Vishwas-

I don't think your statement is true -- seems to me that
the only routers able to detect the change may be downstream
(that is, further along towards D) of R.

John

> -----Original Message-----
> From: Manral, Vishwas [mailto:VishwasMNETPLANE.COM]
> Sent: Thursday, March 07, 2002 12:55 AM
> To: OSPFDISCUSS.MICROSOFT.COM
> Subject: Re: Explained: Regarding
> draft-ietf-ospf-hitless-restart-01.txt
>
>
> John,
>
> I have further narrowed down the case where we need to exit
> hitless restart.
> My new statement is :-
>
> Only when the new nexthop is thru R(because of some new LSA) to a
> destination D, do we actually break off from hitless restart.
>
> Proof: If a path does not traverse thru R no loop can occur.
> This is from
> the simple fact that in that case only the router R has an
> askew routing
> table, so if the path does not traverse thru it, it will not
> result in a
> routing loop.
>
> I guess this considerably reduces the case where we need to
> exit hitless
> restart on any LSA change.
>
> Thanks,
> Vishwas
>
> -----Original Message-----
> From: Moy, John [mailto:John.MoySYCAMORENET.COM]
> Sent: Tuesday, March 05, 2002 11:33 PM
> To: OSPFDISCUSS.MICROSOFT.COM
> Subject: Re: Explained: Regarding
> draft-ietf-ospf-hitless-restart-01.txt
>
>
> Vishwas-
>
> I *think* that you are correct, although I wouldn't
> call your message a proof.
>
> John
>
> > -----Original Message-----
> > From: Manral, Vishwas [mailto:VishwasMNETPLANE.COM]
> > Sent: Friday, March 01, 2002 12:42 PM
> > To: OSPFDISCUSS.MICROSOFT.COM
> > Subject: Re: Explained: Regarding
> > draft-ietf-ospf-hitless-restart-01.txt
> >
> >
> > John,
> >
> > I know, I am replying to a thread that is about 4 months
> > old(a new version
> > of draft is out, however the issue still holds). But this
> > thing has been
> > nagging me for some while and after pursuing thingsand with
> > the help of Aman
> > Shaikh(and a few other at Research at AT&T), I have come up with the
> > following proof.
> >
> > From the previous thread ===>>
> >
> ======================================================================
> > Vishwas>
> > > 6. We talk abt no changes to the contents of the LS
> > > Database(1-5, 7) for a
> > > router to get out of "helper mode" . Why dont we instead have
> > > a check some
> > > thing like a change in the routing table, with the particular
> > > router as the
> > > next hop. In the sense, if any router in helper mode has its
> > > routes changed,
> > > should we get out of "helper mode". Only when the path
> > > traverses thru the
> > > "restarting router" changes, do we need to actually
> update the path.
> >
> > John> I don't think that this is good enough to stop all
> > loops. The helper's
> > routing table may not change, but a larger loop may have
> > formed. It *would*
> > be OK to see whether the *restarting router's* routing table
> > has changed,
> > but
> > that's asking the helper to do quite a lot of extra work.
> > ==============================================================
> > ==========
> >
> > To explain further, if we assume there is no change of
> > cost/link-up/down on
> > any of the helpers, then a change in route in the Helper R for any
> > destination D would cause a change in atleast one
> helper(assuming all
> > routers are in the same area(maybe we can extrapolate to
> > non-ABR's too), if
> > restarting router is not the ABR).
> >
> > To explain it :-
> > =============
> >
> > If we assume that nothing incident on R changed, then I dont
> > think u can
> > find a
> > case where R's path to D changes but none of the helper's
> > path to D changes.
> > At
> > least one helper will see a change in path cost if nothing
> > else. I think
> > this
> > can be proved if u consider that if R's path to D changes and
> > none of its
> > incident links have changed, then something must have
> changed "below a
> > helper"
> > in R's SPT. In that case, that helper should see some change
> > in its path to
> > D as
> > well.
> >
> > Here's an example where R's path to D changes, none its ngb's
> > path to D
> > changes.
> > Nonetheless one ngb sees a change in the path length
> >
> > Suppose graph is like this:
> >
> > link (cost: assumed to be same in both the directions)
> > R<->A (1)
> > R<->B (1)
> > A<->A' (5)
> > B<->B' (4)
> > A'<->D (1)
> > B'<->D (1)
> >
> > Then every router's path to D would look like this:
> >
> > R->B->B'->D (6)
> > A->A'->D (6)
> > B->B'->D (5)
> > A'->D (1)
> > B'->D (1)
> >
> > Suppose cost of B<->B' changes to 6 from 4.
> >
> > Then every router's path to D would be like this:
> >
> > R->A->A'->D (7)
> > A->A'->D (6)
> > B->B'->D (7) <= Change in path cost.
> > A'->D (1)
> > B'->D (1)
> >
> > So, what I mean to say is, that we need not exit hitless
> > restart unless
> > either a link on the helper either goes down/up or we change
> > cost on it, or
> > a route in the helper changes(both the above amount to this too).
> >
> > Please correct me if I am wrong.
> >
> > Thanks,
> > Vishwas
> >
>

• Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]