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