IBGP Load-Balancing

[yuening liao]

Hello Everyone:

Below is my lab topology:

Role of Router:

C1:                   IBGP Peer of RR_T1(non-client)

RR_T1:             RR tier 1(non-client)

RR_T2_1:          RR tier 2, client of RR_T1

RR_T2_2:          RR tier 2, client of RR_T1

C2:                   client of RR_T2_1 & RR_T2_1

all routers reside in BGP AS 1 & OSPF area 0 without additonal configurations.

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C2(config-router)#do sh ip ro os

Gateway of last resort is not set

      2.0.0.0/32 is subnetted, 1 subnets

O        2.2.2.2 [110/3] via 45.0.0.4, 00:05:03, GigabitEthernet2/0

                 [110/3] via 35.0.0.3, 00:05:03, GigabitEthernet1/0

      3.0.0.0/32 is subnetted, 1 subnets

O        3.3.3.3 [110/2] via 35.0.0.3, 00:05:03, GigabitEthernet1/0

      4.0.0.0/32 is subnetted, 1 subnets

O        4.4.4.4 [110/2] via 45.0.0.4, 00:05:03, GigabitEthernet2/0

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C2(config-router)#do sh ip bg 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 2

Paths: (2 available, best #2, table default)

Multipath: iBGP

  Not advertised to any peer

  Refresh Epoch 3

  Local

    12.0.0.1 (metric 3) from 4.4.4.4 (10.0.0.4)

      Origin IGP, metric 0, localpref 100, valid, internal

      Originator: 10.0.0.1, Cluster list: 10.0.0.4, 10.0.0.2

  Refresh Epoch 3

  Local

    12.0.0.1 (metric 3) from 3.3.3.3 (10.0.0.3)

      Origin IGP, metric 0, localpref 100, valid, internal, best

      Originator: 10.0.0.1, Cluster list: 10.0.0.3, 10.0.0.2

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C2(config-router)# do sh run | s r b

router bgp 1

bgp router-id 10.0.0.5

bgp log-neighbor-changes

network 5.5.5.5 mask 255.255.255.255

neighbor 3.3.3.3 remote-as 1

neighbor 4.4.4.4 remote-as 1

maximum-paths ibgp 2

C2(config-router)#

according path selection rules of BGP all 8 criteria above step 9th is equal, now after configuring maximum-paths ibgp i thought it should result multiple equal-cost paths toward 1.1.1.1, but it didn't. Anybody knows the reason?

thanks

[Giuseppe Larosa · Accepted solution]

Hello Yuening,

look at the output of

show ip route 1.1.1.1

the effect of the command should be to have two BGP routes installed for the prefix, however in show ip bgp 1.1.1.1 you still see a single BGP best path selected.

you see that there is the line declaring multipath iBGP

C2(config-router)#do sh ip bg 1.1.1.1

BGP routing table entry for 1.1.1.1/32, version 2

Paths: (2 available, best #2, table default)

>>>>>Multipath: iBGP

However, also the two BGP asvertisements are different just in the cluster list so the original BGP advertisement is only one with a single BGP next-hop.  You should have the same IP prefix advertised by two different routers with different BGP next-hops to see the command in effect.

Hope to help

Giuseppe

[yuening liao]

hello Giuseppe,

Thank you very much. after manually modifying the next-hop the load-balancing is done.