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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter4 Assigning the Switch IP Address and Default Gateway
Assigning Switch Information
For the switch to successfully download a configuration file, the TFTP server must contain one or more
configuration files in its base directory. The files can include these files:
•The configuration file named in the DHCP reply (the actual switch configuration file).
•The network-confg or the cisconet.cfg file (know n as th e defa ult c on figu ra tio n fi les) .
•The router-confg or the ciscortr.cfg file (These files contain commands common to all switches.
Normally, if the DHCP and TFTP servers are properly configured, these files are not accessed.)
If you specify the TFTP server name in the DHCP server-lease database, you must also configure the
TFTP server name-to-IP-address mapping in the DNS-server data base .
If the TFTP server to be used is on a different LAN from the switch, or if it is to be accessed by the
switch through the broadcast address (which occurs if the DHCP server response does not conta in all the
required information described earlier), a relay must be configured to forward the TFTP packets to the
TFTP server. For more information, see the “Configuring the Relay Device” section on page 4-6. The
preferred solution is to configure the DHCP server with all the required information.
Configuring the DNS
The DHCP server uses the DNS server to resolve the TFTP server name to an IP address. You must
configure the TFTP server name-to-IP address map on t he D NS ser ver. The TFTP se rver co nta ins the
configuration files for the switch.
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 ad dresses 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 when a switch sends broadcast packets that need to b e re spon ded t o
by a host on a different LAN. Examples of broadcast packets that the switch migh t 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 a helper addresses by using the ip helper-address interface configuration command.
For example, in Figure4-2, 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
Note If the Catalyst 3550 multilayer switch is acting as the relay device, configure the interface as a routed
port. For more information, see the “Routed Ports” section on page8-4 and t he “Confi guri ng Laye r
3 Interfaces” section on page 8-22.