BGP Route Reflection and Confederation Notes
Route Reflection (RR) and Confederation
Split-horizon rule of BGP
– Updates received from one iBGP neighbor are never sent to another iBGP neighbor
– This prevents possible looping in the AS
– Also means that a full mesh between iBGP routers is necessary
– Route Reflection and Confederation allow for split-horizon to be bypassed, but also introduces new attributes to prevent loops
– Route Reflection works better on small networks
– Confederation works better on large networks
Clients – iBGP
Non-clients – iBGP
External – eBGP
On the route reflector (will also be referred to as RR), any updates from clients can be reflected to clients, non-clients, and external.
If updates are received from a non-client, the updates can be reflected to clients and external.
The router reflector does not change the next-hop information when reflecting routers to clients.
To prevent loops, RR introduces two attributes
– originator-id
-> router-id of the originating router
– cluster-id
-> router-id of route reflector
Originator-id
– Whenever a client sends an update to a RR, the RR associates the client’s router-id as the originator-id
– If a client receives an update with its own router-id listed as originator-id, the update is dropped
Cluster-id
– Whenever an update is sent out of the cluster, the cluster-id is associated with the update
– When a RR receives an update which lists its own router-id as the cluster-id, the update is dropped
R1(config)# router bgp 100
neighbor 2.2.2.2 remote-as 100
neighbor 2.2.2.2 route-reflector-client
neighbor 3.3.3.3 remote-as 100
neighbor 3.3.3.3 route-reflector-client
Confederation
– Used to bypass the split-horizon rule, but in a different way
– Routers are separated into private ASNs to create an eBGP effect within the iBGP AS
Confederation introduces 2 attributes to prevent loops
– AS_CONFED_SET
-> Orderly list of confederated ASs the update has traversed
– CONFED_External
-> CONFED_Internal
Whenever an update is sent to an eBGP neighbor, the real ASN will be used
AS_PATH_SET is always counted as one, regardless of the number of confederations the update has traversed.
R1(config)# router bgp 65001
bgp confederation peer 65002 65003
-> If 65003 is not a direct neighbor, it does not need to be listed
-> It’s a good practice to list all of the confederation ASNs
bgp confederation indentifier 100
-> The real ASN used the eBGP
neighbor 2.2.2.2 remote-as 65001
neighbor 3.3.3.3 remote-as 65001
neighbor 4.4.4.4 remote-as 65002
neighbor 10.0.0.10 remote-as 200
