Call routing process
One of the functions of the VCS is to route calls to their appropriate destination. It does this by processing
incoming search requests in order to locate the given target alias. These search requests are received from:
n
locally registered endpoints
n
neighboring systems, including neighbors, traversal clients and traversal servers
n
endpoints on the public internet
There are a number of steps involved in determining the destination of a call, and some of these steps can
involve transforming the alias or redirecting the call to other aliases.
It is important to understand the process before setting up your
dial plan
so you can avoid circular references,
where an alias is transformed from its original format to a different format, and then back to the original alias.
The VCS is able to detect circular references. If it identifies one it will terminate that branch of the search and
return a “policy loop detected” error message.
How the VCS determines the destination of a call
The process followed by the VCS when attempting to locate a destination endpoint is described below.
1. The caller enters into their endpoint the alias or address of the destination endpoint. This alias or address
can be in a number of
different address formats
.
2. The destination address is sent from the caller’s endpoint to its local VCS (the VCS to which it is
registered).
3. Any
pre-search transforms
are applied to the alias.
4. Any
Call Policy
is applied to the (transformed) alias. If this results in one or more new target aliases, the
process starts again with the new aliases checked against the pre-search transforms.
5. Any User Policy (if
FindMe
is enabled) is applied to the alias. If the alias is a FindMe ID that resolves to
one or more new target aliases, the process starts again with all the resulting aliases checked against pre-
search transforms and Call Policy.
6. The VCS then searches for the alias according to its search rules:
l
A matching rule may apply a zone transform to the alias before sending the query on to its
Target
. A
Target
can be one of the following types:
o
Local Zone
: the endpoints and devices registered to the VCS.
o
Neighbor zone
: one of the VCS's configured external neighbor zones, or a DNS or ENUM lookup
zone.
o
Policy service
: an external service or application, such as a Cisco TelePresence Conductor. The
service will return some CPL which could, for example, specify the zone to which the call should be
routed, or it could specify a new destination alias.
7. If the search returns a new URI or alias (for example, due to a DNS or ENUM lookup, or the response from
a policy service), the process starts again: the new URI is checked against any pre-search transforms,
Call Policy and User Policy are applied and a new VCS search is performed.
8. If the alias is found within the Local Zone, in one of the external zones, or a routing destination is returned
by the policy service, the VCS attempts to place the call.
9. If the alias is not found, it responds with a message to say that the call has failed.
Cisco VCS Administrator Guide (X8.1.1)
Page 172 of 507
Dial plan and call processing
Call routing process
