Subject: DROther/BDR graceful restart question

From: Ramesh Kandula <[email-masked]>
Date: Fri, 10 Feb 2006 00:30:25 -0500

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

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From: Ashok Chandrashekar Holla <[email-masked]>
Date: Fri, 10 Feb 2006 11:21:29 +0530

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

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From: Acee Lindem <[email-masked]>
Date: Fri, 10 Feb 2006 11:38:35 -0500

Hi Ramesh,

>wait till it forms an adjacency with DR?
You are correct. Only the DR adjacency needs to be reformed. We'll 
clarify if we ever respin
this RFC. The key point is that your pre-restart LSAs must match your 
current state  prior  to
existing graceful restart normally.

Thanks,
Acee