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Understanding the Extended DHCP Relay Agent for EX Series Switches

Extended DHCP Relay and Legacy DHCP/BOOTP Relay

The extended DHCP relay agent options configured with the dhcp-relay statement are incompatible with the DHCP/BOOTP relay agent options configured with the bootp statement. Therefore, you cannot enable both the extended DHCP relay agent and the legacy DHCP/BOOTP relay agent on a switch at the same time.

For information about the legacy DHCP/BOOTP relay agent, see DHCP/BOOTP Relay for Switches Overview.

Interaction Among the DHCP Relay Agent, DHCP Client, and DHCP Servers

1. The DHCP client sends a discover packet to find a DHCP server in the network from which to obtain configuration parameters, including an IP address, for the subscriber.
2. The DHCP relay agent receives the discover packet and forwards copies of the discover packets to each of the two DHCP servers. The DHCP relay agent then creates an entry in its internal client table to keep track of the client’s state.
3. In response to receiving the discover packet, each DHCP server sends an offer packet to the client. The DHCP relay agent receives the offer packets and forwards them to the DHCP client.
4. On receiving the offer packets, the DHCP client selects the DHCP server from which to obtain configuration information. Typically, the client selects the server that offers the longest lease time on the IP address.
5. The DHCP client sends a request packet that specifies the DHCP server from which to obtain configuration information.
6. The DHCP relay agent receives the request packet and forwards copies of this packet to each of the two DHCP servers.
7. The DHCP server requested by the client sends an acknowledgement (ack) packet that contains the client’s configuration parameters.
8. The DHCP relay agent receives the ack packet and forwards it to the client.
9. The DHCP client receives the ack packet and stores the configuration information.
10. If configured to do so, the DHCP relay agent installs a host route and Address Resolution Protocol (ARP) entry for this client.
11. After establishing the initial lease on the IP address, the DHCP client and the DHCP server use unicast transmission to negotiate lease renewal or release. The DHCP relay agent snoops all of the packets unicast between the client and the server that pass through the relay agent to determine when the lease has expired or been released. This process is referred to as lease shadowing or passive snooping.