For subscriber packets to reach a network, some form of forwarding must be specified for a subscriber session. A traffic policy
that allows packets to be routed or forwarded to and from an upstream network is known as a network forwarding policy
Where the network forwarding policy type is routing, forwarding decisions are made at Layer 3, and a VRF (Virtual Routing
and Forwarding) identifier must be specified to indicate which routing table should be used to make the routing decision (each
VRF represents an independent routing context within a single router). Where the network policy type is forwarding, forwarding
decisions are made at Layer 2, which means that all subscriber packets are forwarded to and from a single virtual endpoint
within the system. This virtual endpoint represents a Layer 2 tunnel, and a tunnel identifier determines which tunnel should
be used. If a network forwarding policy is not specified, the global routing table will be used to route traffic.
An ISG service that includes a network forwarding policy is known as a primary service
. Primary services are mutually exclusive and may not be active simultaneously. Upon activation of a new primary service,
ISG will deactivate the existing primary service and any other services dependent on the existing primary service through
association with a service group.