Help with SIP trunk issue, no ACK for 200 OK

[carl_townshend]

Hi All

I am having issues with a SIP trunk between my CUCM and a Mitel system.

The calls work one way but not the other.

If the Mitel calls us to an SCCP phone, all works well.

When they call to a SIP phone the call does not work.

As you can see below 172.30.208.17 is our subscriber, 10.80.16.11 is the Mitel.

The Mitel sends us an invite, we then send a 200 OK 7 times, but we get no ACK from the Mitel, the Mitel then cancels the call.

Why are we sending 7 200 OK messages?

Why would we not get an ACK back from the Mitel?

Is it also normal for the frame sizes to be large (1450) etc

Cheers

[Roger Kallberg]

Isn’t this a duplicate with this post?

https://community.cisco.com/t5/ip-telephony-and-phones/issue-with-sip-trunk-between-cucm-and-mitel-phone-system/m-p/4147890#M391442

[carl_townshend]

Yes but no one appears to be answering that and I also have slightly different info

[Roger Kallberg]

You did get responses, but if none of them helped I would recommend you to reach out to TAC to get professional help with this.

Did you ever reach out to the Mitel guys to see why that system doesn’t respond correctly on the communication sent?

[carl_townshend]

Hi Roger

No, they are also stumped.

Am I right in saying the CUCM will act as the B2BUA and relay all the messages from the phone to the other end etc ?

[Roger Kallberg]

Yes that would be accurate. At least for the signaling part. For RTP it would flow directly between the phone and whatever you have that interlinks the Mitel system to your Cisco UC landscape.

How is this interlinked, do you have a SBC in between?

[carl_townshend · Accepted solution]

Hi

Just for info, I finally got to the bottom of the issue, it was not the CUCM, there is a known issue with the Mitel system, there is a default mtu of 1400 set on the Mitel server, so this drops the 200 OK SDP messages as they were around 1450 bytes.

The fix is for Mitel TAC to amend the registry to 1500

If this is a known issue then why the hell don't Mitel just make 1500 the standard setting, I find this ridiculous!

Cheers

[Roger Kallberg]

Thank you for updating the community of what the solution for this was.

[nnikolaev]

the solution with MTU is the root one. As a workaround, there were 2 options for me - switching trunk to work on the TCP or making the name in the DN settings short. This made it possible to fit into the size of the MTU

I'm leaving this here for myself for the future