IP Routing Features

Configuring DHCP Relay

■routing switch access to an Option 82 DHCP server on a different subnet than the clients requesting DHCP Option 82 support

■one IP Helper address configured on each VLAN supporting DHCP clients

General DHCP-Relay Operation with Option 82. Typically, the first (primary) Option 82 relay agent to receive a client’s DHCP request packet appends an Option 82 field to the packet and forwards it toward the DHCP server identified by the IP Helper address configured on the VLAN in which the client packet was received. Other, upstream relay agents used to forward the packet may append their own Option 82 fields, replace the Option 82 field(s) they find in the packet, forward the packet without adding another field, or drop the packet. (Intermediate next-hop routing switches without Option 82 capability can be used to forward—route—client request packets with Option 82 fields.) Response packets from an Option 82 server are routed back to the primary relay agent (routing switch), and include an IP addressing assignment for the requesting client and an exact copy of the Option 82 data the server received with the client request. The relay agent strips off the Option 82 data and forwards the response packet out the port indicated in the response as the Circuit ID (client access port). Under certain validation conditions described later in this section, a relay agent detecting invalid Option 82 data in a response packet may drop the packet.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DHCP

 

 

 

 

 

 

 

 

 

 

 

 

Relay Agent “1”

 

 

 

 

Option 82

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Switch

 

 

 

 

VLAN 3

VLAN 4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Option 82 Enabled

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch

 

Client

 

 

 

Client

 

 

Client

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Client

 

 

 

 

 

 

 

 

 

 

 

 

 

VLAN 3

VLAN 2

Relay Agent “2”

 

 

 

 

 

 

 

 

 

Client

 

 

 

 

No Option 82

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Relay Agent “3”

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Switch

 

 

 

VLAN 1

VLAN 2

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Option 82 Enabled

 

 

 

 

 

 

 

 

 

 

Client

 

 

 

Client

 

 

 

 

 

 

 

 

 

 

 

Switch

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Relay Agent 1 adds an Option 82 field to a client request, and then forwards the request toward the server. This includes any client requests received from Relay Agent 2 without an Option 82 field.

Note: DHCP Option 82 does not operate with clients on VLAN 4 because DHCP requests from these clients are not routed.

Relay Agent 2 does not add an Option 82 field to client requests before forwarding the requests. However, any client requests received from Relay Agent 3 will be forwarded with the Option 82 fields that were added by Relay Agent 3.

Relay Agent 3 adds an Option 82 field to a client request and then forwards the request.

Figure 7-11. Example of DHCP Option 82 Operation in a Network with a Non-Compliant Relay Agent

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