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Dynamic Routes for Dynamic IP Addressing
8000-A2-GB21-20
Alternatively, NSPs can administer IP addresses to the end users dynamically
(automatically) rather than statically (manually).
The dynamic IP addressing feature consists of the following components:
DHCP relay agent
The DSL card in the DSLAM acts as a DHCP relay agent. The DHCP relay
agent is an intermediary function between the end-user system and the
DHCP server. Its functions are to:
— Detect and forward a DHCP request message from an end-user system
to the appropriate DHCP server.
— If you configure the system for optional authentication:
1. Hold the DHCP request message.
2. Send an authentication request to the authentication server in
RADIUS or XTACACS format.
3. Receive the authentication response. If negative, drop the held
DHCP message. If positive, relay the held DHCP message to the
DHCP server.
— Track the end-user system dynamically allocated IP address and lease
time from the DHCP acknowledgement by updating the routing table
automatically.
Local host (DSLAM) route injection
The DSL card's routing table is used to determine the DSL port on which to
forward incoming packets. This is achieved by examining the packet's
destination IP address and comparing it to the list of IP addresses in the
routing table. The subnet masks are set to 255.255.255.255 for host routes.
The IP address and subnet mask are then used to determine the end-user
system destination port. The DHCP relay agent adds (injects) the end-user
system IP address and subnet mask into the routing table automatically.
Remote host (RTU) route injection (when using an 8546 DSL card and a
5446 RTU only)
The DSLAM also injects the end-user system's IP addresses into the 5446
RTU. The routing table in the 5446 RTU is used to determine if traffic on its
10BaseT (Ethernet) port is local or if it should be sent over the DSL. It
determines this by checking if any of the addresses match the addresses in
its local host routing table. This routing table is automatically updated by the
8546 DSL card after the DHCP relay agent has intercepted the end-user
system's IP address in the DHCP reply message.
November 1997
IP Routing
6-5
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