Activate additional BGP address-family leads to BGP session goes down

[Florian Frotzler]

Hi,

I recently activated the lvpn address-family on an existing BGP neighbor, which already had vpnv4 up and running (5.1.3 SP3). After commiting the change the neighbor went down and then up again. Is this behavior expected? Is it expected that an additional address-family brings the session down?

Regards,

Florian

[smailmilak · Accepted solution]

Hi,

indeed it's expected behavior. Any change in BGP AF will restart the BGP session. I think it's because the BGP neighbor capabilities are changed if you remove or add an AF. 

[Florian Frotzler]

Hi,

Thank you for the quick answer, can you point me to any documentation regarding this behavior?

Regards,

Florian

[smailmilak]

It's probably documented somewhere or it's on a Cisco Live presentation.

I did a quick search but it was not able to find anything.

[AARON WEINTRAUB]

Unfortunately the BGP protocol only allows address families to be introduced at session establishment, so adding or removing AFIs from a session causes a session reset.  Maybe in the future additional enhancements will solve this issue.

[Adam Vitkovsky]

Yes indeed,

New capabilities needs to be exchanged when new address-family is configured, so that BGP speakers know how to pars and interpret each other’s messages correctly.

And since capability field is carried only in BGP OPEN messages, and OPEN messages are exchanged only during BGP session initialization, the TCP session needs to be restarted so that the BGP OPEN messages can be exchanged again.

There was some work in IETF to introduce dynamic capabilities exchange but somewhat did not took off:

https://tools.ietf.org/html/draft-ietf-idr-dynamic-cap-14

https://tools.ietf.org/html/draft-ietf-idr-bgp4-cap-neg-05

adam

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