BGP Confederation Fundamentals – What is BGP Confederation?

BGP Confederation is specified in RFC 5065, which describes the use of Autonomous System Confederations for BGP – Border Gateway Protocol.

This topic and many other simple and advanced BGP topics are covered in Orhan Ergun’s BGP In-Depth course from a design and hands-on perspective.
BGP confederations help with this scalability issue by allowing the engineer to subdivide the autonomous system into smaller sub-autonomous systems.
There are generally two design methods when considering BGP confederations.
BGP Confederation usually is designed in two ways:
1. Same IGP (OSPF, IS-IS, EIGRP, etc.) in each Sub-AS
2. Different IGP in the sub-AS
There are pros and cons of each method as usual. Implementing the BGP confederation significantly reduces the total number of BGP sessions.
Implementing BGP confederations involves quite a change to BGP configurations and the architecture itself, adding more complexity to achieve a stable and scalable BGP design.
Migrating a network to a BGP confederation will be disruptive. Routers that are part of a sub-AS will need to change their BGP configuration to use the sub-AS instead of the real AS numbers.
Figure – BGP Confederation 
BGP routers within a sub-AS peering are IBGP peers., BGP routers in different sub-AS are EBGP peers which means that the AS number is prepended when an update travels between the sub-AS.
In the above figure, as you can see, the entire BGP network is divided into Sub-AS (Sub- Autonomous Systems). Since IBGP runs in each sub-AS, IBGP rules still apply in each Sub-AS. So, in each sub-AS, either full-mesh IBGP or Route Reflector can run. In the figure, Sub-AS 65001 have Full-mesh IBGP and Sub-AS 65002 has IBGP Route-Reflection.
If a router has to send an update towards its IBGP neighbor within a sub-AS, it will not change the AS_PATH attribute. BGP between the sub-ASs is called as intra-confederation EBGP
EBGP routes that are exchanged between the sub-ASs are also known as confederation external routes, which are preferred over IBGP routes when it comes to best path selection.
If BGP has to choose between two paths to the same destination, one path leading inside the sub-AS, and another outside the sub-AS but within confederation, it will choose the external path – towards the neighboring sub-AS.
 If it has to choose between the confederation EBGP route and the EBGP route that leads outside the confederation, BGP will choose the second one
Implementing BGP confederations involves quite a change to BGP configurations and the architecture itself, adding more complexity to achieve a stable and scalable BGP design.
Migrating a network from full-mesh IBGP or Route-Reflection to a BGP confederation will be disruptive because Routers that are part of a sub-AS will need to change their BGP configuration to use the sub-AS instead of the real AS numbers.

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