Specific case - How to remove private AS numbers from BGP prefixes

[TelesEC]

Hello everyone,

I have a question regarding the suppression of the BGP private AS numbers in the prefixes announced by the partners or by the subsidiaries.

I have a couple of routers in a colocation facility and there they interconnect with external networks, we attribute a private AS number to each subsidiary or partner and the problem is the following:

The prefixes advertised by the external router traverse their own infrastructure (which includes service providers) therefore the AS_PATH of the prefixes has a mix of public and private AS numbers.... those private AS sometimes overlap with ours which could generate problems... how can I suppress the private AS of the external partners?

I have tried the following:

1. Use remove-private-as but it doesn't work when there is a mix of private and public AS numbers.
2. Use remove-private-as all but it doesn't work because the partner uses a private AS assigned by me.
3. I could replace the private AS numbers with mine through a route-map or RPL, but by replacing the private AS, I would also replace the partner's one and that would cause a loss of information...

Any other idea?

Thanks in advance.

[Giuseppe Larosa]

Hello @TelesEC ,

option 3 is the way to go   but you need to use BGP confederations so that you will have two distinct AS path attribute the external one and the Confed AS path segment , then in peering with true eBGP your public AS number will be used and the Confed AS path segment is not propagated.

this way can work.

Hope to help

Giuseppe

[vencislav.metev]

Hi,

Can you try with command "remove-private-as all replace-as"?

You might need to upgrade to the latest IOS version.

Regards,
Ventsi

[TelesEC]

Hola!

Thank you for your response.

In Cisco IOS-XE the "remove-private-as all" command works only if the ASN of the router where you configure the command is a public ASN... In my case, the routers have private ASNs.
The documentation does not specify if this limitation has been removed... in Cisco IOS-XR I do not find this problem.
I do not have virtual routers in higher versions that allow me to test this command.

Regards,

Andrés

[TelesEC]

Hi!

Thanks for your answer and for the idea, we do not use BGP confederations but it would be interesting to analyze the idea.

Regards,

Andrés