ISDN BRI Multiple lines Backup

Unanswered Question
Aug 13th, 2008

There is a MPLS WAN link connectivty between one cetral site and two remote sites.Now the planning is putting 4BRI lines as a bundle(to use aggregate bandwidth) for backup link .Whenever MPLS link goes down BRI lines (4Nos)have to dial and estlablish the link.This same setup has to work for both remote sites.The cetral site router is 2811 with NM-8B-S/T .The ospf routing protocol is running at the sites.Please suggest a good solution.


I have this problem too.
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spremkumar Wed, 08/13/2008 - 22:30


you want all these bri lines to get connected to the service provider network or directly onto your router ???

if its between two of your routers then you need to create 2 identical dilers for the remote sites and make the isdn bri interfaces as a member of these dialer interfaces.

under the dialer interfaces you can configure dialer min and max links which can help you out to reserve the isdb b channels for each remote sites.


mpoonghat Mon, 08/25/2008 - 22:00

Hi Prem,

Thanks for your reply.I had tested in the same way only.But the issue is MPLS interface is an fast ethernet interface .If I use backup interface command on this interface then it dials the ISDN line once the line protocol is down .This method is OK for in the case of initiating connection from remote offices.But If the Mainoffice link is down ,it can dial only one remote location because the main router use same FE interface for both MPLS connection.

If I use static route floating method it failover to ISDN when destnation is not reachable but whenever the MPLS line comes up it doesnt fail back to MPLS.This happens because of redistribute static subnets is there in ospf config.

Please reply if you think any other method for this issue.



spremkumar Tue, 08/26/2008 - 23:04


If your primary link is terminating onto an ethernet port and your secondary on serial/isdn then you need to use enhanced object tracking feature to trigger the backup link to forward the traffic.

I am attaching a sample config for your reference...


rtr 1

type echo protocol ipIcmpEcho PE-IP-address

timeout 1000

frequency 3

threshold 2


rtr schedule 1 life forever start-time now

track 100 rtr 1 reachability


ip route track 100

ip route 254


interface Ethernet 0/0 Primary Link Configuration

description Ethernet Primary Link

ip address


interface dialer 1 Backup Link Configuration

description ISDN Backup Link

ip address


would suggest you to cehck and do a bit of study about object tracking before you implement the same in your network..


mpoonghat Fri, 09/05/2008 - 21:10

Hi Prem,

Can I use this reachability method in my scenario which has OSPF routing for MPLS and static routing for other LAN Subnets.And static route distribution also there in OSPF .Can you elaborate Object Traking more on this scenario ?

Thanks in advance.



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