Cisco Cisco Expressway Maintenance Manual
External policy overview
The Cisco Expressway (Expressway) supports CPL (Call Processing Language) for implementing complex
policy decisions. CPL is designed as a machine-generated language and is not immediately intuitive; while
the Expressway can be loaded with CPL to implement advanced call policy decisions, complex CPL is
difficult to write and maintain.
policy decisions. CPL is designed as a machine-generated language and is not immediately intuitive; while
the Expressway can be loaded with CPL to implement advanced call policy decisions, complex CPL is
difficult to write and maintain.
The Expressway’s external policy feature allows policy decisions to be taken by an external system which
can then instruct the Expressway on the course of action to take (such as whether to fork a call and so on).
Call policy can now be managed independently of the Expressway, and can implement features that are
unavailable on the Expressway. The external policy server can make routing decisions based on data
available from any source that the policy server has access to, allowing companies to make routing
decisions based on their specific requirements.
can then instruct the Expressway on the course of action to take (such as whether to fork a call and so on).
Call policy can now be managed independently of the Expressway, and can implement features that are
unavailable on the Expressway. The external policy server can make routing decisions based on data
available from any source that the policy server has access to, allowing companies to make routing
decisions based on their specific requirements.
When the Expressway is configured to use an external policy server the Expressway sends the external
policy server a service request (over HTTP or HTTPS), the service will send a response back containing a
CPL snippet which the Expressway will then execute.
policy server a service request (over HTTP or HTTPS), the service will send a response back containing a
CPL snippet which the Expressway will then execute.
Using an external policy server
The main areas where the Expressway can be configured to use an external policy server are:
n
Call Policy (also known as Admin Policy) – to control the allowing, rejecting, routing (with fallback if calls
fail) and forking of calls.
fail) and forking of calls.
n
Search rules (policy can be applied for specific dial plan search rules).
Each of these areas can be configured independently of each other as to whether or not to use a policy
service. If a policy service is used, the decisions made by the policy service replace (rather than supplement)
those made by the Expressway.
service. If a policy service is used, the decisions made by the policy service replace (rather than supplement)
those made by the Expressway.
When configuring policy services:
n
Up to 3 external policy servers may be specified to provide resiliency (and not load balancing).
n
Default CPL can be configured, to be processed by the Expressway as a fallback, if the service is not
available.
available.
n
The status and reachability of the service can be queried via a status path.
External policy request parameters
When the Expressway uses a policy service it sends information about the call request to the service in a
POST message using a set of name-value pair parameters. The service can then make decisions based
upon these parameters combined with its own policy decision logic and supporting data.
POST message using a set of name-value pair parameters. The service can then make decisions based
upon these parameters combined with its own policy decision logic and supporting data.
The service response must be a 200 OK message with CPL contained in the body.
The following table lists the possible parameters contained within a request. It also indicates, where relevant,
the range of accepted values.
the range of accepted values.
Cisco Expressway Administrator Guide (X8.5.1)
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Reference material
External policy overview