Planning BGP Confederations

This topic explains how to plan a BGP backbone for a configuration that includes BGP confederations.

Planning BGP Confederations

• Divide the transit AS into smaller areas.

- Follow the physical topology of the network.

• Define the AS number for each area.

- Use AS numbers reserved for private use

(64512 -65535).

• Verify the Cisco IOS release level.

- All routers have to support BGP confederations.

• Convert each area into an AS.

- A total rewrite of the BGP configuration is required.

»2-5 Systems, Inc. All

BGP v3.2—6-4 |

The physical topology of the AS serves as a guide to design the confederation. You should introduce no additional single points of failure when implementing the confederation. If the physical topology is redundant, good practice is to have redundant intra-confederation EBGP sessions. If the physical topology is not redundant, introducing a nonredundant set of sessions does not add a single point of failure—the network was already nonredundant.

You need to make the following preparations before migrating from a full mesh of IBGP sessions to a confederation design:

■ Identify a group of core routers that can serve as a central member-AS.

■ Identify several groups of more peripheral routers where, within each group, routers are well connected. Let each group be its own member-AS.

■ Make a plan for assigning AS numbers (64512 to 65535) to your member-AS. The plan must uniquely identify each member-AS within the confederation. No member-AS is seen from outside the confederation, so you do not need to coordinate the plan with any other AS.

■ Make sure that no router is lacking support (the correct Cisco IOS release level) for a BGP confederation. If any router lacks support for this feature, it will break the network.

■ Remove the original BGP configuration with the original official AS number.

Note Support for the BGP confederation feature is included in Cisco IOS software releases from

Release 10.3.

6-68 Configuring BGP on Cisco Routers (BGP) v3.2 © 2005, Cisco Systems, Inc.

Configuring BGP Confederations

This topic explains how to configure BGP confederations on a BGP backbone. An example of a BGP confederations configuration is also included in this topic.

After you have done the preparation that is necessary to migrate from a full mesh of IBGP

sessions to a confederation design, you need to complete the following configuration steps:

Step 1 Make a new BGP configuration that uses the internal member-AS number according to the AS number plan for the confederation.

Step 2 Specify the original official AS number as the identifier of the confederation. This information will be used by egress confederation routers whenever you are communicating with other external autonomous systems.

Step 3 Specify a list of the member-AS numbers being used. The router uses this information to distinguish between intra-confederation EBGP behavior and true EBGP behavior.

Step 4 Configure all the IBGP sessions in the full mesh within the member-AS.

Step 5 Configure intra-confederation EBGP sessions between each member-AS in a way that introduces no additional single point of failure.

Step 6 Configure true EBGP sessions with external autonomous systems.

Note Removing the original BGP configuration and creating a new BGP configuration will always cause interruption in network availability. Migration to BGP confederation has to be a well-planned process.

© 2005, Cisco Systems, Inc. Scaling Service Provider Networks 6-69


no router bgp as-number router bgp member-as-number

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