Before You Begin - ZyXEL Communications NXC5200 User Manual

SIP ALG
• SIP clients can be connected to the LAN. A SIP server must be on the WAN. The
SIP server and SIP clients must be in different networks.
• Using the SIP ALG allows you to use bandwidth management on SIP traffic.
• The SIP ALG handles SIP calls that go through NAT or that the NXC routes. You
can also make other SIP calls that do not go through NAT or routing. Examples
would be calls between LAN IP addresses that are on the same subnet.
• The SIP ALG supports peer-to-peer SIP calls. The firewall (by default) allows
peer to peer calls from the LAN zone to go to the WAN zone and blocks peer to
peer calls from the WAN zone to the LAN zone.
• The SIP ALG allows UDP packets with a specified port destination to pass
through.
• The NXC allows SIP audio connections.
• You do not need to use STUN (Simple Traversal of User Datagram Protocol
(UDP) through Network Address Translators) for VoIP devices behind the NXC
when you enable the SIP ALG.
• Configuring the SIP ALG to use custom port numbers for SIP traffic also
configures the application patrol to use the same port numbers for SIP traffic.
Likewise, configuring the application patrol to use custom port numbers for SIP
traffic also configures SIP ALG to use the same port numbers for SIP traffic.
Peer-to-Peer Calls and the NXC
The NXC ALG can allow peer-to-peer VoIP calls for both H.323 and SIP. You must
configure the firewall and NAT (port forwarding) to allow incoming (peer-to-peer)
calls from the WAN to a private IP address on the LAN (or DMZ).
VoIP Calls from the WAN with Multiple Outgoing Calls
When you configure the firewall and NAT (port forwarding) to allow calls from the
WAN to a specific IP address on the LAN, you can also use policy routing to have
H.323 (or SIP) calls from other LAN or DMZ IP addresses go out through a
different WAN IP address. The policy routing lets the NXC correctly forward the
return traffic for the calls initiated from the LAN IP addresses.

15.1.3 Before You Begin

You must also configure the firewall and enable NAT in the NXC to allow sessions
initiated from the WAN.
NXC5200 User's Guide
Chapter 15 ALG
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