Configuring SIP, SPCP, and NAT
Network Address Translation (NAT) and Cisco IP Phones
Cisco Small Business SPA 300 Series, SPA 500 Series, and WIP310 IP Phone Administration Guide 114
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 <number> 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 90.
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.
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