What are Local SIP Ports and Duplicate Ports?
Local SIP Ports
Setting local SIP ports allow you to define what port the phone will be assigned to in the NAT process. This typically will be the local port, however, depending on the firewall in use, this may be used for the external port assignment as well. The router or firewall may override this setting over time.
Typically the purpose of setting local SIP ports and defining what source port will be used for the phones serves two purposes:
- Ensuring stability in the port assignment by generally keeping the device on a single defined port that will not change (can be overridden by a firewall at times).
- Ensuring unique ports are assigned – if multiple phones use the same port, our server will broadcast the traffic to the IP and port, but the traffic may not route correctly if the firewall does not translate it correctly.
Duplicate Ports - NAT Mapping Issues
This means that multiple devices are using the same port assignment defined by the router or firewall NAT process to communicate with the PBX. This will cause multiple devices to communicate over the same socket. Typically this will manifest both over the internal port and the external port.
This can cause a multitude of issues including, but not limited to:
- Device connectivity issues
- Call drops
- Ring group call failures
Duplicate Ports Example:
In this example, traffic is sent to the IP and port. While we know which device we are sending traffic to, we depend on the socket that is created (A socket is the IP and port of the destination and the source) at this point is dependent upon the firewall or router to send the traffic to the correct destination. Most devices will not do this correctly when multiple phones are using the same port.
Unique Ports Example:
By having unique port assignments, the firewall is able to route traffic correctly once it is received from the 8x8 servers. These unique port assignments create unique sockets allowing for correct routing from the router or firewall.
This issue can be resolved on most firewalls and routers in use. However, there are always exceptions in which the NAT issue may not be able to be addressed. Additionally, certain firmware revisions may change how the NAT process functions causing this issue to occur on devices it did not previously.
Below is a list of devices that may have this issue among others if left unconfigured:
Network Appliances with Potential Compatibility Issues for VoIP Services