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BGP Features – Route reflectors & Confederations
BGP Split Horizon explanation
Border Gateway Protocol has two features which enable improving scalability. Thanks to them BGP AS domain with a lots of routers will be working more efficient and seamlessly. They are similar to each other and because of that, they are very often confused. But first, in order to understand RR and Confederations we have to understand how Split Horizon rule works in BGP.
“Split Horizon” – in plain words – NEVER ADVERTISE NETWORKS ON THE INTERFACE THAT YOU HAVE LEARNED ABOUT THEM. It makes sense, take a look at the picture below.
“Split Horizon in BGP” works differently a little bit. Because in BGP AS (Autonomous System) domain “full mesh” topology is required between iBGP routers ( internal routers), it works : if any iBGP router has learned about any prefix then has to broadcasts this prefix to every other iBGP router in domain in the same time. Thanks to this, the the other routers don’t have to advertise this prefix again, because each of them has got those networks already. BGP Split Horizons says – IF YOU HAVE LEARNED ABOUT GIVEN PREFIX FROM THE OTHER iBGP ROUTER … KEEP IT TO YOURSELF AND DON’T TRY TO PASS ALONG THAT INFORMATION TO THE OTHER iBGP ROUTERS, BECAUSE THEY GOT EXACTLY THE SAME PREFIX ALREADY.
ROUTE REFLECTORS – RR (Route Reflector) router breaks above Split Horizon rule and sends updates to defined clients and to the eBGP neighbour routers. Route Reflector behaves differently, depends on which type of routers it has learned prefixes from :
eBGP router – RR router sends to ALL iBGP and eBGP peers
iBGP NON-CLIENT – RR router sends updates to eBGP and CLIENTS
iBGP CLIENT – RR router sends updates to all peers, except that one it got update from.
We may have deployed 2 RR and then eBGP routers that got prefix will send update to both of them and RRs will send them further to Clients and non-Clients. iBGP routrs will have 2 copies of the same route.
RR routers may work in cluster in order to not allow RR routers sending the same information between them (loop prevention mechanism). We may have a few clusters in the same Autonomous System. Cluster-ID is just another attribute. As we see below RR can significantly reduces the traffic.
Configuration only on router reflector, we don’t configure anything on clients and non clients:
neighbor 1.1.1.1 route-reflector-client
neighbor 2.2.2.2 route-reflector-client
bgp cluster-id 10
CONFEDERATIONS
Confederations are considered as the next level of Route Reflectors. In plain words this is Autonomous System in Autonomous System. Confederations use private AS (64512 to 65535). Full mesh is required within Confederation AS, unless we use Route Reflector, because Confederations and Route Reflectors may be deployed at the same time. When eBGP router from the confederation is peering with an external eBGP peer from different AS then Private AS of confederation is stripped.
Regarding configuration we have to change real AS number to Private, bgp identifier is the real AS number
router bgp 64555
bgp confederation indentifer 300
bgp confederation peers 64666 64777