From: Jim Graves (jtg@xxxxxxxxxx)
Date: Thu Jun 14 2001 - 11:11:21 GMT-3
Eric,
There's a bunch of ways to do this. Which way you choose will depend on
your requirements. For example:
* Should the line come up for normal traffic?
* Are you backing up a serial line?
* Should the line stay down no matter what if the primary route is
available, or
* Is it acceptable for the line to come up when OSPF floods LSAs (ie, a
network somewhere has come up or down)?
* How do you want the backup line to act when the primary is down? Should
it come up and stay up, or only stay up when there's application (i.e.,
non-routing protocol) traffic?
The most common ways to configure DDR for OSPF are:
1. Use the "backup-interface" command. This will force the line down and
keep it down as long as the primary line is up. When the primary goes
down, the backup line (i.e., ISDN) comes up. You don't necessarily need
any special OSPF configuration when using this method, unless you want the
backup interface to come up only for application traffic.
2. Use ospf demand-circuit. This will tell OSPF that the line is a demand
circuit, and that periodic hellos shouldn't be exchanged. This only needs
to be done on one side of the line, as long as you can tolerate a certain
amount of line uptime at the start of things for one router to tell the
other that the line is a demand circuit. If everything else is configured
properly (most importantly, redistribution), a line configured with OSPF
demand-circuit will come up for initial negotiation, then stay down except
when the better routes disappear or when there's an LSA flood. That last
bit is an important point - if the demand-circuit is in area 0 (or any
other area that gets LSA floods), a network flap somewhere else in the
network will bring up the demand-circuit. OSPF demand-circuit only
supresses hellos, not LSA flooding.
3. Use dialer-watch. Dialer-watch picks a route to watch. If that route
disappears, the backup line is brought up. When configured properly,
dialer-watch will keep the line down when the primary route is up. With
dialer-watch, you don't want to use the ospf demand-circuit command,
because that sets DNA on the routes. If the watched route doesn't age, it
doesn't disappear, and dialer-watch isn't triggered. Also, with
dialer-watch you want to exclude OSPF from the dialer-list on the
dialing/watching side of the connection. This prevents OSPF hellos or LSAs
from bringing up the line. Leaving OSPF in the dialer-list on the other
side prevents the other end from dropping the connection. Finally,
dialer-watch requires an explicitly configured idle-timeout. This is
probably a bug, but in the IOS versions I used, the default idle-timeout
wasn't recognized by the dialer-watch process.
You can mix and match these methods somewhat, too. It all depends on how
you want the line to act.
At 07:49 PM 6/13/2001 -0400, Eric Wang wrote:
>Hi,
>
>I have two routers connected by using serial connection in OSPF area 1.
>One is ABR. One is internal router. I'd like to configurate ISDN BRIs on
>these routers for backup. No static route allowed. Can anyone tell me how
>to do it?
>
>Thanks
>**Please read:http://www.groupstudy.com/list/posting.html
---------------------------
Jim Graves
CCIE #7524, CISSP, MCSE
Network Systems Consultant
Lucent Worldwide Services
Alpha Pager: 1-800-467-1467
**Please read:http://www.groupstudy.com/list/posting.html
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