From: Chris Fontes (cfontes@atrion.net)
Date: Thu Mar 25 2004 - 13:47:12 GMT-3
I did not say they were incompatable. I said there is no need to configure
both at the same time.
Demand circuit will form an ospf adjacency when first configured and
exchange info then bring the link down, after that they will not send Hellos
or LSA refreash over ISDN link. If the primary goes down and the topology
changes, LSAs will be flooded out the ISDN interface(and every other int)
and OSPF will converge and start routing over the ISDN link. There is no
need for Dialer Watch in this situation.
-----Original Message-----
From: Joseph D. Phillips [mailto:jphillips@ufcwdrugtrust.org]
Sent: Thursday, March 25, 2004 11:24 AM
To: Packet Man; ccielab@groupstudy.com
Subject: OSPF Demand circuit and Backup
DDR doesn't work that way. Packets arriving at a BRI interface will count as
interesting traffic, if you have your dialer list configured correctly, but
the dialing mechanism will not know what to do with them. It won't know
which phone number to dial for that route.
That's my understanding from many months of practical experience. The demand
circuit option doesn't do much. It just keeps hellos from keeping the link
up.
From all the Cisco stuff I read, Cisco support for it is rather tepid. You
can create your own demand circuit by advertising the BRI link in OSPF and
using this as your dialer-list:
dialer-list 1 pro ip list 101
access-list 101 den ospf a a
access-list 101 per ip a a
I have used dialer watch and ospf demand-circuit together without any
trouble. They are perfectly compatible.
-----Original Message-----
From: Packet Man [mailto:ccie2b@hotmail.com]
Sent: Thursday, March 25, 2004 08:02
To: Joseph D. Phillips; ccielab@groupstudy.com
Subject: RE: OSPF Demand circuit and Backup
Hey Joseph,
I see what you're saying but once the primary link goes down, doesn't that
trigger ospf to flood lsa's out all it's interfaces so that all routers can
sync their lsa databases?
And, once all lsa databases are in sync, don't all routers then know that
the former primary link isn't available and therefore when the spf process
is run, any paths that previously used the primary link will now use the
ISDN link instead?
And, if that's true, doesn't that mean that Dialer Watch isn't needed? Or,
put another way, wouldn't configuring Dialer Watch, in addtion to OSPF
demand circuit be reduntant?
If this isn't true, can you describe a scenario where both Dialer Watch and
OSPF demand circuit over the same isdn link would be needed?
Thanks, pm
>From: "Joseph D. Phillips" <jphillips@ufcwdrugtrust.org>
>Reply-To: "Joseph D. Phillips" <jphillips@ufcwdrugtrust.org>
>To: "Group Study (E-mail)" <ccielab@groupstudy.com>
>Subject: OSPF Demand circuit and Backup
>Date: Thu, 25 Mar 2004 07:44:06 -0800
>
>Dialer watch and OSPF demand-circuit accomplish two different purposes.
>
>OSPF demand only keeps the BRI link from flapping after the BRI's IP
>network is defined under the OSPF router process.
>
>By itself, it does not provide any reachability to any networks.
>
>If you want a true backup of a frame-relay network, you'd need
>dialer-watch, and you'd have to create a dialer map statement for each
>network which would be lost if the primary connection goes down.
>
>
>-----Original Message-----
>From: Packet Man [mailto:ccie2b@hotmail.com]
>Sent: Thursday, March 25, 2004 07:12
>To: ccielab@groupstudy.com
>Subject: OSPF Demand circuit and Backup
>
>
>Hi all,
>
>When an ISDN circuit is configured as an OSPF demand-circuit to backup
>another OSPF link in the same area, should and can any other backup method
>such as Dialer Watch also be configured on the same ISDN link?
>
>My sense is that there's no need to configure any other backup method and
>doing so would only complicate things and lead to unexpected results.
>
>Here's what I believes happens in such a situation (without 2nd method
>configured).
>
>Primary link goes down.
>
>This causes ospf to see a topology change has occurred.
>
>OSPF floods lsa over all links including ISDN circuit which brings up ISDN
>circuit.
>
>OSPF updates the route table to reflect new topology.
>
>Now, user traffic that would have used the primary link now uses the ISDN
>link.
>
>Is this simplified chain of events correct? Are there other significant
>things that occur that we should be aware of?
>
>Thanks in advance, pm
>
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