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New Member

Cisco 1721 with ISDN backup when Ethernet0 is down

Hi there,

I've got a Cisco 1721 and a BRI interface. The 1721 is connected to the Internet via SDSL on interface Ethernet0. I'd like to use the BRI interface as ISDN backup when the SDSL line is down. Could anyone check my config and tell me if thi the config should work?

interface BRI0

description ISDN-Backup

no ip address

no ip unreachables

ip nat outside

encapsulation ppp

dialer pool-member 1

isdn switch-type basic-net3

isdn point-to-point-setup

no cdp enable

ppp authentication pap chap

ppp multilink


interface Ethernet0

description $ETH-WAN$

ip address <ip address>

ip load-sharing per-packet

backup delay 1 1

backup interface Dialer0

load-interval 30



interface FastEthernet0

description $ETH-LAN$

ip address <ip address>

ip directed-broadcast

load-interval 30

speed auto



interface Dialer0

ip address negotiated

encapsulation ppp

dialer pool 1

dialer idle-timeout 90

dialer fast-idle 60

dialer string <number>

dialer load-threshold 128 outbound

dialer-group 1

ppp authentication chap pap callin

ppp chap hostname <username>

ppp chap password 7 <password>

ppp pap sent-username <username> password 7 <password>

ppp multilink

Thank you!

Regards, J. Schroeder


Re: Cisco 1721 with ISDN backup when Ethernet0 is down

You will need a floating static route to initiate the BRI connection when the main link goes down:


Re: Cisco 1721 with ISDN backup when Ethernet0 is down

Its good in theory but I think you will find that your ethernet port never goes down.

You have a slim chance when you terminate the dsl connection directly on the router but even then these interfaces tend to stay up.

There are a couple of methods used to solve this nornally. Best is if you can use a routing protocol but unlikely if you are small customer of a isp. One of the new methods to solve this problem is to use policy routing with tracking.

Be aware you also have to have nat configured to use the proper outside address since this is almost always different in configuration such as yours.

New Member

Re: Cisco 1721 with ISDN backup when Ethernet0 is down

Okay, thank you! I also thought of tracking an IP address. But could you tell me how to change the config in that way?


Re: Cisco 1721 with ISDN backup when Ethernet0 is down

Check below for the tracking info. & config. sample., try to search "Configuring the Default Route for the Primary Interface Using Static Routing" :

In order to enable the tacking, you have to ensure the ISDN is working and there is interesting packet to trigger the interface.

Hope this helps.