Background

Recently I implemented a dedicated Lync Mediation server with two NIC’s. In this case I had to use a dedicated Mediation server because the IP Trunk provider was direct SIP. The Mediation server came with one NIC in the server subnet and the other NIC in the telephony subnet.

The problem

After implementing the Lync Topology correct and validated ok, we had one way audio with PSTN calls or no audio at all. All the calls got connected though. I started deep diving into logs and network configuration. While searching the web I found some very good articles about this regarding OCS 2007 R2. I found that there is little or no change in how to configure the Lync Mediation server versus configuring the OCS Mediation server network vise.

The Symptoms

During troubleshooting I saw the following errors:

• Event ID 25073
  • The Mediation Server service has received a call that does not support comfort noise from the Gateway peer
• In the SDP packet on the gateway side we saw that RTP traffic was proposed to go over the server subnet IP and not the Gateway subnet IP
  • No audio could be established because of routing issues
• RTP traffic from the gateway came from an other IP than the SIP IP
  • The Gateway had a media proxy with a different IP than the SIP IP

Reading material

I found these article good reading during the troubleshooting

OCS Mediation Server NIC Configuration: http://blogs.pointbridge.com/Blogs/schertz_jeff/Pages/Post.aspx?_ID=55
Forum discussion about two NIC’s in same subnet: http://social.microsoft.com/Forums/en/communicationsservertelephony/thread/6abb5010-5fd3-4940-8b91-0d2b0184a9fe
Communications Server Mediation Server: Dual NIC Issue: http://technet.microsoft.com/en-us/library/ff518437.aspx

The solution

Make sure the following is true

• Default gateway is only configured on the NIC in the server subnet, under IPv4 IP configuration
  • Use no default gateway on the Voice Gateway NIC
• Make sure that the Voice Gateway NIC is on top when running ipconfig
  • If its not, make it so

• Add static persistent routes for both the SIP and RTP IP against the correct NIC id
  • route ADD 77.88.99.80 MASK 255.255.255.255 10.150.80.241 IF 13 -p
  • route ADD 77.88.99.83 MASK 255.255.255.255 10.150.80.241 IF 13 -p
• I was also considering the Strong Host model, but that is when several NIC’s need to share a default gateway, don’t think it applies here
  • but just in case, here is the commands to disable it
    • netsh interface ipv4 set interface 13 weakhostsend=enabled
    • netsh interface ipv4 set interface 13 weakhostreceive=enabled
  • Read this article to learn more about it: http://blogs.pointbridge.com/Blogs/schertz_jeff/Pages/Post.aspx?_ID=78

Conclusion

In this case it was the missing route to the RTP media proxy IP that was missing. The symptom of the calls being established but no audio is evidence of that SIP traffic is going through ok, but RTP traffic is not being routed correct. The wrong IP being suggested was also evidence towards routing issues. Adding the missing route solved the problem. The Event ID 25073 also disappeared when the routing issue was resolved.

Regarding dedicated Lync Mediation Servers they should have a single NIC when the Voice Gateway is in the same subnet as the Lync Server themselves. Two NIC’s should only be considered when they can be placed in totally different subnets.