Configuring The Relay Device; Obtaining Configuration Files - Cisco Catalyst 2975 Software Configuration Manual

Chapter 3 Assigning the Switch IP Address and Default Gateway
The DNS server can be on the same or on a different LAN as the switch. If it is on a different LAN, the
switch must be able to access it through a router.

Configuring the Relay Device

You must configure a relay device, also referred to as a relay agent, when a switch sends broadcast
packets that require a response from a host on a different LAN. Examples of broadcast packets that the
switch might send are DHCP, DNS, and in some cases, TFTP packets. You must configure this relay
device to forward received broadcast packets on an interface to the destination host.
If the relay device is a Cisco router, enable IP routing (ip routing global configuration command), and
configure helper addresses by using the ip helper-address interface configuration command.
For example, in
On interface 10.0.0.2:
router(config-if)# ip helper-address 20.0.0.2
router(config-if)# ip helper-address 20.0.0.3
router(config-if)# ip helper-address 20.0.0.4
On interface 20.0.0.1
router(config-if)# ip helper-address 10.0.0.1
Figure 3-2
DHCP server

Obtaining Configuration Files

Depending on the availability of the IP address and the configuration filename in the DHCP reserved
lease, the switch obtains its configuration information in these ways:
•
OL-19720-02
Figure 3-2, configure the router interfaces as follows:
Relay Device Used in Autoconfiguration
Switch
(DHCP client)
10.0.0.2
10.0.0.1
20.0.0.2 20.0.0.3
TFTP server
The IP address and the configuration filename is reserved for the switch and provided in the DHCP
reply (one-file read method).
The switch receives its IP address, subnet mask, TFTP server address, and the configuration
filename from the DHCP server. The switch sends a unicast message to the TFTP server to retrieve
the named configuration file from the base directory of the server and upon receipt, it completes its
boot-up process.
Cisco router
(Relay)
20.0.0.1
20.0.0.4
DNS server
Catalyst 2975 Switch Software Configuration Guide
Assigning Switch Information
3-9