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Cisco TelePresence Video Communication Server (VCS)

Traversal call license usage on a Cisco VCS

Traversal call license usage on a Cisco TelePresence Video Communication Server (Cisco VCS)

Any call where the Cisco VCS takes the media as well as the signaling is a traversal call and will use a traversal call license on that Cisco VCS. The following types of call require the Cisco VCS to take the media, and are therefore traversal calls:

  • for a Cisco VCS Control, calls to or from a traversal server (known as firewall traversal calls)
  • for a Cisco VCS Expressway, calls to or from a traversal client (firewall traversal calls); traversal clients include other Cisco VCSs, gatekeepers, Border Controllers, or traversal-enabled endpoints
  • calls that are gatewayed (interworked) between H.323 and SIP on the local Cisco VCS
  • calls that are gatewayed (interworked) between IPv4 and IPv6 on the local Cisco VCS
  • for Cisco 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)
  • calls that have a media encryption policy applied
  • encrypted calls to and from Microsoft OCS Server 2007 / Lync Server 2010 where the Microsoft OCS/Lync B2BUA is not being used. If the B2BUA is used, the B2BUA application always takes the media but the call is not classified as a VCS traversal call and does not consume a traversal call license (it may still consume a non-traversal license if the VCS takes the call signaling). Note that the Enhanced OCS Collaboration option key is required for encrypted calls to OCS/Lync.

All other calls are non-traversal (local) calls.

Note that a call may be a traversal call on one Cisco VCS but a local (non-traversal) call on another Cisco VCS in the call route, depending on whether each Cisco VCS in the route is required to take the media.

Traversal calls use more resource than non-traversal calls, and the numbers of each type of call are licensed separately. The VCS has one license for the maximum number of concurrent traversal calls it can take, and another for the maximum number of concurrent non-traversal calls. You can increase the number of each type of call available on your VCS (or VCS cluster) by purchasing and installing the appropriate option key. While every deployment is different, as a guideline we recommend that your system has a 10:1 ratio of registrations to concurrent call licenses.

When do calls on a Cisco VCS Expressway use a local (non-traversal) call license?

Usually a Cisco VCS Expressway needs a traversal call license to route calls, even for calls between devices directly registered to it. This is because endpoints behind firewalls need the Cisco VCS Expressway to receive and forward media in order to guarantee that the media is routable between the endpoints in the call.

The only situation where a call passing through a Cisco VCS Expressway is classified as a local (non-traversal) call is when:

  • there is no SIP to H.323 interworking required, and
  • there is no IPv4 to IPv6 interworking required, and
  • the call is not being routed from the Cisco VCS Expressway through a Traversal Zone, and
  • neither endpoint is traversal enabled (in other words, neither endpoint has registered with Assent or H.460.18/19 and neither endpoint has made the call requesting Assent or H.460.18/19), and
  • the call is received from and sent to:
    • an endpoint or a Neighbor Zone directly connected to the Wide Area Network, or
    • an endpoint or a Neighbor Zone behind a firewall that has a SIP or H.323 application-level gateway (ALG) that supports video calling (which makes the signaling appear as though the endpoint is directly connected to the Wide Area Network), or
    • locally registered ICE-enabled endpoints (if using Cisco VCS version X5 or later)

    Note: to aid firewall traversal, all Cisco TelePresence endpoints are traversal enabled and so a traversal license will always be needed when at least one of the endpoints involved in the call is a Cisco TelePresence endpoint.

    Note: a non-traversal call on a Cisco VCS Expressway will consume a traversal license if there are no non-traversal call licenses available (if using Cisco VCS version X5 or later). In this situation, the call will remain a non-traversal call � the VCS Expressway will not take the media, even though it is using a traversal license.


This article applies to the following products:

  • Cisco Video Communication Server

August 23, 2012TAA_KB_168