Cisco Systems 4500 IP Unnumbered Interface Support with DHCP Server and Relay Agent, DHCP Option 82

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 14 Configuring IP Unnumbered Interface
About IP Unnumbered Interface Support
IP Unnumbered Interface Support with DHCP Server and Relay Agent
The IP unnumbered interface configuration allows you to enable IP processing on an interface without
assigning it an explicit IP address. The IP unnumbered interface can “borrow” the IP address from
another interface that is already configured on the Catalyst 4500 series switch, which conserves network
and address space. When used with the DHCP server/relay agent, this feature allows a host address
assigned by the DHCP server to be learned dynamically at the DHCP relay agent.
Figure 14-1 shows a sample network topology implementing the IP Unnumbered Interface feature. In
this topology, IP routes are dynamically established by the aggregation switch when the DHCP server
assigns IP addresses to the hosts.
Figure 14-1 Sample Network Topology Using the VLANs over IP Unnumbered Interfaces Feature
DHCP Option 82
DHCP provides a framework for passing configuration information to hosts on a TCP/IP network.
Configuration parameters and other control information are carried in tagged data items that are stored
in the options field of the DHCP message. The data items are also called options. Option 82 is organized
as a single DHCP option that contains information known by the relay agent.
The IP Unnumbered Interface feature communicates information to the DHCP server using a suboption
of the DHCP relay agent information option called agent remote ID. The information sent in the agent
remote ID includes an IP address identifying the relay agent and information about the interface and the
connection over which the DHCP request entered. The DHCP server can use this information to make
IP address assignments and security policy decisions.
Figure 14-2 shows the agent remote ID suboption format that is used with the IP Unnumbered Interfaces
feature.
95961
Multilayer
Ethernet
switch
Aggregation
router
GE
IP unnumbered
interface
IP/MPLS
GE
DHCP server
GE
GE
DSL DSL
DSL access multiplexer
with Gigabit Ethernet (GE) uplink
DSL
DSL
DSL
802.1Q VLAN