BGP Confederation

What is a BGP Confederation?

BGP Confederations is an another mechanism to avoid iBGP Full Mesh topology like Route Reflectors. As a basic definion, BGP Confederaition mechanims is Autonomous Systems (AS) in Autonomous Systems (AS). In other words, in this BGP mechanism, there are  Sub Autonomous Systems inside the iBGP Topology. The top Autonomous System is still exist and the new Sub ASs are connected to this AS and form BGP Confederation.

Here, the routers in the same Confederation is called Confederation iBGP Peers and the routers in another Confederation is called Confederation eBGP Peers.

BGP Confederation Sub ASs aer created with the Private AS Numbers between 64512 and 65535.

BGP Confederations Topology

With BGP Confederations, BGP Autonomous System can be divided into small Autonomous Systems. This reduces the connection number that can be evry high without using Confederations and Route Reflectors. So, the confederation mechanism in BGP is very important especially for the large networks.

BGP Confederations Outside the AS

In the Autonomous System, there can be one more Sub Confederations. These Confederations can connect together like eBGP neighbors. And the routers inside any Confederation can exchange routes like they are iBGP neighbors. And routers in SubASs should be connected as Full Mesh.

Next Hop, Local Preference and Metric values ara preserved in this mechanism.  Only AS Path is changed.  Although the mechanims is like this inside the top Autonomous System, from the outside of AS, BGPConfederation that has different Sub ASs, seems as one Autonomous System (AS).

How to Prevent Loops in BGP Confederation?

In the BGP Confederation mechanism, we should use a Loop Preventation mechanism to avoid Routing Loops. In Confederations, to avoid Routing Loops, AS Path Path Attribute is used.

BGP AS Path Attribute is a Well Known Mandatory Path Attribute. Basically it has a list consist of AS Number of the originating router and the routers it traverse during its destination. From the beginning to the destination, at every AS, AS Number is prepended to this AS List.

Normally two parameters are used with AS Path. These AS Path parameters are given below:

• AS_SET
• AS_SEQUENCE

AS_SET is the list of ASs that is unordered and AS_SEQUENCE is the list of ASs as ordered.

With Confederations and Sub ASs, two additional parameters are added to AS Path attribute of BGP. These new parameters are :

• AS_CONFED_SET
• AS_CONFED_SEQUENCE

Like the parameters before, these parameters are also has the similar meanings. AS_CONFED_SET is the list of ASs that is unordered and AS_CONFED_SEQUENCE is the list of ASs as ordered.

Loop preventatin mechanism with AS Path Path Attribute is similar to normal Autonomous Systems. In a BGP Confederation, if an advertised iBGP route comes from a SubAS to another SubAS (eBGP), with AS_CONFED_SEQ, the router checks that if the same Sub AS is inside the AS Path or not. If the same Sub AS is in the AS Path, it is discarded.

BGP Confederation Traffic Selection

Here, we have different scenarious about the path selection in a BGP Topology that use BGP Confederations. Now, let’s check these scenarious one by one.

• If there are two path to the same destination, one is through the same Confederation Sub AS and the other is through another SubAS in the Confederation, the outside path through another SubAS is selected.

• If there are two path to the same destination, one is through another SubAS (Confederation eBGP) and the other is through Normal eBGP, the Normal eBGP path through another AS is selected.

In this lesson, we have talked about an important mechanism on BGP Networks. We have learned how to prevent Full Mesh iBGP Topology with BGP Confederations.

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