Interface Configuration
7210 SAS-X OS Services Guide Page 935
1. During boot-up, the client device sends a DHCP discover message to get an IP address from
the DHCP Server. The message contains:
→ Destination MAC address — broadcast
→ Source MAC address — MAC of client device
→ Client hardware address — MAC of client device
If this message passes through a DSLAM or other access node (possibly a 7210 SAS device),
typically the Relay information option (Option 82) field is added, indicating shelf, slot, port, VPI,
VCI and other fields, to identify the subscriber.
DHCP relay is enabled on the first IP interface in the upstream direction. Depending on the
scenario, the DSLAM, BSA or the BSR will relay the discover message as a unicast packet
towards the configured DHCP server. DHCP relay is configured to insert the giaddr in order to
indicate to the DHCP server in which subnet an address should be allocated.
2. The DHCP server will lookup the client MAC address and Option 82 information in its data-
base. If the client is recognized and authorized to access the network, an IP address will be
assigned and a DHCP offer message returned. The BSA or BSR will relay this back to the
client device.
3. It is possible that the discover reached more than one DHCP server, and thus that more than
one offer was returned. The client selects one of the offered IP addresses and confirms it
wants to use this in a DHCP request message, sent as unicast to the DHCP server that offered
it.
4. The DHCP server confirms that the IP address is still available, updates its database to indi-
cate it is now in use, and replies with a DHCP ACK message back to the client. The ACK
also contains the Lease Time of the IP address.
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