Cisco 8851, Administration Manual

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NAT Mapping with SIP-ALG Router
NAT mapping can be achieved by using a router that has a SIP Application Layer Gateway (ALG). By using
a SIP-ALG router, you have more choices in selecting an service provider.
NAT Mapping with the Static IP Address
You can configure NAT mapping on the phone to ensure interoperability with the service provider.
• You must have an external (public) IP address that is static .
• The NAT mechanism used in the router must be symmetric. for more information, see Determining
Symmetric or Asymmetric NAT, on page 62 .
Use NAT mapping only if the service provider network does not provide a Session Border Controller
functionality.
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 a value for the NAT Keep Alive Intvl field.
Step 4 Enter the public IP address for your router in the EXT IP 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.
Configure NAT mapping with STUN
If the service provider network does not provide a Session Border Controller functionality and if the other
requirements are met, it is possible to use Session Traversal Utilities for NAT (STUN) to discover the NAT
mapping. The STUN protocol allows applications operating behind a network address translator (NAT) to
discover the presence of the network address translator and to obtain the mapped (public) IP address (NAT
addresses) and the port number that the NAT has allocated for the User Datagram Protocol (UDP) connections
to remote hosts. The protocol requires assistance from a third-party network server (STUN server) located on
the opposing (public) side of the NAT, usually the public Internet. This option is considered a last resort and
should be used only if the other methods are not available. To use STUN:
• The router must use asymmetric NAT . See Determining Symmetric or Asymmetric NAT, on page 62
Cisco IP Phone 8800 Series Multiplatform Phones Administration Guide
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SIP and NAT Configuration