eBGP vpnv4 route ping failure

[ElshanMammadli5597]

We have faced routing issue with

bgp

 We can see

vrf routes

which learns from

ebgp peer

Routes installed in

vrf routing

table but ping is fail. between

ebgp peers  runs ISIS as IGP

Between

ebgp peers

have one node and it is runs as transit role. in this transit routers have isis and mpls. remote

ebgp peer learns vrf router

from

ibgp peer. 

We can see  routes in both side. Everthing seems normally but ping is fail

Simple topology have attached

[Harold Ritter · Accepted solution]

Hi @ElshanMammadli5597 ,

1. Can you replace

ebgp-multihop 10 by ebgp-multihop 10 mpls on both NCS55A2 and Remote N540L

2. Can you provide the output for

show cef vrf azercell 10.22.99.0/24 det from NCS55A2

Regards,

[ElshanMammadli5597]

Hi @Harold Ritter 

It seems that the issue resolved.

[Harold Ritter]

Glad to hear that the issue is resolved @ElshanMammadli5597 .

This is due to the default behavior with the

ebgp VPNv4 session in IOS-XR

which is to generate an

implicit-null

label for the remote ASBR. When eBGP multi hop is used between the ASBRs, this label overrides the one learnt via LDP, which causes traffic to be black holed. The

mpls

keyword is required in this specific scenario to prevent the implicit-null label to override the LDP label (or BGP-LU label).

Regards,

[ElshanMammadli5597]

Hi  @Harold Ritter .

Thank you professional approach and detailed information. 

[Harold Ritter]

You are very welcome @ElshanMammadli5597 and thanks for the feedback