Using F5 Big-IP load balancer with OCS mediation server

One of the interesting things I have done recently is used a F5 Big-IP load balancer in conjunction with an OCS mediation server to load balance across multiple PSTN gateways. While this is a Microsoft unsupported configuration for OCS, it does indeed work and works very well. This configuration of course works only for outbound calls from the mediation to a gateway of some sort. In our configuration, we used it to alternate route outbound 911 across three gateways in a specific order. Something one mediation server is incapable of doing.

Using the F5, we created priority groups and placed one gateway in each one. At the mediation server, we pointed the PSTN gateway next hop to the F5 VIP for the priority groups. The F5 is capable of a SIP heartbeat check so it knows when one of the gateway goes down which then forces it to use the next priority group/gateway in the list. I am not sure what other applications this would be suitable for but with the current limitations surrounding the mediation servers ability to only contact one PSTN gateway I thought it was note worthy.


  1. Hi,

    did you have to configure any certificates on the F5 so that a call from the mediation server can reach the frontend servers?

  2. Hi Edi,

    The configuration I am refering to is about load balancing between a mediation server and a PSTN gateways. Not so much the interaction between the front end and medaitions. For the configuration I have outlined you do not require certificates installed.


  3. Hi Chris, we are thinking use a F5 boad balancing income call between Nortel PBX and OCS Mediation servers. Looks like just your reverse. But I am not sure if possible or need?

  4. I am pretty sure this would be possible to do but I am not sure why you would do it. To have both inbound and out bound you would need to create two VIP's on the F5. Havng never configured a Nortel PBX for such a task I am not sure what options you have available that are native for redundancy, multiple trunks for instance.

    For this purpose we were only requiring outbound and with our f5's already inplace it made sense to use them and not buy another gateway. In your case if you are looking for both inbound and outbound you may be better off with a qualified gateway rather than the F5's.


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