920 Configuring L2 and L3 Relay FeaturesThe PowerConnect DHCP Relay Agent enables DHCP clients and servers toexchange DHCP messages across different subnets. The relay agent receivesthe requests from the clients, and checks the valid hops andgiaddr fields inthe DHCP request. If the number of hops is greater than the configurednumber, the agent discards the packet. If thegiaddr field is zero, the agentmust fill in this field with the IP address of the interface on which the requestwas received. The agent unicasts the valid packets to all configured DHCPservers. Each server responds with a unicast BOOTREPLY addressed to therelay agent closest to the client as indicated bygiaddr field. Upon reception ofthe BOOTREPLY from the server, the agent forwards this reply as broadcastor unicast on the interface where the BOOTREQUEST arrived. Thisinterface can be identified by thegiaddr field or option 82.The PowerConnect M6220/M6348/M8024/M8024-k switch DHCPcomponent also supports DHCP relay agent options to identify the clientinterface. If configured, the relay agent inserts these options when forwardingthe request to the server and removes them when sending the reply to theclients.If an interface has more than one IP address, the relay agent uses the primaryIP address configured as its relay agent IP address.What Is L2 DHCP Relay?In Layer 2 switched networks, there may be one or more infrastructure devices(for example, a switch) between the client and the L3 Relay agent/DHCPserver. In this instance, some of the client device information required by theL3 Relay agent may not be visible to it. In this case, an L2 Relay agent can beused to add the information that the L3 Relay Agent and DHCP server needto perform their roles in address and configuration and assignment.Before it relays DHCP requests from clients, the switch can add a Circuit IDand a Remote ID. These IDs provide information about the circuit and portnumber connected to the client. This information is added as suboptions inthe DHCP Option 82 packets as defined in sections 3.1 and 3.2 of RFC3046.The switch removes this option from packets that it relays from L3 Relayagents/DHCP servers to clients.These sub-options may be used by the DHCP server to affect how it treats theclient and also may be used by the relay agent to limit broadcast replies to thespecific circuit or attachment point of the client.
