Chapter 19 Configuring Dhcp Feature - Cisco Catalyst 3550 series Software Configuration Manual

Understanding DHCP Features
Option-82 Data Insertion
In residential, metropolitan Ethernet-access environments, DHCP can centrally manage the IP address
assignments for a large number of subscribers. When the DHCP option-82 feature is enabled on the
switch, a subscriber is identified by the switch port through which it connects to the network (in addition
to its MAC address). Multiple hosts on the subscriber LAN can be connected to the same port on the
access switch and are uniquely identified.
Figure 19-1
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 19-1 DHCP Relay Agent in a Metropolitan Ethernet Network
(DHCP client)
When you enable the DHCP information option 82 on the switch, this sequence of events occurs:
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•
•
•
Catalyst 3550 Multilayer Switch Software Configuration Guide
19-2
is an example of a metropolitan Ethernet network in which a centralized DHCP server
Service-provider
network
Switch
Catalyst 3550 switch
(DHCP relay agent)
Subscribers
Host A
The host (DHCP client) generates a DHCP request and broadcasts it on the network.
When the switch receives the DHCP request, it adds the option-82 information in the packet. The
option-82 information contains the switch MAC address (the remote ID suboption) and the port
identifier, vlan-mod-port or snmp-ifindex, from which the packet is received (the circuit ID
suboption).
The 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 a circuit ID. Then the DHCP server
echoes the option-82 field in the DHCP reply.
Router
30.0.0.1
Distribution
layer
Access layer
VLAN 10
10.0.0.1
ip helper-address 30.0.0.2
Host B
(DHCP client)
Chapter 19 Configuring DHCP Features
DHCP
server
30.0.0.2
78-11194-09