SIP-Peer Trunk Registration Failed - OPTIONS Packet is Redirected to Incorrect Destination


  • Firmware version: Any
  • Model: S-Series

Problem Description

S100--> Mirotik Router--> SIP Peer-to-Peer trunk -->S300
S300 can be registered to S100, but S100 cannot be registered to S300.
1. Disable the "Qualify" of SIP trunk
To avoid the unstable local network connection, the “Qualify” option of SIP trunk is recommended to disabled, so that PBX will not base on the "Option" message to detect whether the other side still keeping alive or not.
But this issue still existing after taking this measure.
2. Get the PCAP packet from S100
  • S100 receives the "Option" packet from IP address x.x.100.128 but not S300's source IP address x.x.120.203.
  • S300‘s source IP address should be the IP address that lists in the Via filed, but the IP address x.x.100.128 is the IP address of local Router.
At this point, we can make sure the source IP address of S300 is modified by the network device, like Router or Switch. Customer may need to disable the SIP-ALG, SIPPassthrough even restart the Router.
3. Troubleshooting in the Router.
After verifying the source address with the IP of the via field of "OPTIONS" packet, S300 have been sending the "Option" packets to the dedicated trunk provider's private IP address, but not it's own public ip address, it's turn to be the issue.
  • S100 sends the "OPTIONS" packet to the ISP through its own private IP address of WAN port, the correct destination should be the public IP address of S300
  • S300 sends the "Option" packet to the ISP through it's own private IP address of WAN port, the correct destination should be the public IP address of S300
At this point, customer needs to make sure the network segments of LAN, WAN and ISP that had added to NAT's "Local Identification Box", so that packets that go out through the LAN port of PBX will take the public network address as the source address.
4. After correcting the global NAT settings in both PBXs:
  • S300 receives the "OPTIONS" request from S100, but the source IP address is  x.x.6.186 (Router's exit IP address)mceclip4.png
  • At the same time line, S100 sends the "OPTIONS" request to S300, but it does not response get any response from S300, such as "200 ok"
Brief conclusion:
The "OPTIONS" message and the "200 OK" message between the S100 and the S200, it's unable to reach out the correct destination base on the “Via" field in the message.
Disable related options on the router that may modify the source IP address of PBX.
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