I was commiserating with a co-worker recently about how challenging SIP trunks can be when they’re new to you. Specifically we were talking about what port and protocol a device listens on, and what port and protocol the peer device will listen on. Somehow, I agreed that I’d write a blog post about this topic, so here it is!
Our Gateway has an FQDN, which must resolve to the IP of the peer device. You could also use the IP address here. In larger environments, a meaningful FQDN can be helpful vs trying to memorize or find your (probably outdated) list of IP addresses and gateways.
At the bottom, we see a list of trunks that are configured for this gateway in SfB. You can have more than one, but that’s a more advanced configuration and we’re shooting for basic understanding. Similarly, you can ignore the IPv4 addresses and Alternate Media entries.
We’ve got a name for the trunk, which also happens to be the FQDN of the gateway. This is the default that SfB creates, you can change it during (but not after) the creation process.
The PSTN Gateway line has the FQDN and SfB Site (in brackets, HQ here) of the gateway/peer device.
The Listening port and SIP Transport Protocol indicate what protocol and port the gateway device is listening on. We’ll see that in the gateway configuration in a minute.
Mediation Server indicates the FQDN of the mediation server (or mediation pool, if you’ve got a pool of them).
As promised, here’s the configuration on the other end of the trunk, in this case from an AudioCodes SBC, showing what port it’s listening on:
Here you can see that the device is listening on TCP port 5060. Note that it also appears to be listening on port 0 for UDP and TLS. That’s just AudioCodes configuration for “not listening”
And here’s the entry from the Proxy Set on the AudioCodes SBC showing the SfB mediation pool, and what port and protocol it’s listening on. Proxy Sets on the AudioCodes devices are the equivalent of the Gateway in SfB topology.
Here you can see that the AudioCodes expects the peer device at 10.12.1.101 to be listening on TCP port 5068.
Okay, so why do I have an IP address here, but an FQDN in SfB? Well, you can use either, as we discussed above. However, one thing that we do stumble across (too) often, is someone poking the DNS records for the SfB mediation pool. They don’t understand that “FEPool1.example.com” *should* in fact have one entry per front-end server, so they decide to do some “cleanup”.