From: Packet Man (ccie2b@hotmail.com)
Date: Thu Mar 25 2004 - 14:19:29 GMT-3
Hi all,
Just another twist on this topic.
Assume that the primary link is up and the ISDN link is configured as OSPF
demand-circuit and that both have the same cost.
In this instance, would ospf load balance over the 2 links? Why yes or no?
>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 08:26:56 -0800
>
>Seeing's believing. Try just using demand-circuit and see if you can ping
>everything you want.
>
>Can I get a witness from some of the big brainiac CCIE's here, please? :)
>
>Scott? Brian1? Brian2?
>
>
>-----Original Message-----
>From: Chris Fontes [mailto:cfontes@atrion.net]
>Sent: Thursday, March 25, 2004 08:20
>To: Joseph D. Phillips; Group Study (E-mail)
>Subject: RE: OSPF Demand circuit and Backup
>
>
>Hi all
>
>Not sure I agree with your whole explanation Joseph.
>
>OSPF demand circuit does keep the link from flapping, but once there is a
>topology change the link will come up, ospf will converge and routing will
>take place over the link. It will provide reachability.
>
>With dialer watch you only need a dialer map statement for the specific
>network(s) that are being watched with the dialer watch-list not for each
>network that would be lost if the primary connection goes down (ie if your
>learning 6 routes over your primary link, you could just watch one of them
>with dialer watch). If the network drops from the routing table, dialer
>watch will bring the ISDN link up, ospf will converge and routing will take
>place over the link.
>
>Each method would provide the same results.
>
>Packet Man I can't think of any reason why you would cofigure both at the
>same time. Its two different methods that provide the same results.
>
>Chris
>
>-----Original Message-----
>From: Joseph D. Phillips [mailto:jphillips@ufcwdrugtrust.org]
>Sent: Thursday, March 25, 2004 10:44 AM
>To: Group Study (E-mail)
>Subject: OSPF Demand circuit and Backup
>
>
>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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