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Configuring ISG Access for IP Subscriber Sessions
Information About ISG Access for IP Subscriber Sessions
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Service Model for VRF Transfers
A primary service is a service that contains a network-forwarding policy (such as a VRF) in its service
definition. Only one primary service at a time can be activated for a session. A secondary service is any
service that does not contain a network-forwarding policy.
When a subscriber for whom a primary service has already been activated tries to select another primary
service, ISG will deactivate all current services (including the current primary service) and activate the
new primary service, and hence switch the VRF.
When a subscriber for whom a primary service has already been activated tries to select a secondary
service, the action taken by ISG depends on whether the secondary service is part of a service group. A
service group is a grouping of services that may be simultaneously active for a given session. Typically,
a service group includes one primary service and one or more secondary services.
Table 1 describes the
action that ISG will take when a subscriber selects a secondary service.
Benefits of Dynamic VPN Selection
The need for switching of a subscriber session between routing and forwarding domains (also called
network services) occurs frequently in markets where so-called equal access networking must be
supported. Equal access networking is often mandated by regulatory rules stating that an access provider
should allow service providers equal access to a retail subscriber network. ISG dynamic VPN selection
facilitates equal access networking by allowing subscribers to transfer between network services.
IP Session Termination
An IP session may be terminated in one of the following ways:
• DHCP Lease Expiry or DHCP Release from client
If DHCP is used to detect a new session, its departure may also be signaled by a DHCP event.
• Application stop
An application command that is used to terminate a session. The application stop command is
typically used to terminate a session when a subscriber initiates an account logoff from a web portal.
An application stop may also result from the actions of an administrator, such as action taken in
response to rogue behavior from a subscriber.
Table 1 ISG Activation Policy for Secondary Services
Primary Service Characteristics Secondary Service Characteristics Resulting Behavior at ISG
Primary service with no service
group attribute Secondary service with service
group Do not bring up the secondary
service.
Secondary service with no service
group Bring up the secondary
service.
Primary service with service
group attribute
Secondary service with different
service group Do not bring up the secondary
service.
Secondary service with same
service group Bring up the secondary
service.
Secondary service with no service
group Bring up the secondary
service.