Cisco Gigabit Ethernet Switch Module for HP p-Class BladeSystem Software Configuration Guide

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Cisco Gigabit Ethernet Switch Module for HP p-Class BladeSystem Software Configuration Guide
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Chapter 16 Configuring DHCP Features
Understanding DHCP Features
Figure 16-1 DHCP Relay Agent in a Metropolitan Ethernet Network
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.
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 Figure 16-2 do not change:
• Circuit-ID suboption fields
– Suboption type
– Length of the suboption type
– Circuit-ID type
– Length of the circuit-ID type
• Remote-ID suboption fields
– Suboption type
– Length of the suboption type
– Remote-ID type
– Length of the remote-ID type
Subscribers
Blade switch
(DHCP relay agent)
Blade Server A
(DHCP client)
Access layer
DHCP
server
Blade Server B
(DHCP client)
126707
VLAN 10