Office Communications Server 2007 – don’t use multiple IPs on the same NIC

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I have been burned 3 times by OCS when trying to bind multiple IP addresses to the same NIC. Here are the three scenarios:

1. Trying to use the same NIC for the internal and external IP address on the Edge Server
   1. We had only 1 available nic, so I put both internal and external IPs on the one NIC
   2. Communication failed totally – couldn’t get an external user to sign in from the outside through the edge
   3. Finally got another NIC for the server. We split the IPs to different NICs
   4. Success.
2. Trying to use multiple IP addresses for the Mediation Server
   1. Had 2 IPs (both on the same subnet) for mediation: one for gateway listening / one for OCS listening
   2. Calls through the mediation server to the PSTN gateway were failing. Callers would hear just dead silence. PSTN gateway was saying "can’t find an available trunk" and returning a SIP 404 error
   3. I looked in the PSTN gateway’s logs and saw that calls were coming in from the mediation server, but from the wrong IP! They were coming from the OCS listening side
   4. Change mediation server to just use the same IP for PSTN and OCS listening
   5. Success.
3. Had a Medaition server with multiple IP addresses. I was only using 1 of them for Mediation
   1. Everything was working fine and dandy for about 3-4 months
   2. We stopped and started the services on all OCS servers (including mediation) during maintenance
   3. All of a sudden, no inbound calls would route.
   4. Outboud calls were even weirder: the destination phone would ring, but no voice in either direction
   5. The mediation server showed a ton of SIP 200 errors: malformed proxy header
   6. I finally installed Wireshark on the mediation server and took a peek. It was disgusting! OCS was trying to use any/all IP addresses on the NIC for communication. The sniffer cap was a total mess – OCS trying to do STUN binding requests to everything it could find. Ugly.
   7. Moved Mediation to a server with just 1 NIC, 1 IP address.
   8. Success.

Lesson learned: NEVER assign multiple IP addresses to a NIC on an OCS server. You may get away with it for a while (see example 3 above) but it will kill you in the end.