Subject: Message Flow after Link Failure

From: Christian Glomb <[email-masked]>
Date: Mon, 9 Sep 2002 10:12:10 -0400

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.

---

From: Acee Lindem <[email-masked]>
Date: Mon, 9 Sep 2002 16:21:24 -0400

Christian,
It is closer to #2 than #1. Assuming a single area topology,
it would be the neighbor inactivity event that triggers
re-origination of router LSAs for A and B. Refer
to section 12.4 in RFC 2328.

--
Acee