From: Kenneth Wygand (KWygand@customonline.com)
Date: Sun Jul 11 2004 - 12:16:53 GMT-3
When using Demand Circuit, the loss of an external OSPF route that was
injected by another routing protocol does become part of the LSA
database (type 5). My thought is that the loss of these routes would
indicate a change in OSPF's LSA database and should trigger a Demand
Circuit call.
However, this brings up an interesting point. What if OSPF demand
circuit is not running in a normal area, but rather running on router
that are both part of a stubby or totally stubby area? Say you have:
R1----R2----R3
Area2--R1--Area0--R2--Area1(totallystubby)--R3
Say R1 and R2 are in area 0. Then say R2 and R3 are in area 1, which is
configured as a totally stubby area. Then say we have an ISDN link as a
backup between R2 and R3 that's also in area 1 (totally stubby area).
Now all R3 knows about is type 1 and type 2 LSA, in addition to a single
type-3 default route from R2. Suppose R1 has two loopback interfaces in
area 2, and area 2 is summarized into a type-3 LSA sent into area 0
(send from R1 over to R2 through area 0). If one of these loopback
interfaces goes down, the summary address (area range) should still be
active in area 0's LSA database, because an active interface (the other
loopback interface) is still within that "area range" on R1. As a
result, the area 2 LSA database on R1 will change, but the area 0 and
area 1 databases will not change on any of the routers, namely R2 and
R3. If R2 and R3 do not see this as a change in their -own- LSA
databases, I do not believe Demand Circuit should trigger a call. Does
anyone agree/disagree with this logic?
One final note - Although I've never seen it before, it appears possible
that OSPF itself does not have to be running over an ISDN circuit
configured as an OSPF demand circuit. While at the moment I can't think
of a good reason why you would want to do this, I'm sure there might be
a potential (and perhaps obscure) reason out there to do this.
Kenneth E. Wygand
Systems Engineer, Project Services
CISSP #37102, CCNP, CCDP, ACSP, Cisco IPT Design Specialist, MCP, CNA,
Network+, A+
Custom Computer Specialists, Inc.
"The only unattainable goal is the one not attempted."
-Anonymous
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Tom Rogers
Sent: Sunday, July 11, 2004 10:49 AM
To: ccie2be; akbar khan; ccielab@groupstudy.com; security@groupstudy.com
Subject: Re: OSPF Demand Circuit with Watch List..
ccie2be,
R U saying if ISDN is running between R1 and R2 and OSPF in between. And
a route is being redistributed from protocol X into OSPF, and that route
goes away, OSPF will not trigger a call?
Akbar,
Can U paste your config for R1 where U have DC and Dialer watch
configured on the same router?
I hope you're aware of ospf should an interesting traffic for DC and
unintersting for dialer watch.
Tom
ccie2be <ccie2be@nyc.rr.com> 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"
To: ;
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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