Cisco 7604 Configuration Manual page 556
Ios software configuration guide
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Understanding DHCP Snooping
Figure 37-1 DHCP Relay Agent in a Metropolitan Ethernet Network
(DHCP client)
When you enable the DHCP snooping information option-82 on the router, this sequence of events
occurs:
The host (DHCP client) generates a DHCP request and broadcasts it on the network.
•
When the router receives the DHCP request, it adds the option-82 information in the packet. The
•
option-82 information contains the router MAC address (the remote ID suboption) and the port
identifier, vlan-mod-port, from which the packet is received (the circuit ID suboption).
•
If the IP address of the relay agent is configured, the router adds the IP address in the DHCP packet.
The router 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,
•
or 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. The DHCP server
then echoes the option-82 field in the DHCP reply.
The DHCP server unicasts the reply to the router if the request was relayed to the server by the
•
router. When the client and server are on the same subnet, the server broadcasts the reply. The router
verifies that it originally inserted the option-82 data by inspecting the remote ID and possibly the
circuit ID fields. The router removes the option-82 field and forwards the packet to the router port
that connects to the DHCP client that sent the DHCP request.
When the previously described sequence of events occurs, the values in these fields in
not change:
Circuit ID suboption fields
•
–
–
–
–
Remote ID suboption fields
•
–
–
–
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Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
37-4
Catalyst switch
(DHCP relay agent)
Subscribers
Host A
Suboption type
Length of the suboption type
Circuit ID type
Length of the circuit ID type
Suboption type
Length of the suboption type
Remote ID type
Length of the circuit ID type
DHCP
server
Access layer
VLAN 10
Host B
(DHCP client)
Chapter 37
Configuring DHCP Snooping
Figure 37-2
OL-4266-08
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