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Backup-load and static routing

When we were configuring ISDN backup for a serial line we encountered a interesting problem.

We are using the backup interface command on a serial line to have an ISDN line kick in when the serial goes down. This works perfectly.

We also like to use the backup load command to bring up ISDN when the serial line has 80 % load.

This works. When the load hits 80 %, the dialer interface is changed from standby to active. But, no ISDN is triggered because there is no actual traffic directed that way. The route across the serial line is still valid because the interface is up.

We use static routing in this case. A floating static route for the ISDN line will not work because the serial interface is staying up in case of bandwith on demand. The serial line as well as the ISDN line are up. In case of 80 % on the serial line, we want to send some traffic over the ISDN line as well.

So, is this even possible? Backupload and static routing? Or do we have to use a routing protocol.


Re: Backup-load and static routing

If you add a static route pointing to the ISDN interface with the same cost, then when the serial interface is NOT down and the load is NOT at 80%, the ISDN interface will be in STANDBY mode and the ISDN static route will not be installed in the routing table. Only once the ISDN comes out of standby will the static route be installed.

However, there have been problems in the past with providing backup AND bandwidth on demand. Another option is to use Multilink PPP, the hidden command "dialer leased", and dialer-profiles. So your config could look like this:

interface Serial1

no ip address

encapsulation ppp

dialer in-band

dialer pool-member 10

dialer leased

ppp authentication chap

ppp multilink


interface Serial0:23

no ip address

encapsulation ppp

dialer pool-member 10 max-link 2

ppp multilink


interface Dialer10

ip unnumbered Loopback0

encapsulation ppp

dialer remote-name HQ

dialer pool 10

dialer disable-multiencaps

dialer string 202

dialer load-threshold 5 outbound

dialer-group 1

ppp multilink

Good luck.