Re: BGP circular routing

On 2021-04-29, Marko Cupać <marko.cupac@mimar.rs> wrote:
> I guess this is not related to bgpd, but I hope there are skilled
> network admins here who can give me advice.
>
> I have a problem with circular routing on a site which talks BGP with
> two upstream providers, with traffic to site which has static default
> route over third ISP:
>
> --> ISP1 --> ISP3 -->
> SITEA SITEB
> <-- ISP2 <-- ISP3 <--

Asymmetric routing (circular suggest that it's looping so you have
no working connecticity, which I tuink ks not what you're describing).

> I tried to prepend self / neighbor to ISP2 - no change (ISP1 has best
> routes for 99% of the prefixes, including to SITEB). I contacted ISP2,
> they said the problem is with ISP3. I contacted ISP3, they said ISP2
> announces my prefix (they're my LIR) so the best route is over them. I
> contacted ISP2 again, they said they prepended my prefix to ISP3, but
> situation is the same.
>
> Is it OK for ISP2 (my LIR) to announce and prepend my prefix? I thought
> I should be in control of that.
>
> Is there anything I can do about the situation?

You can't do much to control incoming traffic though you can sometimes
influence it. But you do control which routes you accept/prefer. If you
want to avoid the assymetric path, you need to prefer ISP2's announcwments
for SITEB, for example you could match and give it a higher localpref.

Is it causing a problem though? This is completely normal and expected
on the internet.

> Thank you in advance,
>