Multipath in BGP

Answered Question
Nov 11th, 2009
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Hi All,


I noticed a really unusual thing. When multipath is enabled between eBGP and iBGP, the AD of iBGP path also appears as 20 instead of 200. However, if eBGP path is not available and only iBGP path is available, the AD becomes 200.


This seems like a bug. Any ideas?


Here's the sample output- Notice network 10.2.2.0 in the both outputs.


PE1#sh ip route vrf CUST1


Gateway of last resort is not set


172.64.0.0/30 is subnetted, 3 subnets

C 172.64.1.0 is directly connected, Serial0/0

B 172.64.2.0 is directly connected, 00:22:35, Serial0/1

B 172.64.3.0 [200/0] via 2.2.2.2, 00:04:33

10.0.0.0/24 is subnetted, 2 subnets

B 10.2.2.0 [20/0] via 172.64.2.1 (CUST2), 00:22:35

[20/0] via 2.2.2.2, 00:00:27

B 10.1.1.0 [20/0] via 172.64.1.1, 00:22:45



PE1#sh ip bgp vpnv4 all 10.2.2.0

BGP routing table entry for 1:1:10.2.2.0/24, version 24

Paths: (2 available, best #2, table CUST1)

Multipath: eiBGP

Flag: 0x800

Advertised to update-groups:

1

65002, imported path from 2:2:10.2.2.0/24

2.2.2.2 (metric 11) from 2.2.2.2 (2.2.2.2)

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

Extended Community: RT:2:2

mpls labels in/out nolabel/20

65002, imported path from 1:2:10.2.2.0/24

172.64.2.1 from 172.64.2.1 (10.2.2.1)

Origin IGP, metric 0, localpref 100, valid, external, multipath, best

Extended Community: RT:1:2



Regards,

Amit.

Correct Answer by Giuseppe Larosa about 7 years 6 months ago

Hello Amit,

I don't think it is a bug.


you have asked to perform iBGP and eBGP multipath if the iBGP prefix would be presented with natural AD 200 it wouldn't be installed in routing table and you would see only the eBGP path coming from CE node.


it is a way to fool IP routing table mantainer


Hope to help

Giuseppe


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shivlu jain Wed, 11/11/2009 - 06:06
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Hi Amit


Please post the diagram.


regards

shivlu jain

Correct Answer
Giuseppe Larosa Wed, 11/11/2009 - 09:55
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Hello Amit,

I don't think it is a bug.


you have asked to perform iBGP and eBGP multipath if the iBGP prefix would be presented with natural AD 200 it wouldn't be installed in routing table and you would see only the eBGP path coming from CE node.


it is a way to fool IP routing table mantainer


Hope to help

Giuseppe


shivlu jain Thu, 11/12/2009 - 01:28
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Amit You are right. I need to simulate the same and will come back with results.

Could you post the IOS name.


regards

shivlu jain

amit.bhagat Thu, 11/12/2009 - 04:02
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Hi Shivlu,


I tried on c2691-spservicesk9-mz.124-25a.bin.


Regards,

Amit.

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