Failed to Make Outbound Call - INVITE Packet Size Larger than Router MTU Requirement

Applicability

  • Firmware version: Any
  • Model: S-Series

Problem Description

User failed to make outbound calls. The PCAP log show the provider doesn't reply the 2nd INVITE packet sent by PBX.

mceclip0.png

Analysis

Further analysis show PBX got the ICMP - Fragmentation needed packet.

mceclip1.png

The MTU of the router required is 1394.

mceclip2.png

First INVITE MTU size is 1090 which is less than the router MTU requirement.

mceclip4.png

Second INVITE MTU size is 1426 which is more than the router requirement. 

mceclip3.png

Solution

  • On PBX side, you can try to minize the codec selection in the SIP trunk> Codec menu. In this we leave the alaw and ulaw, then issue is resolved.
  • On the router side, you might change the MTU side to a proper value.
Have more questions? Submit a request

0 Comments

Please sign in to leave a comment.