By definition, subscriber IP addresses are at least routable in the access network. If the access network is a routed network,
subscriber IP addresses can be used to uniquely identify IP subscribers.
When using a subscriber IP address as the identifier, ISG assumes that the subscriber IP address is unique. If the access
network is deployed with Layer 3 load balancing, redundancy, or asymmetric routing, ISG also assumes that IP traffic from
the same IP subscriber may arrive at different access interfaces. To support this type of deployment, ISG assumes a single
IP address space for all access interfaces connecting to the same access network.
If there is a requirement to support several IP address spaces over a single physical access network, the access network
must use some Layer 2 encapsulation to create a separate logical access network for each IP address space. In this case, ISG
can still have a single IP address space for all the logical access interfaces that connect to a logical access network.
When subscriber IP addresses are private IP addresses, the access network must be able to route such subscriber traffic.
If the subscriber traffic is destined for the Internet, NAT must be performed.
For routed IP subscribers, the subscriber IP address serves as the key for an IP session. ISG associates IP traffic with
an IP session as follows:
In the upstream direction, the source IP address of an IP packet is used to identify the IP session. The source IP address
is the subscriber IP address.
In the downstream direction, the destination IP address of an IP packet is used to identify the IP session. The destination
IP address is the subscriber IP address.
If the IP subscriber is a VPN user, the subscriber IP address must be routable in both the global routing table and the VPN
routing table on ISG.
For an IP subnet subscriber, a subscriber IP address is defined as an IP prefix address instead of a /32 IP host address.
This IP prefix covers a range of IP addresses used by end users but represents a single logical IP subscriber for ISG. In
this deployment, all end users share the same connectivity and services provided by ISG.
To normalize the classification of IP subscribers that have different network masks, ISG uses the network mask in conjunction
with the subscriber IP address for routed IP subscribers.