Subject: Re: Message Flow after Link Failure

From: "Manral, Vishwas" <[email-masked]>
Date: Tue, 10 Sep 2002 03:08:13 -0400

Hi Christian,

I will add to what Acee has already said. I will correct a few things you
have stated.

a) After the RouterDeadInterval when an adjacency to a neighbor is broken
the statement that

"All OSPF information concerning the other router is removed from the
database of
Router A and Router B respectively and both Routers are recalculating their
routing tables."

is wrong, we do not remove the other routers information(I assue u meant
LSA's) at all. We just update our Router LSA to state the neighbor is no
more reachable.

b) The neighbor list in a hello consists of neighbor on that particular
interface not all neighbors on all interfaces.

Or are you talking about all routers A...H on one LAN? If thats the case, if
the LAN becomes partitioned, each partition will have its own DR, and will
work accordingly.

Also check the SPF section Section 16 in RFC2328.

Thanks,
Vishwas

-----Original Message-----
From: Christian Glomb [mailto:[email-masked]]
Sent: Monday, September 09, 2002 7:42 PM
To: [email-masked]
Subject: Message Flow after Link Failure

Dear all,

let's assume we are monitoring the flow of OSPF messages within one OSPF
area. All routers have successfully come up and are running stable, i.e.
only HELLO messages are exchanged between adjacent routers. Now the link
between Router A and Router B fails such that Router A does not receive
HELLOs from Router B and vice versa. After the timer RouterDeadInterval
has expired at each Router, Router A as well as Router B assumes that the
other router (or the link to the other router) has crashed. All OSPF
information concerning the other router is removed from the database of
Router A and Router B respectively and both Routers are recalculating their
routing tables.

However Router A as well as Router B have working adjacencies to other
Routers C...E and F...H respectively and are still sending HELLOs to these
neighbors.

What happens then?

1.
Due to the fact that the ID of Router B is missing in the neighbor list
of the HELLOs send from Router A to Routers C...E and the ID of Router A
is missing in the HELLO neighbor list sent from Router B to Router F...H,
Routers C...E as well as Routers F...H are sending a LINK_STATE_REQUEST to
Router A and Router B respectively, since they notice that the topology has
changed (i.e. one link is missing).
Routers A and B resp. are responding with a LINK_STATE_UPDATE. Then the
Flooding Procedure starts due to the receipt of such a message at Routers
C...E and F...H.

OR

2.
Routers A and B resp. send a LINK_STATE_UPDATE directly to Routers C...E
and Router F...H after having updated their routing tables to trigger the
Flooding Process.

OR

3. I'm completely wrong, it's working completely different.

Thank you for your comments,

Christian.