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New Member

ISDN backup with redundancy

Our WAN is built on frame relay with ISDN backup. So in the event of the frame relay circuit failing the remote router dials the main office.

We have two routers at the main office that can accept ISDN connections from the remote sites.

At present we can split the load over both routers so half of the remote sites dial RouterA and the other half would dial RouterB.

However, if RouterA has failed or is offline for maintenance then we have a problem in that half of the sites won't be able to connect via ISDN should the frame relay circuit fail.

We've puzzled over using dialer profiles and dialer map commands but neither seems to be suitable.

Although dialer profiles can let us us use two dialer string commands to conect to either router at head office we can only have one dialer remote-name command so authentication may fail.

With dialer maps the order is by IP address so we can't set a preference.

Has anyone come up with a solution for this sort of thing or is there no means to do this at all?


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New Member

Re: ISDN backup with redundancy

I do hope that someone offers you a better solution, but I wonder what would happen if you used the same name and password for both central site routers to get around the authentication issue (assuming there isn't anything in your topology that would preclude you from doing so)?

I see some problems with this though. First, it seems like you would have to dynamically assign your IPs to the remote site BRIs or use unnumbered since you don't know which site you will be successfully calling. Also, I think you would need a routing protocol since it isn't firmly predefined which networks are reachable via which other networks/interfaces. So then you run into problems bringing the connection down due to routing updates. I would guess there are probably other issues but those are what immediately come to mind.

New Member

Re: ISDN backup with redundancy

Thanks for the response.

Using the same name for both routers was a thought, however, authentication is done via TACACS so I don't think this can be done.

We're actually using the unnumbered command at the moment. So it doesn't really matter which ISDN router takes the call.

Although we use EIGRP over the frame relay lines, this is blocked over ISDN. Each remote site uses a floating static to point it to an ISDN router at the main office.

As a result when the frame relay circuit goes down, ISDN comes up. When frame relay is restored, ISDN goes down as there is a better router via frame relay.

Because the loopback used in the unnumbered command takes its IP address from the remote subnet, routing isn't really a problem when ISDN comes into play.

This works fine when there's one main office router to dial into it, but using a 2nd ISDN router at head office for redundancy looks an increasingly unlikely option.

New Member

Re: ISDN backup with redundancy

Have you thought of having the central site initiate the Dial in the event of a route loss. Both routers will attempt to dial the down site until a connection is made. (remember a static route floating or not is only injected into the routing table once it can see the other side.) Once one router makes the connection it will redistibute the route eigrp wieght of 170 into the other Dial router and the 2nd router will stop call attempts. provided you are using a weight of 200 in your static routes.

New Member

Re: ISDN backup with redundancy

An interesting thought but unfortunately the remote site must initiate the connection.

Its a politics thing. All boils down to who pays the bill.