BGP - [Part 22] - BGP CONFDERATION

BGP Confederations - 

• BGP has two mechanism to reduce the number of IBGP peering - 

1. Confederations
2. Route Reflector

• Basically BGP confederations used to manage the complexity and scale of large AS. 
• BGP confederations is alternative of  BGP route reflectors, we can used together also.
• BGP confederation divides the Autonomous System into Sub-Autonomous Systems to reduce the number of IBGP peering.
• But still full mesh IBGP required within a Sub-Autonomous System, because it will follow the full IBGP rule.
• EBGP will be used between the Sub-Autonomous System and it will use AS_Path mechanism for loop prevention, but it will be behaves like IBGP.
• Sub-Autonomous System will not appear to the outside, only main Autonomous System number will be appear to outside.
• Check the below topology, we have 8 router in AS 100,  IBGP is running on these router, you can calculate the number of IBGP peering with the full mesh formula (N(N-1)/2). As per below topology N=8. So 8 * (8-1 = 7) / 2  = 28 IBGP peering.

• Check the  below topology, after dividing the Autonomous System into Sub Autonomous System, it reduced the number of IBGP peering, now only 16 peering required

• We can specifiy the Parent or main AS via command "bgp confederation identifier [Parent AS number]"
• We can specifiy the confederation peers AS via command "bgp confederation peer [Confederation Peer AS number]"

Check the below configuration sample

Router1(config)# router bgp 200-----------------------------------200 is Sub- AS
Router1(config-router)# bgp confederation identifier 100----------100 is main AS
Router1(config-router)# bgp confederation peer 400---------------400 is Peer Sub-AS
Router1(config-router)# neighbor 10.10.10.1 remote-as 400

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Anubhav Upadhyay

Hello and welcome to networktopic Blog. My name is Anubhav. I am a Senior Network Egineer. I have created this blog specially to serve interview questions and answer on Network Routing and Switching, I will try my best to serve correct and updated networking knowledge for you as per my corporate experience.

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