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Re-routing traffic on incoming ISDN call

One of our clients has a central 72xx router supporting incoming and outgoing calls with a large estate of ISDN routers connecting over 4 PRI circuits. They are currently trying to implement an HSRP solution with a second 72xx router.

The central network routes outgoing calls to the primary router in the HSRP group, but incoming calls can be presented to either router because of the rotary order on the PRI lines.

If a call is delivered to the secondary router, the network tries to respond via the primary, causing the connection to fail.

Has anyone out there successfully implemented this setup, or know how?

1 REPLY
New Member

Re: Re-routing traffic on incoming ISDN call

I have done something similar, if a little bit different!! In my case we had a two core ISDN routers at the hub, with a network of approx 150 remote site ISDN routers. The ISDN network was addressed, with the ISDN interface of each router having an address within a /24 network. The ISDN connections used PPP, and as part of the negotiation the address of the router that had dialled in was inserted into the routing table as a /32 route, which in turn was redistributed to the second core ISDN router in the centre. Each of these two core ISDN routers then had a static route to the remote site LANs pointing to the remote site ISDN address. If a packet arrived at one of the core routers destined for a remote LAN (we didn't have an active / standby router in our scenario but the principal will be the same) and no call was up, that router would make a call out, again placing the remote /32 address in the routing table. If a packet arrived and a call was already up to one of the core routers, then the router would know from the static route which remote IP address the packet needed to go to, and then following a recursive lookup which of the two core routers had the call up (i.e. was the /32 route 'connected' to this router or advertised by the other router) and so direct the packet accordingly.

As I mentioned, we did not have HSRP running in our scenario, but there should be no reason why it should not work OK anyway. You will just have to be aware that you will have a number of redirected packets as the active HSRP router resends data to the standby router, although this may be partly alleviated through the use of ICMP redirects.

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