Determining Symmetric Or Asymmetric Nat - Cisco 8851 Administration Manual

SIP and NAT Configuration
• A computer running STUN server software is available on the network. You can also use a public STUN
server or set up your own STUN server.
Procedure
Step 1 In the phone web user interface, navigate to Admin Login > advanced > Voice > SIP.
Step 2 In the NAT Support Parameters section, set Handle VIA received, Insert VIA received, Substitute VIA
Addr, Handle VIA rport, Insert VIA rport, and Send Resp To Src Port fields to Yes.
Step 3 In the NAT Support Parameters section, set STUN Enable field to Yes.
Step 4 Enter the IP address for your STUN server in the STUN Server field.
Step 5 Click the Ext(n) tab.
Step 6 In the NAT Settings section, set NAT Mapping Enable to Yes.
Step 7 (Optional) Set NAT Keep Alive Enable to Yes.
The service provider might require the phone to send NAT keep alive messages to keep the NAT ports open.
Check with your service provider to determine the requirements.
Step 8 Click Submit All Changes.
What to Do Next
Configure the firewall settings on your router to allow SIP traffic.

Determining Symmetric or Asymmetric NAT

STUN does not work on routers with symmetric NAT. With symmetric NAT, IP addresses are mapped from
one internal IP address and port to one external, routable destination IP address and port. If another packet is
sent from the same source IP address and port to a different destination, a different IP address and port number
combination is used. This method is restrictive because an external host can send a packet to a particular port
on the internal host only if the internal host first sent a packet from that port to the external host.
This procedure assumes that a syslog server is configured and is ready to receive syslog messages.
To Determine Whether the Router Uses Symmetric or Asymmetric NAT:
Procedure
Step 1 Verify that the firewall is not running on your PC. (It can block the syslog port.) By default, the syslog port
is 514.
Step 2 Click Voice > System and navigate to Optional Network Configuration.
Step 3 Enter the IP address for the Syslog Server, if the port number is anything other than the default, 514. It is not
necessary to include the port number if it is the default.
The address and port number must be reachable from the Cisco IP phone. The port number appears on the
output log file name. The default output file is
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(if port number was not specified).
syslog.514.log