Problems configuring multihop BGP setup

[Ondrej Zajicek]

On Thu, Jul 22, 2021 at 10:04:29PM +0200, Rainer Kulow wrote:
> Hello,
> 
> Thank you very much for your response.
> Actually, this is just an extracted example from a larger setup to show the 
> problem in a simpler way. Therefore, the use of BGP seems perhaps a bit 
> superfluous here.
> 
> In fact, you were absolutely right with your guess.
> everything works as desired.

Hello

Yes, there is the option 'gateway', which controls how immediate next
hops are computed from bgp_next_hop attributes. It uses different
defaults for direct or multihop sessions:

https://bird.network.cz/?get_doc&v=20&f=bird-6.html#bgp-gateway

For multihop, recursive mode is used and bpg_next_hop is resolved through
local / IGP routes, so you need such routes in the routing table (e.g. from
Direct or OSPF protocols).

> > > >If I delete "multihop" and "next hop self" from the template, I get
> > > >all routes, but can only ever ping the closest AS.

IMHO it should work without 'multihop' and 'next hop self' and that would
be the natural approach. My first guess why you can ping only the closest
AS is that you do not propagate /30 connecting networks, just /16
networks, and trying ping from the router is uses src IP from the /30
connecting range (unless explicitly forced to use different one using -I
option), so the destination router cannot answer ping back because it is
missing the non-local /30 route.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, wwwkeys.pgp.net)
"To err is human -- to blame it on a computer is even more so."