Route Reflectors

From a full mesh to a reflector

A full mesh of six BGP speakers needs 15 sessions: N(N−1)/2. A single reflector with five clients needs five client sessions, but becomes a control-plane dependency. Redundant reflectors reduce that failure risk. A reflector advertises selected paths; it does not automatically put all learned alternatives into every client’s table.

Three-router experiment

R1–R2 and R2–R3 are ordinary iBGP sessions in AS 65123. OSPF supplies loopback and transit reachability. R1 originates the host route. R3 can resolve its next hop, but initially R2 cannot advertise an iBGP-learned route to another ordinary iBGP neighbor.

hostname R1
ip cef
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
exit
interface Loopback10
 ip address 198.51.100.1 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.1
 network 10.255.0.1 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.1 0.0.0.0 area 0
exit
router bgp 65123
 bgp router-id 10.255.0.1
 no bgp default ipv4-unicast
 neighbor 192.0.2.2 remote-as 65123
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.2 activate
  network 198.51.100.1 mask 255.255.255.255
 exit-address-family
end

hostname R2
ip cef
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 no shutdown
exit
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.2
 network 10.255.0.2 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.2 0.0.0.0 area 0
 network 192.0.2.5 0.0.0.0 area 0
exit
router bgp 65123
 bgp router-id 10.255.0.2
 no bgp default ipv4-unicast
 neighbor 192.0.2.1 remote-as 65123
 neighbor 192.0.2.6 remote-as 65123
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.1 activate
  neighbor 192.0.2.6 activate
 exit-address-family
end

hostname R3
ip cef
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 no shutdown
exit
router ospf 1
 router-id 10.255.0.3
 network 10.255.0.3 0.0.0.0 area 0
 passive-interface Loopback0
 network 192.0.2.6 0.0.0.0 area 0
exit
router bgp 65123
 bgp router-id 10.255.0.3
 no bgp default ipv4-unicast
 neighbor 192.0.2.5 remote-as 65123
 address-family ipv4 unicast
  no auto-summary
  neighbor 192.0.2.5 activate
 exit-address-family
end

R3# show ip bgp 198.51.100.1/32
% Network not in table

Separate clients and non-clients

The table summarizes iBGP reflection. Ordinary eBGP export rules still apply. Do not reflect a route back to its source as a useful new path; ORIGINATOR_ID and CLUSTER_LIST provide additional loop protection. Reflection normally preserves NEXT_HOP, so IGP reachability remains necessary.

Make R2 the reflector

router bgp 65123
 address-family ipv4 unicast
  neighbor 192.0.2.1 route-reflector-client
  neighbor 192.0.2.6 route-reflector-client
 exit-address-family

Apply these commands on R2 only. R1 and R3 retain ordinary iBGP configuration. R2’s clients are identified by its directly connected neighbor addresses in this lab.

Inspect the reflected route

R3# show ip bgp 198.51.100.1/32
BGP routing table entry for 198.51.100.1/32
Paths: (1 available, best #1, table default)
  Local
    192.0.2.1 from 192.0.2.5 (10.255.0.2)
      Origin IGP, metric 0, localpref 100, valid, internal, best
      Originator: 10.255.0.1, Cluster list: 10.255.0.2

The advertising peer is R2, while the preserved next hop and originator point back toward R1. R3’s OSPF route to the R1–R2 link makes that next hop reachable. The cluster list records reflection history; a reflector rejects a route containing its own cluster ID.

Restore ordinary iBGP

router bgp 65123
 address-family ipv4 unicast
  no neighbor 192.0.2.1 route-reflector-client
  no neighbor 192.0.2.6 route-reflector-client
 exit-address-family

R3# show ip bgp 198.51.100.1/32
% Network not in table

The route disappears after the withdrawal, even though the two sessions remain up. A production design needs reflection or the appropriate full mesh, not this intentionally incomplete chain.