Linksys SPA942 Cisco Small Business Pro SPA and Wireless IP Phone Administrati - Page 112

5 Configuring SIP, SPCP, and NAT Network Address Translation (NAT) and Cisco IP Phones NAT Mapping with Session Border Controller It is strongly recommended that you choose an ITSP that supports NAT mapping through a Session Border Controller. With NAT mapping provided by the ITSP, you have more choices in selecting a router. NAT Mapping with SIP-ALG Router If the ITSP network does not provide a Session Border Controller functionality, you can achieve NAT mapping by using a router that has a SIP ALG (Application Layer Gateway). The WRV200 router is recommended for this purpose, although any router with a SIP-ALG can be used. By using a SIP-ALG router, you have more choices in selecting an ITSP. Configuring NAT Mapping with a Static IP Address If the ITSP network does not provide a Session Border Controller functionality, and if other requirements are met, you can configure NAT mapping to ensure interoperability with the ITSP. Requirements • You must have an external (public) IP address that is static. • The NAT mechanism used in the router must be symmetric. See "Determining Whether the Router Uses Symmetric or Asymmetric NAT," on page 113. • The LAN switch must be configured to enable Spanning Tree Protocol and Port Fast on the ports to which the SPA devices are connected. NOTE Use NAT mapping only if the ITSP network does not provide a Session Border Controller functionality. STEP 1 Click Admin Login and advanced. STEP 2 Click the SIP tab. 110 Cisco SPA and Wireless IP Phone Administration Guide