This article relates to Cisco TelePresence Video Communication Server Expressway.
Q. When do calls on a VCS use a traversal call license?
A. Any call where the VCS takes the media as well as the signaling is a traversal call and will use a traversal call license on that VCS. The following calls require the VCS to take the media, and are therefore traversal calls:
for a VCS Control, calls to or from a traversal server (i.e. firewall traversal calls)
for a VCS Expressway, calls to or from a traversal client (i.e. firewall traversal calls); traversal clients include other VCSs, gatekeepers, Border Controllers, or traversal-enabled endpoints
calls that are gatewayed (interworked) between H.323 and SIP on the local VCS
calls that are gatewayed (interworked) between IPv4 and IPv6 on the local VCS
for VCSs with Dual Network Interfaces enabled, calls that are inbound from one LAN port and outbound on the other
a SIP to SIP call when one of the participants is behind a NAT (unless both endpoints are using ICE for NAT traversal)
Also, a non-traversal call on a VCS Expressway will consume a traversal license if there are no non-traversal call licenses available (if using VCS version X5 or later).
Note that a call may be a traversal call on one VCS but a local (non-traversal) call on another VCS in the call route, depending on whether each VCS in the route is required to take the media.