[c-nsp] OSPF router gets separated from a broadcast domain

[Peter Rathlev]

Hi Gabor,

Yes, I see now, like Christopher describes. If you have a split to A can
see C, C can see A and B and B can see C, and it's on a broadcast media
like Ethernet, with a single DR, then you have the problem.

If would be nice if you could use specific "neighbor" statements and
something like BGPs "next-hop-self" or similar for OSPF.

But as Christopher says, it would be a wicked setup for this to happen,
not that it doesn't happen though.

Would it maybe be possible to use three different subnets in the same
broadcast domain, one for A-C, one for B-C and one for C-B, or does the
OSPF multicast ruin this? I know this doesn't scale very well -- for N+1
routers you'd need N*(N-1)/2 connections. :-|

Regards,
Peter

On Mon, 2008-02-04 at 14:05 +0100, Gabor Ivanszky wrote:
> Peter Rathlev wrote:
> >  When
> > the Dead Interval expires, it will think of the neighbor as down and
> > invalidate all routes learned from it. Only the still connected network
> > is left and announced, but since there are no other OSPF routers on that
> > segment (seen from each of the two) no paths are learned through this
> > segment.
> >
> >
> I thought just the same before we get burnt by this issue. So I am 
> afraid this doesn't work like this(but I am far from to be sure...)
> 
> Router A
> - has "customer" network x.x.x.0 as connected
> - connected to transit network t.t.t.0 with address t.t.t.a
> - loopback: a.a.a.a
> 
> Router B
> - connected to backbone
> - connected to transit network t.t.t.0 with address t.t.t.b
>           assume that we have a connection problem here, so t.t.t.b is 
> up, but cannot reach t.t.t.c and t.t.t.a
> - loopback: b.b.b.b
> 
> Router C
> - connected to backbone
> - connected to transit network t.t.t.0 with address t.t.t.c
> - loopback: c.c.c.c
> 
> 
> Now Router B receives a packet with a destination address x.x.x.x. It 
> makes the routing decision based on it's LSDB, which will be something 
> like this:
> 1. x.x.x.0 is connected to router a.a.a.a
> 2. router a.a.a.a has an interface in network t.t.t.0, namely t.t.t.a
> 3. I (Router B) have also an interface in t.t.t.0: "Hurray, we have a 
> path!"; BTW.   I (Router B) know, that Router C also has an interface in 
> t.t.t.0, so if I (Router B) have my t.t.t.b interface down, I would 
> route toward c.c.c.c. But "luckily", this is not the case this time.
> 4. Router B starts to ARP t.t.t.a without any success and drops the packet.
> 
> The routing decision on all routers will be similar in the same OSPF area.
> 
> 
> 
> 
> I don't know whether it happens like I described above, but I am keen to 
> get to know it.
> 
> 
> cheers,
> Gabor