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ISDN backup for a hub & spoke network model

The network is setup in a hub and spoke scenario. The primary data connections from the main branch to the 5 remote branches are point-to-point T1s. The main branch (Cisco 3745)has two BRIs (4 B channels) and each remote branch (Cisco 2621XM) has one BRI. The main branch ISDN configuration must have the ability to support up to 4 branches at one time (each on a separate B channel).

The issue I have run into is routing. I have configured the IP address on the physical BRI interface but I wonder if that is causing problems when two remote branches are connected to one BRI at the main branch. The main branch router will only hold one route through the BRI interface. I am also having trouble getting the third remote branch connected to the main branch via ISDN because if the first BRI is busy or unavailable, it is not trying the second BRI for connection.

Any suggestions on the cleanest, most effective configuration to implement ISDN backup with authentication based on the scenario described above? I am happy to include more details, just let me know. I have been searching so many documents, they are all running together for me now.




Re: ISDN backup for a hub & spoke network model


Have you tried using Dialer profiles for each remote site ? You can use individual dialer profiles with appropriate ip address (subnet) on each dialer profile. The BRI interfaces will not be bound to any specific site. When a call comes in from a remote site, the dialer profile will choose one of the available BRI interface/Channel. The routes for each site should appear pointing to a different dialer interface.

Hope this configuration info helps.



Re: ISDN backup for a hub & spoke network model

This is a common requirement and can be easily met if your routing protocol is EIGRP (or RIP). The key is to use IP unnumbered for the ISDN. You can use legacy dialer maps or dialers as you like for the actual implementation. Assign a map or dialer string for each BRI at the core at each remote. Just be careful, because some IOS releases seem to require the use of dialers to get calls functional.

If you do it right, your solution can grow with you to support an arbitrary number of remotes and multiple access routers at the hub (no single points of failure that way). There is extensive coverage in my book, see Chapter 7 on hub and spoke specifics. The example listings are all posted on my web site.

Good luck and have fun!

Vincent C Jones

Community Member

Re: ISDN backup for a hub & spoke network model


Thank you very much for your quick response. I used the Dialer Profile information you provided and the configuration worked like a charm. I have had two TAC cases open to get this working and it only took you one answer. You have been very helpful.


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