bird 2.0.4 autodetects /30 as ptp

[Ondrej Zajicek]

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.

-- 
Elen sila lumenn' omentielvo

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."