From: Devi Mallampalli (Devi.Mallampalli@chubb.com.au)
Date: Wed Jun 23 2004 - 01:54:05 GMT-3
Hi Cert,
I had similar confusion few days ago and later I kind of devised my own
little way of nailing down this issue.
This is how I tend to tackle this issue.
Firstly watch out for which L3 protocol is running on ISDN. If it is
OSPF , then look for requirement or PHRASE of "Topology change" , then
we know they are talking about "Demand Circuit". On the other hand , if
L3 is ISIS , then we got to start thinking about other possible methods
such as "Dialer-watch" or "Back-up " interface. In case of Dialer-watch
, always look out for phrases such as "watching a certain route from the
R/table..etc. Where as a Phrase of an "Interface" going down can mapped
to "Back up interface " option. Besides there is an another option of
"Floating Static "which works on manipulating the AD on a Prefix basis
so that she is preferred by Routing table to route the packets across
ONLY in CASE better metric Route vanishes from the R/Table due to
primary link failure. And again when primary path comes back online ,
this Floated route will be ignored because she has lot less desired AD
on it.
But at a high level thumb rule ( from my point of any way , please
correct me if I am wrong), Demand circuit is only associated with OSPF ,
when ever two or routers are involved and were asked to look for any
route , then Dialer-watch is the way to go and whenever they are talking
about an Interface, then Backup interface option is a way to go. And
finally whenever you are talking about having 2 routes to the given
prefix and then want 2nd one to back up the 1st route in case primary is
down , then Floating static is the way to go.
Devi.
-----Original Message-----
From: Cert [mailto:cert77@hotmail.com]
Sent: Thursday, 17 June 2004 10:40 PM
To: ccielab@groupstudy.com
Subject: ISDN doubts..
Hello All,
I have a very basic question about ISDN being used as backup . In the
lab how can I figure out whether they are asking me to configure ISDN
dialer watch or OSPF deman circuit or backup interface. According to me
all the three above meets the requirement of backup for the primary
link. Any help would be highly appreciated. Thanks, Cert.
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