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Community Member

Is this an IOS problem?

I have c1721 router at a customer premises. IOS version is c1700-k9sy-mz.122-32.bin.



ISDN backup is not working automagically. isdn test call from the router(1721) works. dropping the backup statement on the interface(1721) works but if you SHUT the interface on the gateway router they connect to on our side, the ISDN does not come up.

It cannot be IOS on the gateway, as all other backups work fine except for this one.

Ideas? Upgrade IOS on 1721? if so...what IOS to?


Re: Is this an IOS problem?

Hi .

If you shut the physical interface of the serial line, the ISDN will not take up automatically. It should not receive any of the signals like DTS,DSR etc.

Just switch off the modem connected to the serial line or physically remove the cable. The backup interace i.e ISDN will fire automatically.

--Pls rate if this helps--

Re: Is this an IOS problem?


(1)Can u give the config.?

(2)Are u generating interesting traffic?

"dialer-list 1 protocol ip permit"

(3)The floating static routes should be configured -

"ip route dialer1 200"

(4)Is backup delay configured?

"backup delay 1 1"

Pls visit the following link for more troubleshooting...

Hope that will solve the prob.

Pls rate helpfull posts.



Re: Is this an IOS problem?

Hi Brad

If you shut the main interface for which you have configured the bri interface as a backup it wont fire up.

You need to physically switch off modem where your telco line is getting terminated to make sure your isdn is firing..


Community Member

Re: Is this an IOS problem?

shutting the physical interface on our gateway router should have the same effect as dropping the physical line itself?


Re: Is this an IOS problem?

Hi bradle

It does not work if you shut the interface only. Even if the interface is connected, it will detect a CTS signal and will never make out that it is down. I have tested it a number of times.

You will have to manually remove the interface or shut the modem

--Pls rate if it helps--

Hall of Fame Super Gold

Re: Is this an IOS problem?


It is standard behavior for IOS (and always has been) for the backup interface command that it will initiate the backup only when there is a problem. Issuing the shutdown command is not a problem. The IOS assumes that if you issue the shutdown command that you meant it to be down and therefore will not initiate the backup interface. The interface must go down or the line protocol of the interface must go down for the backup interface to become active.



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