CME <=> Avaya CM5

I'm currently studying for my CCNA Voice certification. As part of this I have configured a router with CME (Call Manager Express) which is the mini PBX that you can run on a suitable Cisco router.

I have manage to get most of this running as expected as per the methods shown in the very useful CBT Nuggets videos which I have access to through work.

Yet there was one problem. Whilst I could call from Avaya to Cisco and have a proper voice path, calls from Cisco to Avaya failed. I would hear silence after dialling then eventually a busy tone.

I have been scratching my head for a week now trying to work out what was wrong but debug utilities on the Avaya side are nothing like the Cisco ones.

I took to considering the differences between the ideal network topology as shown in the videos where the CME router is configured as a router on a stick, and my own topology, where I have a router with two interfaces. My router had it's own network for the IP phones whilst the Cisco IP Communicator (CIPC) client that I was using was not on this separate network.

I started thinking about source and destination IP addresses. Now whilst the CIPC client had no issues registering from another network presumably due to the ip source-address command under telephony-service. Yet this did not dictate what source IP address the router would use when talking to dial-peers.

Thus whilst I had configured my Avaya to talk to the CME router via it's dedicated NIC, this only worked for call signalling coming from the Avaya. Call signaling coming from CME to the Avaya had a different source IP than the trunk far end I had configured on the Avaya and thus the Avaya was rejecting the call attempt.

I simply updated the Avaya configuration to use the closest IP address (topology wise) and whalla it works. I can now dial from Cisco => Avaya and all details come across!

Lesson to be learned, don't assume things.