BGP confederation and Router Reflector
BGP requires full mesh neighbor relationship in order to advertise routes to its peers since BGP does not advertise IBGP to its neighbor. In a large environment which contains a lot of BGP routers in the AS, configuring BGP neighbor is a lot of work to do. For example, if in the AS, there are 10 BGP routers, we need to form (10 x 9)/2 = 45 BGP neighbor relationships. That is a painful in the implementation. Here below are functions to help establish the full mesh in the AS.
BGP confederation
The concept of BGP confederation is to have sub-AS inside an original AS. In the sub-AS we form a full mesh networks. Then, join each sub-AS together. In this case, we will have small sub full mesh network and form a entire AS mesh. Configuring BGP confederation requires only few command. Instead of using a real AS number in the router bgp command, we use sub AS number. However, the real AS is configured by bgp confederation-identifier under router bgp command.
Connecting different BGP confederation is a same way as eBGP.
(See example for the configuration )
Route-Reflector
RR achieve the same result as confederation that is iBGP mesh. There are 2 modes in RR: server and client. All clients will establish the neighbor relationship with the server. Server will reflect routes received from one client to others without having client formed neighbor with each other.
The command we use for RR is neighbor x.x.x.x route-reflector client.
Configuration Example
- Configure R1 as AS 100
- Configure R2 - R5 as AS 200
- Configure BGP confederation 65001 and 65002 according to the diagram below
- Configure RR inside AS 65002
- Verify full mesh inside each confederation and AS 200
Configuration
R1
router bgp 100
no synchronization
bgp log-neighbor-changes
neighbor 10.10.10.2 remote-as 200
no auto-summary
!
R2
router bgp 65001
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200
bgp confederation peers 65002
neighbor 3.3.3.3 remote-as 65002
neighbor 3.3.3.3 ebgp-multihop 2
neighbor 3.3.3.3 update-source Loopback0
neighbor 10.10.10.1 remote-as 100
no auto-summary
R3
router bgp 65002
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200
bgp confederation peers 65001
neighbor 2.2.2.2 remote-as 65001
neighbor 2.2.2.2 ebgp-multihop 2
neighbor 2.2.2.2 update-source Loopback0
neighbor 4.4.4.4 remote-as 65002
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 route-reflector-client
neighbor 5.5.5.5 remote-as 65002
neighbor 5.5.5.5 update-source Loopback0
neighbor 5.5.5.5 route-reflector-client
no auto-summary
!
R4
router bgp 65002
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200
neighbor 3.3.3.3 remote-as 65002
no auto-summary
!
R5
router bgp 65002
no synchronization
bgp log-neighbor-changes
bgp confederation identifier 200
network 100.100.100.0 mask 255.255.255.0
neighbor 3.3.3.3 remote-as 65002
neighbor 3.3.3.3 update-source Loopback0
no auto-summary
!
We advertise network 100.100.100.0/24 originated from R5 to BGP.
##################################
Verify neighbor at R3
R3#sh ip bg summary
BGP router identifier 3.3.3.3, local AS number 65002
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
2.2.2.2 4 65001 17 18 1 0 0 00:05:04 0
4.4.4.4 4 65002 23 23 1 0 0 00:19:23 0
5.5.5.5 4 65002 15 15 1 0 0 00:11:24 0
R3#
Verify neighbor at R2
R2#sh ip bgp summary
BGP router identifier 10.10.10.2, local AS number 65001
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
3.3.3.3 4 65002 17 17 1 0 0 00:05:41 0
10.10.10.1 4 100 34 33 1 0 0 00:30:08 0
R2#
Verify that R1 has received network information 100.100.100.0/24 from R5
R2# sh ip bgp
BGP table version is 2, local router ID is 10.10.10.2
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 100.100.100.0/24 5.5.5.5 0 100 0 (65002) i
R2#
R1#sh ip bgp
BGP table version is 2, local router ID is 10.10.10.1
Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,
r RIB-failure, S Stale
Origin codes: i - IGP, e - EGP, ? - incomplete
Network Next Hop Metric LocPrf Weight Path
*> 100.100.100.0/24 10.10.10.2 0 200 i
R1#
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