BGP Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Releases
Introduces BGP confederations, summarizing their characteristics and providing steps for configuring a BGP routing domain confederation to improve network scalability and organization.
In very large networks, managing a full iBGP mesh can be challenging. BGP confederations address this by partitioning an AS into sub-ASs, improving scalability and simplifying management.
BGP routing domain confederations are used to simplify iBGP mesh requirements and improve scalability in large autonomous systems. Key characteristics include:
A confederation divides a single autonomous system (AS) into multiple sub-autonomous systems, called member ASes.
The confederation appears as a single autonomous system to external BGP peers.
Each member AS maintains a full internal BGP (iBGP) mesh within itself. It establishes only limited external BGP (eBGP-like) connections to other member ASes within the same confederation.
Peers in different member ASes use eBGP sessions but exchange routing information as if they were iBGP peers within the confederation.
Routing attributes such as next hop, multi-exit discriminator (MED), and local preference are preserved across confederation boundaries.
A single Interior Gateway Protocol (IGP) can be used for the entire confederation.
This approach reduces the number of required iBGP sessions and simplifies network management while maintaining compatibility with external BGP peers.
Reduce internal iBGP mesh complexity by grouping multiple autonomous systems (AS) into a BGP confederation, enabling easier management and scalability.
A BGP routing domain confederation divides a single autonomous system (AS) into multiple sub-autonomous systems (sub-ASes). This approach simplifies iBGP connectivity by fully meshing only within each sub-AS, while maintaining a consistent external BGP appearance. All sub-ASes in a confederation use the same IGP.
Gather the confederation AS number (the confederation identifier).
Collect the list of sub-AS numbers to include as confederation peers.
Access the router in global configuration mode.
Specify the confederation identifier, and associate all AS numbers that participate in the confederation. Example: |
The router now treats the configured sub-ASes as part of the same BGP confederation. To external peers, the group of sub-ASes appears as a single AS.
This is a sample configuration showing several peers within a BGP confederation. The confederation is made up of three internal autonomous systems with AS numbers 6001, 6002, and 6003. To BGP speakers outside the confederation, it appears as a single autonomous system with AS number 666, as specified by the bgp confederation identifier command.
router bgp 6001
bgp confederation identifier 666
bgp confederation peers
6002
6003
exit
address-family ipv4 unicast
neighbor 172.16.232.55
remote-as 6002
exit
address-family ipv4 unicast
neighbor 172.16.232.56
remote-as 6003
exit
address-family ipv4 unicast
neighbor 172.19.69.1
remote-as 777
On a BGP router in autonomous system 6001, the bgp confederation peers command designates peers from autonomous systems 6002 and 6003 as special eBGP peers within the confederation. As a result, peers at 172.16.232.55 and 172.16.232.56 receive updates with local preference, next hop, and MED attributes unchanged. In contrast, the router at 172.19.69.1 is a standard eBGP peer outside the confederation, so the updates it receives from this peer are treated as regular eBGP updates from autonomous system 666.