bird 2.0.4 autodetects /30 as ptp

[Kenth Eriksson]

On Tue, 2019-03-26 at 09:00 +0100, Cybertinus wrote:
> On 2019-03-26 08:49, Kenth Eriksson wrote:
> > On Mon, 2019-03-25 at 21:09 +0100, Ondrej Zajicek wrote:
> > > CAUTION: This email originated from outside of the organization.
> > > Do
> > > not click links or open attachments unless you recognize the
> > > sender
> > > and know the content is safe.
> > > 
> > > 
> > > 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.
> 
> If you are willing to change the default config with a config
> setting, 
> why not just configure the OSPF interface as ptp or broadcast, as 
> needed? :). Not trying to be a smart-ass here, but just wondering
> what 
> would be the added value of such a config setting?
> 

I see a great value of having a working autodetect of the OSPF
interface type. That gives me the possibility to change my interface
address without the need to touch my bird OSPF configuration.

> > > --
> > > 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."
> 
> Kind regards,
> Cybertinus