On 2022-06-27, Ivo Chutkin <openbsd@bgone.net> wrote:
> Hello guys,
> It is not related to OpenBSD. Since I started my admin "career" with
> OpenBGPD and OpenBSD, I just need some thoughts and advises from anyone
> more experienced.
>
> The situation is as follows:
> I have 2 border routers in main location. All Upstreams,IX-es and
> clients have eBGP sessions. Clients are mostly small regional ISPs.
> We carry customers traffic from main location to their region over L2
> vlans. On all regional POPs, I have L3 switches (Brocade ICX6650).
>
> The idea I have is to make eBGP session with regional ISPs on their
> local POP switch and distribute their prefixes to other ISPs connected
> there. To make some kind of Internet Exchange on regional level or even
> national level for our customers.
>
> As far as I know, all routers (BGP running switches) in a single AS,
> should be connected via iBGP (If I am not mistaken, it is called full
> mesh). But, on main routers, I have number of full feeds that regional
> switches are not capable to handle.
>
> Do you think it could be done somehow without iBGP full mesh or it is
> stupid idea by design?
You do want either full mesh or to use route reflectors, but you don't
need to send a full BGP table to all the routers, you can filter in
various ways (for example, maybe just carry national routes, or peer
routes, or routes with a short AS path length, or...there are lots of
options). But you do need to make sure that other destinations are
still reachable from routers receiving a partial table so in that
case you will want to generate/announce a default route; probably
arrange your filters so that the default route is only sent/accepted
on those routers with a partial table - for the full table routers
you'll usually want _no_ default route so that packets for nonexistent
destinations get "net unreachable" rather than maybe flip-flopping
between a couple of routers which point default to each other.
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