Loop Prevention in RR Setups -
iBGP loop prevention mechanism - Split Horizon Rule
Route learned from an iBGP neighbor cannot be forwarded to other iBGP neighbor.
RR violates this rule so …”introduces risk of loops”
Originator ID
The Originator ID is set by the route reflector
32-bit router ID of the original iBGP speaker that injected the route. For eg R1
Visible in Route Reflector environments
Prevents loops if route comes back to the originator
Cluster ID & Cluster List
Cluster ID is an unique ID assigned to each Route Reflector.
Cluster List is a list of all Cluster IDs the route has passed through
When an RR reflects a route, it adds its own Cluster ID to the Cluster List.
If a router sees its own Cluster ID in the Cluster List → loop detected → route discarded
Cluster 1
R1 , R2 - Client
R3 - Route Reflector
Cluster 2
R4 , R5 - Client
R6 - Route Reflector
BGP Neighborship
R1_Client>show ip bgp summary
BGP router identifier 1.1.1.1, local AS number 100
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 4 100 7 7 1 0 0 00:03:31 0
R2_Client>show ip bgp summary
BGP router identifier 2.2.2.2, local AS number 100
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 4 100 6 8 1 0 0 00:03:48 0
R2_Client>
R3_RR1#show ip bgp summary
BGP router identifier 3.3.3.3, local AS number 100
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
1.1.1.1 4 100 11 11 1 0 0 00:06:41 0
2.2.2.2 4 100 10 8 1 0 0 00:06:13 0
6.6.6.6 4 100 4 4 1 0 0 00:00:08 0
R4_Client>show ip bgp summary
BGP router identifier 4.4.4.4, local AS number 100
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
6.6.6.6 4 100 9 9 1 0 0 00:05:01 0
R5_Client>show ip bgp summary
BGP router identifier 5.5.5.5, local AS number 100
BGP table version is 1, main routing table version 1
Neighbor V AS MsgRcvd MsgSent TblVer InQ OutQ Up/Down State/PfxRcd
6.6.6.6 4 100 9 9 1 0 0 00:05:20 0
R5_Client>
R6_RR2#show ip bgp summary
BGP router identifier 6.6.6.6, local AS number 100
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 4 100 5 5 1 0 0 00:01:10 0
4.4.4.4 4 100 10 10 1 0 0 00:05:43 0
5.5.5.5 4 100 10 10 1 0 0 00:05:43 0
R6_RR2#
=============================================
Now lets advertise a network 100.1.1.1/32 from R1 in BGP
R1_Client(config)#int loopback 100
R1_Client(config-if)#ip address 100.1.1.1 255.255.255.255
R1_Client(config)#router bgp 100
R1_Client(config-router)#network 100.1.1.1 mask 255.255.255.255
R1_Client(config-router)#^Z
Lets check on its RR - R3
R3_RR1#show ip bgp 100.1.1.1/32
BGP routing table entry for 100.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
Advertised to update-groups:
2 3
Refresh Epoch 1
Local, (Received from a RR-client)
1.1.1.1 (metric 11) from 1.1.1.1 (1.1.1.1)
Origin IGP, metric 0, localpref 100, valid, internal, best
rx pathid: 0, tx pathid: 0x0
Lets check on RR2 - R6
R6_RR2#show ip bgp 100.1.1.1/32
BGP routing table entry for 100.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
Advertised to update-groups:
2
Refresh Epoch 1
Local
1.1.1.1 (metric 21) from 3.3.3.3 (3.3.3.3)
Origin IGP, metric 0, localpref 100, valid, internal, best
Originator: 1.1.1.1, Cluster list: 3.3.3.3 ----------> ORIGINATOR ID OF R1 CLUSTER ID OF RR1
rx pathid: 0, tx pathid: 0x0
Now lets check on R4
R4_Client>show ip bgp 100.1.1.1/32
BGP routing table entry for 100.1.1.1/32, version 2
Paths: (1 available, best #1, table default)
Not advertised to any peer
Refresh Epoch 1
Local
1.1.1.1 (metric 31) from 6.6.6.6 (6.6.6.6)
Origin IGP, metric 0, localpref 100, valid, internal, best
Originator: 1.1.1.1, Cluster list: 6.6.6.6, 3.3.3.3 ---> RR2 Cluster ID is also attached
rx pathid: 0, tx pathid: 0x0
R4_Client>
For complete configuration - mail me on networkengineerstuff.com
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