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

[Gabor Ivanszky]

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