Can I achieve ISDN DDR backup by using dialer watch?

[lehpoh]

Can I achieve ISDN DDR backup by using dialer watch? I have tried floating static route ISDN DDR backup in conjunction with OSPF but it seems not stable, the ISDN up and down for few times then remains up thereafter even though there is no traffic passing through ISDN.

[zahmed]

Sure. Check out the following :

http://www.cisco.com/en/US/tech/tk801/tk133/technologies_tech_note09186a008009457d.shtml

http://www.cisco.com/en/US/tech/tk801/tk379/technologies_configuration_example09186a0080094143.shtml

~Zulfi

[lehpoh]

Do you mean that if I use dialer watch method, the ISDN backup will be brought down when there is no interesting traffic passing through, even though the primary link still not yet restored?

Thanks.

[makchitale]

No, the dialer idle-timeout works in the dialer watch scenario as follows:

1)When primary link is DOWN & backup is connected, at the end of each idle timeout (dialer idle-timeout xxx) the router will check to see if the primary link has come UP....if it's not yet UP then will keep the backup connection nailed up.

2)Now as soon as the primary link comes UP, at the end of the idle timer router will check the primary link status, since it being UP & all traffic passing thru the primary link (the route watched will be updated thru the primary connection) the backup will disconnect since it does not have any interesting traffic to reset the idle timer counter.

The debugs in the below URL have good info.

http://www.cisco.com/warp/public/129/bri-backup-map-watch.html

Thanks, Mak.

[tepatel]

Idle-timeout value will keep on reset back to maximum all the time after the expiration idle-timeout. After the expiration of each idle-timeout, status of primary link for watched routes also checked. So if primary link is up and watched routes are there in the routing table, the isdn link will be brought down after the expiration of idle-timeout.

So to answer your question, isdn line will not reset even if there is no interesting traffic because of if idle-timeout value will be rest back to max all the time time till primary link comes up with watched routes.

[lehpoh]

Thanks for prompt reply. So if I want to have a "DDR" ISDN backup, I can ONLY use backup interface method or floating static method?

[makchitale]

If you want an idle backup link to be disconnected & then reconnected for traffic then the back interface / floating static route is the method to use.

Thanks, Mak.

[lehpoh]

If that is the case, under what condition do we use dialer watch since it is not a cost effective backup solution due to continuous backup connection regardless of the existence of interesting traffic.

[makchitale]

Please refer to:

http://www.cisco.com/warp/public/123/backup-main.html

Thanks, Mak.

[tepatel · Accepted solution]

The dialer-watch is mostly used when there is a routing protocol running between branches and central office. Dialer-watch is not limited or based on DDR but based on watched routes. So mostly if you are too concern about the lost routes because of which the the branch office become isolate with central office, you can use dialer-watch.

So in a way you are right about thinking that DDR backup can be achived only via floating static routes (which is layer 3 backup) and using backup interface command (which is layer 1 and 2 backup of primary link)