Cisco 3020 - Catalyst Blade Switch Configuration Manual page 368

Understanding DHCP Features
Figure 17-1
assigns IP addresses to subscribers connected to the switch at the access layer. Because the DHCP clients
and their associated DHCP server do not reside on the same IP network or subnet, a DHCP relay agent
(the Catalyst switch) is configured with a helper address to enable broadcast forwarding and to transfer
DHCP messages between the clients and the server.
Figure 17-1
Blade Server A
(DHCP client)
When you enable the DHCP snooping information option 82 on the switch, this sequence of
events occurs:
The Blade Server (DHCP client) generates a DHCP request and broadcasts it on the network.
•
• When the blade switch receives the DHCP request, it adds the option-82 information in the packet.
The remote-ID suboption is the switch MAC address, and the circuit-ID suboption is the port
identifier, vlan-mod-port, from which the packet is received.
If the IP address of the relay agent is configured, the switch adds this IP address in the DHCP packet.
•
The blade switch forwards the DHCP request that includes the option-82 field to the DHCP server.
•
• The DHCP server receives the packet. If the server is option-82-capable, it can use the remote ID,
the circuit ID, or both to assign IP addresses and implement policies, such as restricting the number
of IP addresses that can be assigned to a single remote ID or circuit ID. Then the DHCP server
echoes the option-82 field in the DHCP reply.
The DHCP server unicasts the reply to the blade switch if the request was relayed to the server by
•
the switch. The switch verifies that it originally inserted the option-82 data by inspecting the
remote ID and possibly the circuit ID fields. The switch removes the option-82 field and forwards
the packet to the switch port that connects to the DHCP client that sent the DHCP request.
When the described sequence of events occurs, the values in these fields in
Circuit-ID suboption fields
•
–
–
–
–
• Remote-ID suboption fields
–
Cisco Catalyst Blade Switch 3020 for HP Software Configuration Guide
17-4
is an example of a metropolitan Ethernet network in which a centralized DHCP server
DHCP Relay Agent in a Metropolitan Ethernet Network
Blade switch
(DHCP relay agent)
Subscribers
Suboption type
Length of the suboption type
Circuit-ID type
Length of the circuit-ID type
Suboption type
DHCP
server
Access layer
VLAN 10
Blade Server B
(DHCP client)
Chapter 17 Configuring DHCP Features
Figure 17-2 do not change:
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