Subject: Question about hitless restart with virtual links.

From: Philip Crocker <[email-masked]>
Date: Thu, 16 May 2002 09:40:22 +0100

Hello,

I have a comment on the OSPF Hitless Restart draft
(draft-ietf-ospf-hitless-restart-02.txt).

I don't think the draft details how to ensure an unplanned hitless restart
if a router has active virtual links.

- In the unplanned restart case, grace LSAs are sent out to attached
interfaces as soon as a router restarts. However, at this point, the router
does not know the IP addresses of the ends of any virtual links it has
configured. These are only discovered when a routing calculation calculates
the end of a virtual link as reachable.

- If a HELLO packet is sent to the virtual neighbor at this point, the
virtual neighbor will reset the virtual adjacency and cause the router's
hitless restart to fail, disrupting routing through the virtual link.

I think there is a pretty simple solution to this problem, which is to make
sure that, if we are attempting an unplanned restart, then whenever a
virtual link is calculated as reachable, we send the neighbor a grace LSA
before any HELLO packet. This should cause the virtual neighbor to enter
helper mode and so not disrupt routing through the virtual link.

This may well be the intention of the draft, but I think that, if this is
the correct behaviour, it should be mentioned specifically in the draft.

Thanks,

Phil.

-------
Phil Crocker
DC-OSPF Development Team
Tel: +44 20 8366 1177
Fax: +44 20 8363 1039
Data Connection Ltd
Email: [email-masked]
Web: http://www.dataconnection.com