Cisco IOS XE, Configuration Manual

Page: 92 / 358

Configuring ISG Access for IP Subscriber Sessions
Information About ISG Access for IP Subscriber Sessions
8
When the access network is a Layer 2 connected network, a subscriber IP address can be either native or
foreign to an access interface. A native subscriber IP address is one that belongs to the subnet
provisioned on the access interface. A foreign subscriber IP address is one that does not belong to the
subnet provisioned on the access interface. A foreign subscriber IP address could result when a retail
ISP assigns an IP address to the IP subscriber from its own IP address allotment, which is different from
the wholesale ISPs, or when an IP subscriber with a static IP address that is native in the home access
network roams to a foreign access network. To support IP subscribers with foreign IP addresses, ISG
must be able to respond to Address Resolution Protocol (ARP) requests that originate from foreign IP
addresses with a MAC address of the ISG itself. Because the access network is Layer 2 connected, ISG
maintains an adjacency to every subscriber.
When the access network is a routed network, a subscriber IP address must be routable in the access
network; otherwise, subscriber traffic will never be able to reach ISG. ISG may not have an adjacency
for each subscriber in this case, but rather an adjacency of the next hop toward a subscriber. The next
hop is determined by the routing process on ISG.
Overlapping IP Addresses
When an access network is deployed without VPN capability, the IP address space in the access network
is shared among all IP subscribers. When the IP addresses are assigned dynamically, care must be taken
to ensure that these addresses do not overlap. In cases where overlapping IP addresses are assigned to IP
subscribers intentionally, the access network should use a Layer 2 separation mechanism to differentiate
the IP address spaces. For example, the access network may put each IP address space in a different
VLAN.
In cases in which the access network serves both local IP subscribers and roaming users, the static private
IP address of a roaming subscriber may overlap the native private IP address of another subscriber. For
example, a public wireless hot spot that generally assigns dynamic IP addresses might want to provide
access to occasional roaming users with statically configured IP addresses. To support this special
overlapping condition, all IP subscribers must be in a Layer 2 connected access network in which
overlapping MAC addresses do not exist. In this case, IP subscribers can be distinguished using MAC
addresses.
ISG Subscriber IP Address Assignment Using DHCP
When ISG is in the path of DHCP requests (as either a DHCP server or a DHCP relay), ISG can influence
the IP address pool and DHCP server that are used to assign subscriber IP addresses. To enable ISG to
influence the IP addresses assigned to subscribers, associate a DHCP address pool class with an address
domain. The DHCP address pool class must also be configured in a service policy map, service profile,
or user profile that is associated with a subscriber. When a DHCP request is received from a subscriber,
DHCP uses the address pool class that is associated with the subscriber to determine which DHCP
address pool should be used to service the request. As a result, on a per-request basis, an IP address is
provided by the local DHCP server or relayed to a remote DHCP server that is defined in the selected
pool.
IP Subscriber Identity
IP subscriber identity is closely related to IP session initiation because ISG must uniquely identify an IP
subscriber at the moment that it creates the IP session. However, the need to identify an IP subscriber
goes beyond the session initiation phase. The following sections describe how ISG uniquely identifies
IP subscribers: