ProCurve 2610-PWR Technical Information download pdf (Page 289)

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.

Switch

Client

DHCP

Option 82

Server

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.

Client

Switch

Relay Agent “2”

VLAN 1

VLAN 2

VLAN 3

VLAN 2

VLAN 3

VLAN 4

Switch

Client

Switch

Client

Switch

Client

Relay Agent “1”

Relay Agent “3”

No Option 82

Option 82 Enabled

Client

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

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