Problem in ISDN backup

[preeda]

Hello all,

Now, our branch offices use Cisco router 2521 lease line using frame relay protocol and analog modem for dial back up. Recently, we are trying to migrate to ISDN back up, and we found some difficulty with ISDN as following. I am basic user, so probably some of you can suggest me the better solution. Thanks a lot !

1) How should I test the ISDN properly ?? I think ISDN is difficult to test. Now, I have to shut the main link to confirm, wheter the ISDN is ready to use. Anyone know how to test ISDN backup with out interupting user ?

2) Sometimes main link is down, but ISDN doesn't make a call, until somebody shutdown the modem for main link and reset the router. I suppose it to dial right after main link is down. Do anyone has any comment on this

Thanks alot for your help

 

[ccie9545]

In terms of testing the backup, probably the easiest way is to do it when noone is there.....while you are closed. If there is not a time like this, consider what would be the best time to do it, and schedule it in advance so everyone knows that it is going to happen. Bring down the main link, if the ISDN fails, bring the main link right back up again.

Do you have the configurations? It really depends on how you are implementing the backup. If you dont have configs can you tell us how you are implementing the backup... Erik Rudnick, CCIE No. 9545
mailto:erik@kuriosity.com

 

[l3stuff]

floating static routes where designed for this application. Basically you point a static route at the ISDN Dialer, when the dynamic routing protocol hellos are missed the static route (which has a higher admin distance than the IGP) kicks in, dials the connection and provides your back up circuit

 

[ccie9545]

i would agree floating static routes are a perfect implementation for this situation..... Erik Rudnick, CCIE No. 9545
mailto:erik@kuriosity.com

 

[preeda]

Hello Erik,

Thanks a lot for your response. Here is our config, that we are using. If I understand it correctly, backup delay 60 120 means to let router wait for 60s before making a call after main link down. When the mainlink comes up wait another 120s to disconect ISDN line.

By the way, sometimes it doesn't work accordingly, since after main link down we need to shut down the mainlink completely (switch off modem) seems like the router still sense some signal from mainlink.

interface Serial0
description main link
backup interface BRI0
backup delay 60 120
no ip address
encapsulation frame-relay IETF
load-interval 30
frame-relay traffic-shaping
frame-relay class 128K
!
interface Serial0.xxx point-to-point
ip address x.x.x.x 255.255.255.252
ip ospf cost 100
frame-relay interface-dlci xx
!
interface Serial0.yyy point-to-point
ip address y.y.y.y 255.255.255.252
ip ospf cost 110
frame-relay interface-dlci yyy
!
username aaaa password 7 bbbbbbbbb
isdn switch-type basic-net3
!
interface BRI0
description dial backup
ip unnumbered Loopback0
ip rip send version 2
ip rip receive version 2
encapsulation ppp
dialer idle-timeout 2000000
dialer string zzzzzzzzz
dialer-group 1
ppp authentication chap
!

Thanks for looking through my configuration.
Have a good day !!
Preeda

 

[preeda]

Hello I3stuff, Erick,

As you can see from my config. We use rip instead of static, since we have around 500 branches to manage. The problem, we are facing is for some branch ISDN doen't make a call after mainlink down. Not until we ask someone there to shut the mainlink completely.

Thanks,
Preeda

 

[ccie9545]

Onr problem with using backup interface with frame relay is that backup interface looks at the status of the line protocol. For instance, if you do a show interface you will see the line being up or down nad the line protocol being up or down. backup interface will watch the line protocol and when the line protocol goes down, it will consider the main link to be down and initiate the call. The problem can occur if your line protocol doesn't actually go down, but you lose connectivity for one reason or another (say your provider misconfigures DLCI's or something like that) the line protocol will remain up and the isdn link will still think that the main link is up. YOu could try using a floating static routes. Add static routes, or a single static default route, that points to the dialer interface. having teh higher administrative distance will ensure that the route will not get used if the RIP route is in the routing table. Erik Rudnick, CCIE No. 9545
mailto:erik@kuriosity.com