Cisco Catalyst 3550 Series User Manual

19-2
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-09
Chapter19 Configuring DHCP Features
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 is an example of a metropolitan Ethernet network in which a centralized DHCP server
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
When you enable the DHCP information option 82 on the switch, this sequence of events occurs:
• 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.
Service-provider
network
Router
30.0.0.1
Subscribers
Switch Distribution
layer
Catalyst 3550 switch
(DHCP relay agent)
Host A
(DHCP client)
Access layer
DHCP
server
30.0.0.2
Host B
(DHCP client)
74006
VLAN 10
10.0.0.1
ip helper-address 30.0.0.2

Cisco Catalyst 3550 Series Specifications

General

Brand	Cisco
Model	Catalyst 3550 Series
Category	Network Router
Language	English