Re: DROther/BDR graceful restart question

[Ashok Chandrashekar Holla <ashok_ch at HUAWEI.COM>]

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
>
>