Configuring The Relay Device - Cisco Catalyst 2950 Software Configuration Manual

Chapter 4 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information
You can configure the IP addresses of the DNS servers in the lease database of the DHCP server from
where the DHCP replies will retrieve them. You can enter up to two DNS server IP addresses in the lease
database.
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 an 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 Figure 4-2, configure the router interfaces as follows:
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 4-2 Relay Device Used in Autoconfiguration
Switch
Cisco router
(DHCP client)
(Relay)
10.0.0.2
10.0.0.1
20.0.0.1
20.0.0.2 20.0.0.3 20.0.0.4
DHCP server TFTP server DNS server
Catalyst 2950 and Catalyst 2955 Switch Software Configuration Guide
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