BGP: Route Reflector

Technology: Routing

Area: IGP/EGP

Vendor: Cisco

Title: BGP Route Reflector

Software: 12.X , 15.X, IP Services

Platform: Catalyst 3560, 3750, 3850, 4500, 6500, ISR/ASR Routers

Functionality of route reflections was designed to avoid a necessity of configuring a fully meshed iBGP peering across entire autonomous system. The router which plays this role reflect all prefixes received from every iBGP neighbor without modification of attributes. In that manner route reflector could be outside the transit traffic and be central point only for network control packets. It is worth to mention that in general only the best route is reflected which has negative impact on multi path load balancing scenarios.

To prevent the routing loops route reflector is obligated to attached below attributes to every prefix:

1. ORGINATOR – BGP router-id of the neighbor from which it learned the prefix, the prefix
   is discarded when ORGINATOR field matches BGP_ROUTER_ID,
2. CLUSTER_LIST – either BGP router-id of route-reflector or manually specified parameter,
   the prefix is discarded when CLUSTER_LIST already contains one of above values.

To configure router R1 as a route-reflector for R2 and R3 iBGP neighbors:

R1# configure terminal
R1 (config)# router bgp 100
R1 (config-router)# bgp router-id 1.1.1.1
R1 (config-router)# neighbor 10.10.12.2 remote-as 100
R1 (config-router)# neighbor 10.10.12.2 route-reflector-client
R1 (config-router)# neighbor 10.10.12.2 description R2
R1 (config-router)# neighbor 10.10.13.2 remote-as 100
R1 (config-router)# neighbor 10.10.13.2 route-reflector-client
R1 (config-router)# neighbor 10.10.13.2 description R3
R1 (config-router)# exit

R2# configure terminal
R2 (config)# router bgp 100
R2 (config-router)# bgp router-id 2.2.2.2
R2 (config-router)# neighbor 10.10.12.1 remote-as 100
R2 (config-router)# redistribute connected route-map LOOPBACK_PREFIXES
 

R3# configure terminal
R3 (config)# router bgp 100
R3 (config-router)# bgp router-id 3.3.3.3
R3 (config-router)# neighbor 10.10.13.1 remote-as 100

Verification commands:

R1# show bgp ipv4 unicast summary
Neighbor        V    AS MsgRcvd MsgSent   TblVer  InQ OutQ Up/Down  State/PfxRcd
10.10.12.2      4     100      40      64        5    0    0 00:07:53        2
10.10.13.2       4     100     48      47        5    0    0 00:07:53        0

R1# show bgp ipv4 unicast
Network          Next Hop            Metric LocPrf Weight Path
*>i192.168.0.0      10.10.12.2                0    100      0 ?
*>i192.168.1.0      10.10.12.2                0    100      0 ?

R1# show bgp ipv4 unicast 192.168.0.0
BGP routing table entry for 192.168.0.0/24, version 31
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Advertised to update-groups:
1
Local, (Received from a RR-client)
10.10.12.2 from 10.10.12.2 (2.2.2.2)
Origin incomplete, metric 0, localpref 100, valid, internal, best
R1# show bgp ipv4 unicast update-group 1
BGP version 4 update-group 1, internal, Address Family: IPv4 Unicast
Route-Reflector Client
Has 2 members (* indicates the members currently being sent updates):
10.10.12.2        10.10.13.2

R3# show bgp ipv4 unicast neighbors 10.10.13.1 routes
Network          Next Hop            Metric LocPrf Weight Path
*>i192.168.0.0      10.10.12.2                0    100      0 ?
*>i192.168.1.0      10.10.12.2                0    100      0 ?
R3# show bgp ipv4 unicast 192.168.0.0
BGP routing table entry for 192.168.0.0/24, version 48
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
Local
10.10.12.2 from 10.10.13.1 (1.1.1.1)
Origin incomplete, metric 0, localpref 100, valid, internal, best
Originator: 2.2.2.2, Cluster list: 1.1.1.1
 

 Without route reflector functionality configured, R1 will not reflect prefixed from R2 to R3:

R1# show bgp ipv4 unicast 192.168.0.0
BGP routing table entry for 192.168.0.0/24, version 10
Paths: (1 available, best #1, table Default-IP-Routing-Table)
Flag: 0x820
Not advertised to any peer
Local
10.10.12.2 from 10.10.12.2 (2.2.2.2)
Origin incomplete, metric 0, localpref 100, valid, internal, best