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Traversal and non-traversal calls


Can somebody explain in simple words what are traversal and non-traversal calls?, what is the difference? And also Does the VCS control below need traversal licenses?,


VCS Control

Thanks in advance.


Accepted Solutions
Tomonori Taniguchi
Cisco Employee

If you have two C-series Endpoint (i.e. CTS-SX20) both registered on VCS Control and also Endpoint able to reach out directly each other (has routing), then in general the call will treat as non-traversal call, so no traversal license will be required.

However if those Endpoint calling each other but using different protocol (i.e. one Endpoint registered on VCS Control only with H323 and other Endpoint registered on VCS only with SIP), or different IP version (i.e one with IPv6 and other with IPv4 registration), then VCS Control will remain the media routing in order to handle such interworking scenario which will require traversal call license.

View solution in original post

Tomonori Taniguchi
Cisco Employee

On VCS Control…,

Non-Traversal Call: VCS only involve and take call signaling

Traversal Call: VCS involve both call signaling and media routing.

- Traversal Call between VCS-Expressway

- IPv4/IPv6 Interworking Call

- SIP/H323 Interworking Call

- B2BU Call (i.e. MS Lync integration)


Will a SIP to SIP(P2P) call between Jabber and Endpoint disconnect when VCS loss network connection or failure? (both registered to same VCS)

Will a SIP to SIP(P2P)call between endpoint and endpoint disconnect when VCS loss network connection or failure?  (both registered to same VCS)

Will a SIP to SIP (P2P) call between endpoint and endpoint disconnect when VCS loss network connection or failure? (one endpoint registered to VCSc and other is external endpoint call travel thru VCSe)

All scenario, call will disconnect either next SIP transaction or call TTL.

Thanks All!

1. How to know or calculate the required traversal and non-traversal license?

2. How the VCS calculate the traversal and non-traversal license utilization in percentage(%) in status page?

Every call that goes through the VCS will consume either a traversal or non-traversal license, the type of license the call consumes depends on how the call is made and if the media (audio and video) is routed through the VCS, take a look at VCS Traversal and Non-Traversal Call License Usage.

The percentages on the status page display the current and peak license usage based on the available licenses for each license type installed on the VCS.

As Tomonori has mentioned, if the VCS loses network connectivity, all calls that are either registered or simply route their call through that particular VCS will disconnect, this is because the VCS handles the signalling and in some cases the media between endpoints.

what if we have cluster of five VCS and one VCS fails which had couple of active calls and registration.

I understood that only the Traversal call will drop when VCS fails since the media and signal flowing thru VCS and Non-Traversal call media would flow separate.

The active calls on the VCS that fails will disconnect, if your DNS is setup correct, the endpoints will then register with another VCS in the cluster.

In which scenarios rtp-media will flow thru the VCS?

When the rtp-media flows thru VCS will the video and audio quality degrade ?

In point to point call after signaling will the VCS continues to send signal between endpoints or will there be live check between VCS and endpoints?