From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Sun Jul 11 2004 - 13:38:30 GMT-3
At 11:23 AM -0400 7/11/04, David Buechner wrote:
>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.
It's also worth noting that the demand circuit feature is useful on
extremely low bandwidth but non-dial links, such as a 2400 bps
tactical radio channel. While its original purpose was indeed for
dialing, it has other applications.
>
>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.
Not a lab-specific but real-world observation, it also is a Cisco
proprietary feature and may not be practical in a mixed vendor
enviroment.
>
>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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