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

[Kevin.X.White at corusgroup.com]

Used to happen to me all the time!
 We call it the Switch in the middle problem! (although the actual fault in
our case usually involves an inground fibre and an mechanical digger)
 Design it out by not having sprawing L2 nets
 If all your long, damage prone links are in their own subnets you can
leave it all up to your IGP

 Kevin

Enno Rey wrote:
>
>> Now one of the routers gets separated from the network, while it's
>> physical interface remains in "up" state, which is easily possible
>> especially with ethernet. Now the routers can not see each other, so
>> both of them became DR, and start to announce the network. The problem
>> is with the announcement of the separated router, which could
>> potentially create a black-hole in the network.
>>
>
> hmm... maybe I do not get your scenario here... but if that router "gets
separated" (whatever action you refer to here in detail, especially with
its interface staying "up")... how can it still announce anything to the
network it just got separated from?
>
> What kind of "separation" do you mean?
>
>
>
I try to describe it with a simplistic scenario:
you have a L2 network made up of two ethernet switches(SWITCH A and
SWITCH B). You have hosts, servers, etc. and OSPF ROUTER A connected
directly to SWITCH A, and you have OSPF ROUTER B connected to SWITCH B.
You have SWITCH A and SWITCH B connected with a link. All L3 interfaces
connected to this L2 broadcast domain share the same prefix: NET X.
ROUTER A & ROUTER B have their own uplinks to the backbone, making NET
X  reachability redundant. Now everything works as expected.

At some point in time the link between SWITCH A and SWITCH B breaks
down.  Now you have ROUTER B(and SWITCH B of course) separated from the
other parts(hosts, servers, ROUTER A) of NET X. Since ROUTER B still has
it's interface "up", and it is configured with an IP address from NET X,
it continues to announce NET X. If this announcement is more
attractive(in terms of OSPF metrics) than the similar announcement made
by ROUTER A, then the traffic destined to NET X will flow to ROUTER B,
and become blackholed.

I hope I made the issue clearer. ;)

regards,
Gabor
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