sip phones are not working

[sebastian.cirnu]

Hello, I want to ask for some help, cause I don't understand anymore the behaviour of my phones. I have 4 Cisco 8841 and 1 Cisco conference phone 8831. The phones are registering very well with the Cisco 4331 ISR router and I can place inside calls without any problems. 

The problems comes for the outbound calls to a PSTN line. The Cisco 4331 router connects via separate VLAN to another Cisco 2811 router that acts as CUBE.  I checked all the dial-peer configuration and all is ok. The inbound calls from the PSTN are working great, but when I try to place a call outbound, the conference phone is working without problems, the 8841 ones are not. The configuration is the same for all the phones regarding the dial-peer for outbound call.

I don't understand anymore what is the problem, maybe one of you here can give me a hint or a place where should I start looking.

Thanks in advance!

[jabritt]

Post your configuration, firmware for the 8841 phones.

What is the call flow and protocols in use ?

[sebastian.cirnu]

Hello, thank you for your reply. I managed to figure it out. I started capture with wireshark on both ends of the trunk and see that the packet size for the invite request was too big (like 1460). I'm still asking me the question why in the name of the lord the conference phone (8831) has the packet size at about 1200 and the other ones (8841) has the size over 1400. The problem was that our trunk was made through a EVPL line between our datacenter and office headquarters and the MTU set over the EVPL was 1380 and that's why the packet was dropped. Now, after setting the MTU everything works great, but I still need to figure it out why the invite packet is so big, but I think that i can decrease the MTU on the both ends and let the packet be frangmented, because the invite is a TCP request and it should work ok with fragmentation.

In the future if anyone of you face this kind of problem just monitor the packet size and the MTU.

Thank you!

[jabritt]

Well normally size MTU is 1550 so its still smaller then the normal MTU. Depending upon the phone configuration, trunk configuration, parameters being relayed in initial invite the size may vary, and different phones may support different features.