From: Tom Lijnse (Tom.Lijnse@globalknowledge.nl)
Date: Thu Mar 25 2004 - 14:28:49 GMT-3
Hi all,
I tend to agree with Chris here. Both techniques can accomplish the same goal and they don't need to be configured simultaneously. (Of course you can configure both at the same time and it will still work, so in that respect they're "compatible").
The behavior is somewhat different though and especially you have to pay attention to your interesting traffic definitions. For both features you will have to configure the regular DDR stuff. In addition you need to take the following into account:
1) OSPF on Demand Feature
The idea is to have any ip traffic defined as interesting traffic (including OSPF). OSPF will trigger the link, exchange databases across the ISDN and after that hellos will be suppressed and unless there's a topology change OSPF will not trigger the link. You have to make sure that the cost of the ISDN link is higher than the cost of the primary, so regular user traffic will be routed across the primary. Now if the primary goes down, OSPF will trigger the ISDN to exchange the newly generated LSAs and then recalculate routes. All user traffic will now be routed across the ISDN. If there's no user traffic the ISDN will go down, because OSPF still suppresses hellos. Any new interesting packets will trigger the link, since the routing table.
2) Dialer Watch Feature
You configure regular DDR, but you explicitly exclude OSPF from your interesting traffic definitions. We don't want OSPF triggering the link, since the dialer watch feature is going to trigger the link when the watched routes disappear from the routing table. So basically what happens is that OSPF initially only forms an adjacency across the primary link, not across the ISDN, so all user traffic is routed across the primary. When the primary goes down the watched route disappears and dialer watch kicks in. Now OSPF forms an adjacency across the ISDN, exchanges the database and recalculates to route user traffic across the ISDN.
So each of the two methods accomplishes the same overall goal: Rerouting user traffic across the ISDN when the primary goes down. No need to configure them both at the same time.
Regards,
Tom Lijnse
CCIE #11031
Global Knowledge Netherlands
P.S. sorry for repeating some things already mentioned in this thread. The replies came in while I was composing this mail. I figured this post might serve as a final summary so I decided to hit the "send" button anyway.
-----Original Message-----
From: Chris Fontes [mailto:cfontes@atrion.net]
Sent: Thursday, March 25, 2004 5:20 PM
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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