iBGP requires full-mesh between all peers — N×(N−1)/2 sessions. A Route Reflector (RR) breaks this requirement: clients only peer with the RR, which re-advertises routes to all other clients.
Topology
R1 is the Route Reflector. R3 and R4 are RR clients.
Configurations
R1 — Route Reflector
interface Loopback0
ip address 1.1.1.1 255.255.255.255
ip ospf 1 area 0
!
interface Loopback1
ip address 100.100.100.100 255.255.255.255
!
interface Ethernet0/0
ip address 10.1.2.1 255.255.255.0
!
interface Ethernet0/1
ip address 10.1.3.1 255.255.255.0
ip ospf 1 area 0
!
router ospf 1
router-id 1.1.1.1
!
router bgp 65000
bgp log-neighbor-changes
network 100.100.100.100 mask 255.255.255.255
neighbor 3.3.3.3 remote-as 65000
neighbor 3.3.3.3 update-source Loopback0
neighbor 3.3.3.3 route-reflector-client
neighbor 4.4.4.4 remote-as 65000
neighbor 4.4.4.4 update-source Loopback0
neighbor 4.4.4.4 route-reflector-client
R3 — RR client
interface Loopback0
ip address 3.3.3.3 255.255.255.255
ip ospf 1 area 0
!
interface Loopback1
ip address 200.200.200.200 255.255.255.255
!
interface Ethernet0/0
ip address 10.3.4.3 255.255.255.0
ip ospf 1 area 0
!
interface Ethernet0/1
ip address 10.1.3.3 255.255.255.0
ip ospf 1 area 0
router ospf 1
router-id 3.3.3.3
!
router bgp 65000
bgp log-neighbor-changes
network 200.200.200.200 mask 255.255.255.255
neighbor 1.1.1.1 remote-as 65000
neighbor 1.1.1.1 update-source Loopback0
R4 — RR client
interface Loopback0
ip address 4.4.4.4 255.255.255.255
ip ospf 1 area 0
!
interface Ethernet0/0
ip address 10.3.4.4 255.255.255.0
ip ospf 1 area 0
!
interface Ethernet0/1
ip address 10.2.4.4 255.255.255.0
!
router ospf 1
router-id 4.4.4.4
!
router bgp 65000
bgp log-neighbor-changes
neighbor 1.1.1.1 remote-as 65000
neighbor 1.1.1.1 update-source Loopback0