From: David Buechner (dbuechn@attglobal.net)
Date: Sun Jul 11 2004 - 12:23:42 GMT-3
Tim,
Actually, I have to disagree with you on this. OSPF demand circuit and
dialer watch serve two different purposes.
OSPF demand circuit changes how OSPF handles transactions over a particular
connection. It does two things. First, periodic hellos are not necessary
to maintain an adjacency over a demand circuit. Second, the periodic
refresh of LSAs is suppressed. To support this the LSAs learned over a
demand circuit are marked "Do Not Age". With demand circuit configured the
*only* OSPF traffic generated on a link is new/changed LSAs being flooded
in response to a topology change. Note however, that this flooding does
not necessarily activate the link! What this flooding will do is use the
link if it is active.
Dialer watch is one of the methods that can be used to activate a
link. Other possibilities include defining interesting traffic, defining
your ISDN interface as a backup interface, or receiving a call from another
router. Dialer watch is not an OSPF specific technology, but will work
with routes learned from any source. In particular what it does is watch
for a route to be removed from the main routing table. When this occurs
dialer watch activates the interface on which dialer watch is configured.
Does this help? Holler if you have questions!
David Buechner, CCIE #13539
At 07:22 AM 7/11/2004, ccie2be wrote:
>Hi Akbar,
>
>I have thought about the same thing not so long ago.
>
>Here's how I understand this.
>
>In many respects, these 2 commands are functionally redundant - in other
>words, they accomplish the same thing but use different methods to do so.
>
>That said, I can imagine there are situations where you have to use dialer
>watch instead of ospf demand circuit. One possible situation is where the
>watched route isn't an ospf route. I've never actually seen this situation,
>but here's my thinking.
>
>ospf demand circuit will trigger a call only when there's a change in the
>ospf topology. But, suppose dialer watch is configured to watch a route
>that isn't an ospf route or a route redist into ospf. In this case, the
>ospf topology might not change so with ospf demand circuit configured, a
>call is NOT triggered. But, with dialer watch configured, a call is
>triggered if the watched route disappears from the route table.
>
>Again, let me emphasize that I've never actually seen a network where dialer
>watch would work but ospf demand circuit wouldn't work, but I can imagine
>Cisco could design a lab in which this could be the case.
>
>However, if the network is a pure ospf network, then I believe either method
>would work and there would be no reason to configure both. But, if you were
>configuring dialer watch to trigger the isdn circuit, then you would need to
>make sure ospf traffic was defined to not be interesting traffic because
>otherwise the ospf hello's would keep bringing up the isdn circuit.
>
>I would certainly be interested in knowing if others have a different
>opinion.
>
>HTH, Tim
>
>
>----- Original Message -----
>From: "akbar khan" <ciscokhan@hotmail.com>
>To: <ccielab@groupstudy.com>; <security@groupstudy.com>
>Sent: Saturday, July 10, 2004 1:52 PM
>Subject: OSPF Demand Circuit with Watch List..
>
>
> > Hello Folks, Is it reasonable to use dialer watch-list with Demand
> > Circuit. Ex: R1---ISDN---R2 & R1----FR----R3-----FR----R2 (Hub &
> > Spoke) running OSPF on all the links. I have configured R1 BRI for Demand
> > circuit to fire for any ospf topology change. In addition I am watching
> > the Loopback of R2 with watch list. My question is that if there is a
> > toplogy change somewhere in the ospf domain except the loopback of R2
> > does my R1 will trigger the call with the stipulaiton that it is already
> > configured with watch-list.. I mean can my BRI will trigger the call for
> > both topolongy change or in the loss of R2 Loopback...? Many thanks and
> > Regards in Advance, Akbar khan
> >
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