On Sat, Nov 13, 2021 at 12:11:08AM +0000, Stuart Henderson wrote:
> I have a pair of -current routers running bgpd (let's call them rtr-a
> and rtr-b) on a subnet which also has some vpn gateways and firewalls.
>
> These routers provide a carp address which the vpn gateways are using
> as default route. There are some networks behind the vpn gateways (a
> /32 to accept incoming vpn connections and some other prefixes that vpn
> clients are numbered from).
>
> rtr-a and rtr-b have static routes to those networks, and they have
> network statements in bgpd.conf to announce them to their ibgp peers
> ("network 172.24.232.0/21 set nexthop XXX" etc) so the paths are reachable
> from the rest of the network. (This is replacing an existing setup using
> ospf, trying to remove routing protocols from machines that don't really
> need them).
>
> It is working but something seems a little odd - the paths are announced
> from both routers briefly and show up on the rest of the network from
> both rtr-a and rtr-b. But after a few seconds, rtr-b receives these
> paths from rtr-a, and then rtr-b stops announcing them itself. (they
> stop showing in "bgpctl sh rib out" on rtr-b; "bgpctl sh nex" does
> correctly identify the associated nexthops as connected/UP).
>
> Is this expected/correct behaviour?
>
> I'd prefer to have them announced from both rtr-a and rtr-b, so there's
> no blackhole period if rtr-a is restarted while rtr-b figures out that
> it should start announcing them, etc. (No need for tracking carp state
> in this case, I'm not using stateful pf rules on the traffic involved).
>
> If rtr-b stops seeing the prefixes from rtr-a (either by taking down
> the ibgp session, or by filtering) I see the announcements from both
> rtr-a and rtr-b again. So the obvious workaround is to filter, but
> I thought I'd ask first in case it's something that is better handled
> by code changes rather than config.
>
This reminds me of the knob "bgp best external" on some routers.
Juniper:
https://www.juniper.net/documentation/us/en/software/junos/bgp/topics/ref/sta
+tement/advertise-external-edit-protocols-bgp.html
Cisco:
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/x
+e-17/irg-xe-17-book/bgp-best-external.html
The difference is that in your case it's an internal route ....
Remi
