From: Saru Vijayakumar (v_saru@hotmail.com)
Date: Tue Nov 19 2002 - 22:05:00 GMT-3
Before justifying the usage and comparison between Demand Circuit and Dialer
Watch, I think it helps to note that:
1)While they both use DDR technology to cut down on conneciton costs, the
Demand Circuit is not exactly a backup feature. It is used when you want to
have an OSPF backbone at the central site and you want to connect
telecommuters or branch offices to the central site. In this case, OSPF On
Demand Circuit allows the benefits of OSPF over the entire domain without
excessive connection costs. Periodic refreshes of Hello and LSA updates are
prevented from enabling the on-demand circuit when there is no "real" data
to transmit.
It may function like a backup feature in a situation where the router's
forwarding decision points to the primary link for all destinations while
the Demand Circuit feature running on the ISDN/OSPF link keeps the circuit
quiet.
2)Dialer Watch is a backup feature that integrates dial backup with routing
capabilities. Dialer Watch provides reliable connectivity without relying on
defining interesting traffic to trigger outgoing calls at the central
router. Therefore, dialer watch can also be considered regular DDR with no
requirement for interesting traffic, just lost routes. By configuring a set
of watched routes that define the primary interface, you are able to monitor
and track the status of the primary interface as watched routes are added
and deleted.
I would think your network topology is the deciding factor (in the real
world) as to use Demand Circuit or not. Comparing the pros and cons between
Backup Interface, Floating Static and Dialer Watch is probably more
appropriate.
However, as far as the exam goes you would need to interpret the question
correct based on how it is worded and your understanding of the concept.
HTH.
>From: "Ajit" <ajitmohanraj@vsnl.com>
>Reply-To: "Ajit" <ajitmohanraj@vsnl.com>
>To: "Sara Li" <saralilin@hotmail.com>, <ccielab@groupstudy.com>
>Subject: Re: DDR in OSPF network
>Date: Wed, 20 Nov 2002 03:28:08 +0530
>
>You brought up an xxcellent thought provoking point. My reply to your
>question would be as under :-
>
>ip ospf demand circuit :
>Primal aim is to prevent the ospf's hello's from bring up a ddr link.
>
>dialer-watch.
>primal aim is to 'watch' a user defined set of routes and provide an
>alternate path
>
>I think, that when we have a 'grey' solution like this, it might be best to
>see it from Cisco;s point of view and WHY they brought in a type of
>solution
>to addres what TYPE of problem.
>
>My guess is that the proctor would think likewise !
>
>Just my 2 cents !!
>----- Original Message -----
>From: "Sara Li" <saralilin@hotmail.com>
>To: <ccielab@groupstudy.com>
>Sent: Tuesday, November 19, 2002 2:51 PM
>Subject: DDR in OSPF network
>
>
> > In a OSPF network, in order to acheive the dial-on-demand as a backup
> > link, we can use either dialer-watch or on-demand-circuit. my question
> > is: when to use which? how do we justify which on to use? how do you
> > compare these two methods in term of adv vs disadv? Thanks.
> >
> > ------------------------------------------------------------------------
> >
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