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ISDN Backup for a MLPPP

I have two serial interfaces configured as MLPPP. I want to configure an ISDN interface to work as a backup for the MLPPP in case of both serials goes down. and I need to use the full 128Kbps isdn (mlppp).

When I configured this network, when I disconnet the two lines the ISDN goes UP but when I return the lines again the serials stays protocol down until disconnect the ISDN manually.

I correctly configured the Backup timer but it seems that 2 mlppp at the same time in the router is not possible (Cisco 2620).

how can I solve this?

Community Member

Re: ISDN Backup for a MLPPP

Not sure why the serial stays up/down until the ISDN is disconnected, that should not happen. Try configuring the isdn with the backup interface, or another option could be to use a floating static route. Here's a link that might be helpful:

Community Member

Re: ISDN Backup for a MLPPP

The ISDN backup interface behavior is normal if the backup for the mlppp is a single BRI channel. but if I enable mlppp on the ISDN dialer to enable the 128Kbps (2B). Problems appears.

It seams that Only one MLPPP is allowed in the router(2600).

so here is what happens:

- if 2 BRI Channels configured as a backup for one serial. no problems.

- if one BRI Channel configured as a backup for a MLPPP (2 Serials). no problems.

- if 2 BRI Channels configured as a backup for a MLPPP (2 Serials). the ISDN goes UP if the MLPPP link is down but when the Serials comes back Up the MLPPP for the serials stays down as protocol until the ISDN is manualy disconnected. it means it waits until the other MLPPP is down.


Re: ISDN Backup for a MLPPP

Sounds like both MLPPP links (primary and dial backup) are getting confused because both ends are using the same names to identify the circuits. You could try configuring the dial backup to use different "usernames" for the calling and called routers (look up the commands "ppp authentication chap" and "ppp chap hostname" et al.).

Disclaimer: This could have nothing to do with your problem and merely be a waste of time. You are obligated to pay for this advice by letting us know what actually works.

Another Disclaimer: If I did guess right, there are almost certainly other approaches you could use to avoid confusing MLPPP, and others might be more efficient or robust.

Good luck and have fun!

Vincent C Jones

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