bird 2.0.4 autodetects /30 as ptp

[Kenth Eriksson]

On Mon, 2019-03-25 at 21:09 +0100, Ondrej Zajicek wrote:
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> On Mon, Mar 25, 2019 at 03:06:29PM +0000, Kenth Eriksson wrote:
> > It seems as bird autodetects a /30 OSPF interface as ptp.
> > 
> > bird> show ospf interface
> > ospfv2_1:
> > Interface eth0 (10.210.138.32/30)
> >         Type: ptp
> >         Area: 0.0.0.0 (0)
> >         State: PtP
> >         Priority: 0
> >         Cost: 10
> >         Hello timer: 10
> >         Wait timer: 40
> >         Dead timer: 40
> >         Retransmit timer: 5
> > bird>
> > 
> > Is this really correct behaviour? The peering Cisco router thinks
> > the
> > /30 network is a broadcast interface...
> 
> Well, OSPF RFC speaks about broadcast and ptp networks/interfaces,
> but it
> is not really explicit about which networks are 'broadcast' and which
> are
> 'ptp'. One could think about medium types (ethernet vs. serial link),
> physical topologies (star/bus vs. ptp link) or addressing (unnumbered
> or 'ptp' addresses vs /30 vs wider prefix).
> 
> BIRD uses the third approach and autodetect unnumbered / ptp / /30 as
> ptp, while wider prefix as broadcast (or PtMP / NBMA if the interface
> does not support multicast). That is because in such cases it is not
> possible to have more than two nodes in the same L3 network, so it is
> not
> necessary to use the more complicated and error prone broadcast mode.
> 
> There is RFC 5309 that discusses this issue, which implies that was
> perhaps meant more in the first sense (medium types).
> 
> It is possible that other implementations use other heuristics, which
> causes compatibility issues in default setting. Perhaps we could
> change
> it (in some major release) if there is a clear consensus about the
> proper
> behavior.
> 

I can see the rationale of saying that a /30 network can run with OSPF
interfaces in ptp mode since there are only 2 hosts on it. That said, I
*believe* the Cisco routers defaults to broadcast mode for a /30 which
would cause some interop issues with the default settings. I don't know
the default values for other vendors. 

Is it possible to change the autodetect using a function in the bird
configuration file? E.g. if the prefix length of the interface is 31 or
32 then use ptp mode else use broadcast mode.

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