Re: Graceful Restart

[Acee Lindem <[e-mail]>]

Hi Krishna,

Krishna Rao wrote:
> Hi,
> 1) Since the aim is to keep the restarting router
>    in the network, so during graceful restart:
>
>    If router analyses its own router/network lsa (if
>    it was DR) and does necessary operation, that should
>    be enough.
>    Why to again elect itself as DR ?
>
> 2) So suppose, if it doesn't elect itself as DR and
>    new DR(some other router) is elected.
>    But if network topology hasn't changed, only change
>    might be in terms of new DR/BDR and new adjacencies
>    with those DR/BDR.
>    But that does not in anyway change network topology in
>    terms of reachability of routes.
>    So graceful restart should not be aborted !!!!
>
>    Though Network Lsa will change.
>    But how does it makes "Router LSA" of restarting
>    router inconsistent, in terms of reachability
>    of "helpers", to restarting router???
>
> 3)Finally when it exits the graceful restart:
>   It originates router Lsas, calculates SPF etc.
>
> Or i am missing some point ???

With the current approach, the restarting router doesn't
re-originate any LSAs until it exits graceful restart. Rather,
the pre-restart LSAs are used. Hence, a change in DR will result
in a topology and an inconsistency with the pre-restart LSA.

Thanks,
Acee

>
> krishna
>
>
>
> On Fri, 04 Apr 2003 Acee Lindem wrote :
>
>> John, Krishna,
>>
>> John has it pretty much right but there is one additional key
>> point. If the restarting router doesn't succeed its role as
>> DR on interfaces where it was previously DR the specification
>> is broken. The restarting router uses his own pre-restart
>> router and network LSAs to determine when graceful restart
>> has converged. A DR election would change the topology and
>> the restarting router would detect an LSA inconsistency and
>> exit graceful restart prematurely.
>>
>> Thanks,
>> Acee
>>
>> John Smith wrote:
>>
>>> Krishna,
>>> my 2 euros (dollars are out !!!)
>>>
>>>
>>>> draft-ietf-ospf-hitless-restart-07.txt:
>>>> 2.  Operation of restarting router:
>>>>
>>>> Point 2) If the restarting router determines that it was
>>>> Designated
>>>>          Router on a given segment immediately prior to the
>>>> restart,
>>>>          it elects itself as Designated Router again
>>>>
>>>> Q1)Why does this need to be done while restarting?
>>>>    AND
>>>>    How does it help ??
>>>
>>>
>>>
>>> Its helps in maintaining sanity in the network. Others think
>>> that this router is still up
>>> and continue trusting him as DR. When he comes up he should not
>>> break this trust and
>>> should ideally start operating as a DR.
>>>
>>> The basic concept is that if he resigns from DRship then the BDR
>>> will take over and they
>>> will need to again re-elect the BDR .. which is a messy affair.
>>> The whole point of
>>> graceful restart is to avoid such events !!
>>>
>>>
>>>> Q2)What if normal DR/BDR election takes place "on exiting
>>>>    graceful restart"
>>>
>>>
>>>
>>> Why do you want to land in this mess? You will again need to
>>> re-issue your LSAs, etc.
>>> Everybody will have to re-synch .. and the whole purpose of
>>> graceful restart will be
>>> defeated.
>>>
>>> Others more informed about this protocol can correct me whatever
>>> i missed out!
>>>
>>> JS
>>>
>>>
>>>
>>> __________________________________________________
>>> Yahoo! Plus
>>> For a better Internet experience
>>> http://www.yahoo.co.uk/btoffer
>>>
>>
>>
>> --
>> Acee
>
>
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--
Acee