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Re: Reestablish adjacencies after Graceful-Restart

Hi Debopam,

Debopam Bandyopadhyay wrote:

Thanks a lot for the help, Acee. It has cleared a major chunk of my design
requirements.
Finally, I have one question related to a hypothetical case (which may be
required for interoperability).

I will go back to the previous example:

           X ----- lan ----- Y

In the topology, we have two routers, X and Y; and Y happens to be the DR in
the network segment.

Now Router X restarts. After 'T' seconds, Router X becomes fully adjacent
with Router Y. 'T is very much less than X's Grace Period.
Let's say, X is unable to trace its pre-restart Router-LSA in its database
(because Y had not sent it during the DD-exchange). So my question is:
1> Would X now exit Graceful-restart at this point, or
2> Would X wait for its pre-restart Router-LSA to be received from Y (in the
form of LS-Updates), until grace period expires; or
3> Would it be implementation dependent?


While not every scenario is not explicitly documented, not having a pre-restart LSAs and
a full neighbor would be fall into the category of an inconsistent LSA. Hence, you should
terminate graceful restart as specified by 2) in section 2.2 of RFC 3623. Keep in mind,
that a full adjacency implies a complete database exchange.

Hope this helps,
Acee


Thanks,
Debopam

----- Original Message ----- From: "Acee Lindem" <acee at CISCO.COM>
To: <OSPF at PEACH.EASE.LSOFT.COM>
Sent: Monday, May 09, 2005 5:50 PM
Subject: Re: Reestablish adjacencies after Graceful-Restart


Debopam Bandyopadhyay wrote:



This question is a bit long, backdated and maybe, queer. But it is a bit


critical for me.


It is regarding RFC 3623 (Graceful OSPF Restart), Section 2.2. "When to


Exit Graceful Restart", Point 1 "Router X has reestablished all its
adjacencies".


I am trying to identify the corresponding event from RFC 2328 for Router X.


Is it one of the following:


a> Upon installation of an LSA in the database [RFC2328, Sec 13.2] [and


thereby tracing the database for all the relevant LSAs which indicate the
previous adjacencies],


or
b> State of neighbor Y changing to Full; [and thereby tracing the database


...]


   or
c> Implementation specific

Let me explain this with a simplified example:

          X ----- lan ----- Y

In the topology, we have two routers, X and Y; and Y happens to be the DR


in the network segment.


Now Router X restarts. After 'T' seconds, Router X becomes fully adjacent


with Router Y.


At this point, three scenarios are possible:
1> X had received all the relevant LSAs (i.e. router-LSAs of X, Y and


Network-LSA of Y) BEFORE becoming fully adjacent with Y.


Hence, the graceful-restart exit event has already been fired BEFORE X


became fully adjacent with Y.


2> X had received all the relevant LSAs BEFORE becoming fully adjacent with


Y;


but graceful-restart exit event gets fired EXACTLY when X became fully


adjacent with Y.


3> X had completed receiving all the relevant LSAs AFTER becoming fully


adjacent with Y;


Hence, the graceful-restart exit event gets fired sometime AFTER X


became fully adjacent with Y.




Hi Depopam,

As stated in RFC 3623 section 2.2, a restarting router uses its
pre-restart router
LSA to determine whether or not it has re-established all its
pre-restart adjacencies.
Adjacency formation implies database synchronization and reception of
all LSAs within
the flooding domain of the router with whom the adjacency is being
formed. Hence,
#2 above is the only scenario which makes sense.

Hope this helps,
Acee




Thanks,
Debopam