Subject: Re: DROther/BDR graceful restart question

From: Nitin Kakkar <[email-masked]>
Date: Fri, 10 Feb 2006 11:57:01 +0530

Ashok> For the second part:
When a BDR undergoes GR, it will almost certainly lose its BDR status
since BDR status is not maintained across GRs. So, when it comes up, it
will run fresh election and the behavior is as per election algorithm
(it may become a DROther).

>> Doesn't this defy the whole purpose of GR?
Nitin

-----Original Message-----
From: Mailing List [mailto:[email-masked]] On Behalf Of Ashok
Chandrashekar Holla
Sent: Friday, February 10, 2006 11:21 AM
To: [email-masked]
Subject: Re: DROther/BDR graceful restart question

Hi Ramesh,

For the first part:
When a DROther comes up after restart, there is no need for it to 
'remember' that it was DROther before restart. The restarting router 
learns the DR and BDR  using the usual hello mechanism. This behavior is

no different from a non graceful restart.
When it runs the election algorithm, it will determine that it is 
currently DROther. Once this is done, it will complete forming 
adjacencies with DR and BDR of the network and then exit GR.

 From the perspective of learning the pre-restart topology, there is no 
need for a DROther to wait until it completes adj formation with both DR

and BDR. However it is inevitable that this be done, since a helper 
router (DR and BDR) in some scenarios, will declare a 'Peer down' in 
their LSA if they see a flushed grace lsa before adjacency formation is 
complete (basic protocol behavior).

For the second part:
When a BDR undergoes GR, it will almost certainly lose its BDR status 
since BDR status is not maintained across GRs. So, when it comes up, it 
will run fresh election and the behavior is as per election algorithm 
(it may become a DROther).

Hope it helps.
- Ashok

Ramesh Kandula wrote:

>Hi All,
>
>Suppose a DROther router restarts gracefully on a broadcast network. 
>Before restart it has adjacencies with DR and BDR. According to clause
in
> 
>2.2.1 (RFC 3623), it is supposed to wait till it forms adjacencies with

>
>BDR and DR before exiting restart mode.
>
>However there is no way for it to determine who the BDR was before
restar
>t 
>using the pre-restart router lsa (and/or pre-restart network lsa). Or
am 
>I 
>missing something? Actually why is it necessary to wait (i.e for
exiting 
>
>restart mode) till it forms an adjaceny with BDR? Isn't it enough for
it 
>
>wait till it forms an adjacency with DR?
>
>A similar issue arises with BDR restart as well. According to the same 
>
>clause it is supposed to wait till it forms adjacencies with all the 
>routers on the network. However it has no way of determining that it
was 
>
>the BDR using just the pre-restart lsas.
>
>Thanks,
>Ramesh
>
>  
>

---

From: Ashok Chandrashekar Holla <[email-masked]>
Date: Fri, 10 Feb 2006 12:09:51 +0530

Hi Nitin,
Although a modification was suggested in  
http://www.ietf.org/internet-drafts/draft-holla-ospf-update-graceful-restart-00.txt 
, after the discussions in this forum, I now feel that the existing 
mechanism need not be changed :).
Retaining BDR status is not necessary and provides little value addition 
(only the avoidance of the linear time bound election algorithm overhead).
- Ashok