set local-preference 70
   route-map set-customer-local-pref permit 20
   match community 80
   set local-preference 80
   route-map set-customer-local-pref permit 30
   match community 90
   set local-preference 90
   route-map set-customer-local-pref permit 40
   match as-path 1
   set local-preference 100
   !






Chen & Bates                 Informational                      [Page 9]