announcing BGP prefixes from the same AS at different sites?

Raphael Mazelier raph at
Wed Sep 10 18:45:28 CEST 2014

Back to basics :

1) Your IGP should be use to propagate loopback (and interco if needed) 
of your router. You may use multiple path between your router for 

2) Your IBGP should be a full mesh of your loopback core router using 
BGP (unless using RR). A IBGP session should not failed, if so you are 
loosing your route associated to the router. The redundancy is done by 
the IGP (if you have multiple path)

I agreed that disabling as-path loop is an hacky workarroud, which may 
not work, and make thing unclear. Bad idea for me.

So two options :

- make some tunnel, and establish an IGP/IBGP session with it.
- or use statics routes (with tracking optionnaly).


Le 10/09/2014 15:38, Thomas Johnson a écrit :
> Thank you for the feedback. I was aware of disabling aspath loop
> detection, but have not yet explored it; based on my reading, it seemed
> like a hacky workaround. Are there any drawbacks to this approach, aside
> from the obvious possibility of creating loops?
> With regard to an IBGP mesh, it seems that if a BGP session in my mesh
> fails, routing of prefixes between the affected routers fails. Is this
> the expected behavior? Is this where I should be using an IGP to route
> around the failed IBGP?
> Thanks!
> On Mon, Sep 1, 2014 at 11:54 AM, Raphael Mazelier <raph at
> <mailto:raph at>> wrote:
>     Le 30/08/2014 01:36, Tom Daly a écrit :
>         Hi,
>         It sounds like you might want to try disabling aspath loop
>         detection for
>         eBGP. Checkout the bird option called "allow local as [number]" as
>         documented at
>         <>. This
>         will allow you to import prefixes with your own ASN in the aspath.
>         However, one possible pitfall is that your upstream ISP's router
>         might
>         try to be smart - and not send prefixes with your aspath in them
>         too.
>         Older Foundry boxes do this and there is a per BGP peer knob you
>         have to
>         turn.
>         Overall, this should let you run everything over eBGP and run
>         two simple
>         island networks.
>         Cheers,
>         Tom
>     Unfortunately proper ISPs make their job good and does not advertise
>     your own route. I have a Pop in the same configuration of yours.
>     Two solution : IBGP in a GRE tunnel, it's uggly, but It works.
>     Or static route :)
>     --
>     Raphael Mazelier
>     AS39605

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