Cisco SPA509G Administration Guide - Page 132

Configuring SIP, SPCP, and NAT Network Address Translation (NAT) and Cisco IP Phones 4 • Insert VIA received: yes • Insert VIA rport: yes • Substitute VIA Addr: yes • Send Resp To Src Port: yes • STUN Enable: Choose yes. • STUN Server: Enter the IP address for your STUN server. STEP 5 Click the Ext tab. Configure the following: • NAT Mapping Enable: Choose yes. • NAT Keep Alive Enable: Choose yes (optional). NOTE Your ITSP may require the phone to send NAT keep alive messages to keep the NAT ports open permanently. Check with your ITSP to determine the requirements. STEP 6 Click Submit All Changes. NOTE You also need to configure the firewall settings on your router to allow SIP traffic. See "Configuring SIP," on page 105. Determining Whether the Router Uses 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, then 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. NOTE This procedure assumes that a syslog server is configured and is ready to receive syslog messages. Cisco Small Business SPA300 Series, SPA500 Series, and WIP310 IP Phone Administration Guide 131