Frame relay backup with ISDN

[wsherlock]

I have an issue that someone may be able to help with.

We have a solution that uses a Cisco router to connect via 2 seperate PVC's to different locations for resilience. We use EIGRP and bandwidth statements to define which PVC to use as the primary.

We also use an ISDN 2e line as a backup in the event both PVC's fail.

Initially we were using dialer watch to achieve this which worked OK.

Now we want to remove this as the routing updates put a major overhead on the circuits. The PVC's are only 256K.

The initial thought was to use a backup interface command on the serial interface. The problem with this is that if the failure is centralised on one of the PVC's then the Serial interface will not necessarily go down and the backup will not kick in.

Can anyone think of a simpler way of doing this.

Any help would be appreciated.

[tomanderin]

what about using a subinterface per PVC, then adding the backup commands to the subinterfaces

[vcjones]

Frame relay per se does no reliably report loss of link function, so you need to run some protocol with a keep alive function over each frame relay PVC to detect when it has failed. It is easiest to do this with a routing protocol, use of which should be minimal overhead (on the order of one packet every five seconds when in steady state).

If your routing protocol is clogging your links, there is something seriously wrong with your setup and changing the mode of ISDN backup is not going to fix it.

There is a burst of activity on a link when it first comes up and EIGRP (or OSPF, you don't state what your are using) first comes up. You can control this by configuring individual subinterfaces for each PVC and assigning each subinterface the correct (256Kbps) bandwidth.

If that does not resolve your problems, you may need to call in an expert.

Good luck and good hunting.

Vincent C Jones

www.networkingunlimited.com