OSPF unexpectedly adds /128 interface route

[Ondrej Zajicek]

On Mon, May 11, 2026 at 12:48:48PM +0100, Lexi Winter wrote:
> Ondrej Zajicek wrote in <agG8FCXbKhBjqsvg at feanor>:
> > On Sun, May 10, 2026 at 01:42:07PM +0100, Lexi Winter wrote:
> > > i have an interface with a /64 address configured:
> > >[...]
> > > however, BIRD only adds a /128 route into the OSPF database
> > 
> > There are two reasons why it could generate /128 route instead of /64.
> > Either BIRD thinks there is link down in the interface, or it is
> > confused by its type and handles the interface as PtMP.
> 
> thanks.  setting 'check link no' fixed the problem, but i'm not sure
> i understand why.
> 
> on the host, the interface is up:
> 
> 	bridge0.500: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
> 
> but BIRD considers it down (LinkDown):
> 
> 	bridge0.500 up (index=28)
> 		MultiAccess Broadcast Multicast AdminUp LinkDown MTU=1500
> 		fe80::1/64 (Preferred, scope link)
> 		fd13:480d:2ffa:3::1/64 (Preferred, scope site)
> 
> looking at another interface that BIRD considers LinkUp:
> 
> 	bridge0 up (index=27)
> 		MultiAccess Broadcast Multicast AdminUp LinkUp MTU=9000
> 
> i see it has the LOWER_UP flag in ifconfig:
> 
> 	bridge0: flags=1008843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST,LOWER_UP> metric 0 mtu 9000
> 
> so, it seems like the problem is that the host system isn't setting this
> flag on the vlan interface, even though it probably should be set since
> the underlying interface is up.
> 
> does that seem like a reasonable explanation for this behaviour?  if so,
> i will go away and see if we can fix the host :-)

Yes, that looks like the right explanation. Not sure if it is a missing
implementation in kernel or a bad assumption on BIRD part.

-- 
Elen sila lumenn' omentielvo

Ondrej 'Santiago' Zajicek (email: santiago at crfreenet.org)
"To err is human -- to blame it on a computer is even more so."