Cisco Cisco TelePresence Video Communication Server Expressway
To: <sip:64.100.0.20>
Max-Forwards: 70
Content-Type: application/sdp
Content-Length: 2825
v=0
s=-
c=IN IP4 10.0.10.2
c=IN IP4 10.0.10.2
b=AS:2048
…
…
…
As can be seen from the example above, endpoint B will see that the SIP INVITE was received from IP 64.100.0.10 (NAT
router), so the endpoint will know where to send its reply messages for the INVITE itself.
router), so the endpoint will know where to send its reply messages for the INVITE itself.
The c-line within the SDP of the SIP INVITE is however still set to c=IN IP4 10.0.10.2, which means that endpoint B will
attempt to send RTP media to the IP address 10.0.10.2, an address which is not routable on the Internet.
attempt to send RTP media to the IP address 10.0.10.2, an address which is not routable on the Internet.
The result in this scenario will therefore be that endpoint A will never receive media sent by endpoint B (while endpoint B
will normally receive media from endpoint A, since endpoint B is assigned with a publicly routable IP address).
will normally receive media from endpoint A, since endpoint B is assigned with a publicly routable IP address).
Similar behavior will be seen in H.323 calls, since H.323 uses the same principles as SIP in terms of embedding IP
address and port references within the message payload.
address and port references within the message payload.
How Does VCS Expressway Address This Challenge?
To ensure that call signaling and media connectivity remains functional in scenarios where the VCS Expressway is
deployed behind a NAT (as in the example above), the VCS Expressway will have to modify the parts of SIP and H.323
messages which contain references to its actual LAN2 network interface IP address (10.0.10.2) and replace these with
the public NAT address of the NAT router (64.100.0.10).
deployed behind a NAT (as in the example above), the VCS Expressway will have to modify the parts of SIP and H.323
messages which contain references to its actual LAN2 network interface IP address (10.0.10.2) and replace these with
the public NAT address of the NAT router (64.100.0.10).
This can be achieved by enabling Static NAT mode on selected network interfaces on the VCS Expressway. The Static
NAT mode feature on the VCS Expressway is made available with the Advanced Networking option key.
NAT mode feature on the VCS Expressway is made available with the Advanced Networking option key.
This option key allows the use of two network interfaces (LAN1 and LAN2), and on a VCS Expressway it allows Static
NAT mode to be enabled on one or both of these interfaces. You do not have to use both interfaces, but we recommend
that you do. If you choose to use a single interface, and enable static NAT on that interface, read
NAT mode to be enabled on one or both of these interfaces. You do not have to use both interfaces, but we recommend
that you do. If you choose to use a single interface, and enable static NAT on that interface, read
When static NAT has been enabled on an interface, the VCS will apply static NAT for all outbound SIP and H.323 traffic
for this interface, which means that H.323 and SIP devices have to communicate with this interface using the static NAT
address rather than the local interface address.
for this interface, which means that H.323 and SIP devices have to communicate with this interface using the static NAT
address rather than the local interface address.
When the Advanced Networking key is installed on the VCS Expressway, the IP configuration page (System > Network
interfaces > IP) has additional options, allowing the user to decide whether to Use dual network interfaces, to nominate
which interface is the External LAN interface, to enable Static NAT mode on selected interfaces and configure an IPv4
static NAT address for each interface.
interfaces > IP) has additional options, allowing the user to decide whether to Use dual network interfaces, to nominate
which interface is the External LAN interface, to enable Static NAT mode on selected interfaces and configure an IPv4
static NAT address for each interface.
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Cisco VCS Expressway and VCS Control - Basic Configuration Deployment Guide