Chapter 2 Advanced Routing for FortiOS 5.0 : Border Gateway Protocol (BGP) : BGP background and concepts : Parts and terminology of BGP : Confederations
  
Confederations
Confederations were introduced to reduce the number of BGP advertisements on a segment of the network, and reduce the size of the routing tables. Confederations essentially break up an AS into smaller units. Confederations are defined in RFC 3065 and RFC 1965.
Within a confederation, all routers communicate with each other in a full mesh arrangement. Communications between confederations is more like inter-AS communications in that many of the attributes are changed as they would be for BGP communications leaving the AS, or eBGP.
Confederations are useful when merging ASs. Each AS being merged can easily become a confederation, requiring few changes. Any additional permanent changes can then be implemented over time as required. The figure below shows the group of ASs before merging, and the corresponding confederations afterward as part of the single AS with the addition of a new border router. It should be noted that after merging if the border router becomes a route reflector, then each confederation only needs to communicate with one other router, instead of five others.
Figure 105: AS merging using confederations
Confederations and route reflectors perform similar functions — they both sub-divide large ASes for more efficient operation. They differ in that route reflector clusters can include routers that are not members of a cluster, where routers in a confederation must belong to that confederation. Also, confederations place their confederation numbers in the AS_PATH attribute making it easier to trace.
It is important to note that while confederations essentially create sub-ASs, all the confederations within an AS appear as a single AS to external ASs.
Confederation related BGP commands include:
config router bgp
set confederation-identifier <peerid_integer>
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