Summary

© 2004 Cisco Systems, Inc. All rights reserved. BGP v3.1—4-2

4-42 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

Return Path Selection in a Multihomed AS

This topic describes the need to influence BGP return path selection in a service provider environment.

Return Path Selection in a Multihomed AS

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• Requirement: The return traffic to the customer must arrive over the highest-speed access link.

© 2004 Cisco Systems, Inc. All rights reserved. BGP V3.1—4-3

It is fairly easy for an AS to select the appropriate path for outgoing traffic. It is much more complicated to influence other autonomous systems to select the appropriate path for traffic that is returning to a specific AS.

To configure the preferred path only for outgoing traffic and not for incoming (return) traffic is likely to result in asymmetrical traffic flow as well as suboptimal performance of the return traffic. In the figure, outgoing traffic is directed to the high-speed line (2 Mbps) as a result of configuring local preference or weight. However, the return traffic from AS 387 would take the default path over the low-speed line (64 kbps). The low-speed line would be a limiting factor in the overall performance that the network could achieve.

In this example, AS 213 requests AS 387 to send packets toward network 10.0.0.0/8 via AS 462. The reason for this request is to improve network performance and minimize delay (assuming, of course, that the connectivity between AS 387 and AS 462 is better than the direct 64-kbps link between AS 387 and AS 213).

Copyright © 2004, Cisco Systems, Inc. Route Selection Using Attributes 4-43

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

Result: The return traffic flows over the path with the shortest AS-path length.

© 2004 Cisco Systems, Ii

If no BGP path selection tools are configured on the route to influence the traffic flow, AS 387 will use the shortest AS path. This action will result in unwanted behavior because the return traffic to AS 213 will be sent over the low-speed WAN link.

AS 213 announces network 10.0.0.0/8 over EBGP sessions to both AS 462 and AS 387. When AS 213 sends EBGP updates, it changes the AS-path attribute according to BGP specifications. Both AS 462 and AS 387 receive a BGP update for network 10.0.0.0/8 with the AS path set to 213.

Because AS 462 selects the route for network 10.0.0.0/8 that it received from AS 213 as its best route, AS 462 will use that route and forward it on to AS 387. According to BGP specifications, AS 462 also changes the AS-path attribute. AS 387 receives the route to network 10.0.0.0/8 from AS 462 with an AS path set to 462 213.

AS 387 has now received two alternative routes to network 10.0.0.0/8 (the direct route from AS 213 and the route through AS 462). Because nothing is configured in AS 387 to influence the flow of traffic, the router will use the BGP route selection rule of shortest AS path to select the best return path to network 10.0.0.0/8.

4-44 Configuring BGP on Cisco Routers (BGP) v3.1 Copyright © 2004, Cisco Systems, Inc.

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

Q: How do you select the proper return path from AS 387? A: Use local preference in AS 387 Q: Will the administrator of AS 387 configure it? A: Unlikely

© 2004 Cisco Systems, Ii

Remember that the incoming traffic flow (from the perspective of AS 213) will be a result of the route selection for outgoing traffic in AS 387. The traffic that is going out from AS 387 will end up as incoming traffic in AS 213.

If AS 387 configures some changes that cause the route selection process for outgoing traffic to prefer to reach network 10.0.0.0/8 via AS 462, it would result in behavior matching the desired administrative policy for AS 213, which specifies that incoming traffic to the AS should be received over the high-speed link.

One way to accomplish the desired administrative policy in AS 213 is to configure the router in AS 387, which is receiving EBGP updates directly from AS 213, to assign a local preference value less than the default value (100) to all routes that are received from AS 213. The router in AS 387 is also configured specifically not to set local preference on EBGP routes that are received from AS 462. This configuration results in assignment of the default value of 100 to all routes received from AS 462. When the route selection process in AS 387 selects the best route to reach network 10.0.0.0/8, the difference in local preference values causes AS 387 routers to select the path via AS 462 as the best.

However, all the configuration work to complete this process must be performed in AS 387. The network administrators of AS 387 would be required to modify the router configurations in AS 387 to satisfy the administrative policy requirements of AS 213. All changes must be documented and maintained according to the rules and procedures that have been adopted by AS 387.

If AS 387 is a major ISP, it is most likely that the network administrators are too busy doing other things than to tailor router configurations that are based on the demand of a single leaf (nontransit) AS that lacks bandwidth on a redundant connection.

Copyright © 2004, Cisco Systems, Inc. Route Selection Using Attributes

The PDF files and any printed representation for this material are the property of Cisco Systems, Inc., for the sole use by Cisco employees for personal study. The files or printed representations may not be used in commercial training, and may not be distributed for purposes other than individual self-study.

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