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8000-A2-GB21-20
1. The end-user system requests an IP address by broadcasting a DHCP
request message to the DHCP server.
2. The DSLAM performs a DHCP relay by acting as a DHCP relay agent. The
DHCP relay function of the DSLAM acts as an intermediary between the
end-user system and the DHCP server, and works with DHCP servers that
support structured subnetting. At this point, the following events occur:
A. The DHCP relay within the DSLAM intercepts the end user's DHCP
request for an address.
B. If a domain name is detected, the DHCP relay determines if the
domain has been configured to the DHCP server.
C. It determines if authentication is required.
If authentication is not required, it injects a gateway address into the
message and forwards it to the DHCP server.
If authentication is required, the DHCP relay agent determines the
authentication type (Radius or XTACACS). Then, it holds the original
message and creates an authentication request message and passes the
message to the appropriate authentication server.
3. An authentication response is received by the DSLAM. If the authentication is
confirmed, the DHCP relay agent inserts the gateway address (i.e., the e1a
IP address associated with the domain name) into the original DHCP request
message. Then, it forwards the message to the DHCP server within the
configured service domain.
4. The original DHCP request (with the gateway address) is relayed to the
DHCP server.
5. The DHCP relay function of the DSLAM intercepts the DHCP ACK
(acknowledge) message. At this point, the following events occur:
— The DHCP relay agent extracts the IP address and lease time
information from the DHCP ACK message.
— The IP address is injected to the RTU (if the DHCP relay agent is an
8546 DSL card and the RTU is a 5446 RTU).
— The DHCP relay agent injects the IP address, subnet mask of
255.255.255.255, lease time, and port number into the routing table. The
routing tables are updated automatically.
6. After successful completion of these events, the DHCP ACK message is
forwarded to the end user.
7. The IP addresses are automatically deleted from the DSLAM routing tables
when the end user releases the IP address (by sending a DHCP release
message) or the lease time expires without a renewal. Once the DHCP relay
has deleted the configuration information, the end user will no longer be able
to access the NSP.
To regain access to the NSP, the end user must initiate a DHCP discover or
request again to the DHCP server, and a new IP address will be assigned.
November 1997
IP Routing
6-7
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