bgp router with multi asn's - neighbor config ?

[Ondrej Filip]

On 2.9.2014 23:36, Kai wrote:
> G'day!

Hi!

> 
> For the configuration we want to set up, I couldn't find any details in
> the docs, the wiki or the list archive. So please allow me to ask my
> questions here (questions see below).
> 
> desired setup:
> 
> We want to establish a router ('A') announcing two different ASNs
> (as1,as2) to it's neighbours. In fact we want to setup three routers A,
> B and C, all of them corporately announcing as1 and as2.
> 

You cannot have two BGP relations to a single peer. I see two options:

1) You will announce two different AS paths with the same first AS:

 < as1 >
 < as2 as1 >

so you can see as2 is announced "behind" as1.

2) You can announce two same as paths:

 < as1 >
 < as2 >

But the peer must disable check on 1st AS in AS paths. Cisco command
"no bgp enforce-first-as"


In both case the configuration is:
 protocol bgp {
   local as <as1>;
   neighbor X as as9;
   (...)
 }

Different is synthesis of propagated prefixes.

		Ondrej


> I'd guess the config on router A should include some lines like these:
>  protocol bgp {
>    local as <as1>;
>    neighbor X as as9;
>    (...)
>  }
>  protocol bgp {
>    local as <as2>;
>    neighbor X as as9;
>    (...)
>  }
> X being one of the neighbours and announcing as9.
> 
> Question 1: Is this basically the correct approach?
> 
> Question 2: What would be the correct config on router X (A's neighbour)
> in this setup? Would it look like:
>  protocol bgp {
>    local as <as9>;
>    neighbor A as as1;
>    (...)
>  }
>  protocol bgp {
>    local as <as9>;
>    neighbor A as as2;
>    (...)
>  }
> 
> Question 3: Does anyone have a pointer to a text/tutorial/whatever that
> explains in detail what config is needed for this kind of setup? 
> 
> Best regards, Kai
>