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. > -- > Elen sila lumenn' omentielvo > > Ondrej 'Santiago' Zajicek (email: [email protected]) > OpenPGP encrypted e-mails preferred (KeyID 0x11DEADC3, > wwwkeys.pgp.net) > "To err is human -- to blame it on a computer is even more so."
