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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter22 Configuring IP Unicast Routing
Configuring IP Addressing
Beginning in privileged EXEC mode, follow these steps to disable classless routing:
To restore the default and have the switch forward packets destined for a subnet of a network with no
network default route to the best supernet route possible, use the ip classless global configuration
command.
Configuring Address Resolution Methods
You can control interface-specific handling of IP by using address resolution. A devi ce using IP can have
both a local address or MAC address, which uniquely defines the d ev ice o n i ts loca l se gmen t or L AN,
and a network address, which identifies the network to which the device belongs. The local address or
MAC address is known as a data link address because it is cont ai ned i n t he d ata link l ayer (La yer 2)
section of the packet header and is read by data link (Layer 2) devices. To communicate with a device
on Ethernet, the software must determine the MAC address of the device. The process of determining
the MAC address from an IP address is called address resolution. The process of determining the IP
address from the MAC address is called reverse address resolution.
The switch can use these forms of address resolution:
•Address Resolution Protocol (ARP) is used to associate IP address with MAC addresses. Taking an
IP address as input, ARP determines the associated MAC address and then stores the IP
address/MAC address association in an ARP cache for rapid retrieval. Then the IP datagram is
encapsulated in a link-layer frame and sent over the network. Encapsulation of IP datagrams and
ARP requests or replies on IEEE 802 networks other than Ethernet is spe cif ied by t h e Subne tw ork
Access Protocol (SNAP).
•Proxy ARP helps hosts with no routing tables determine the MAC addresses of hosts on other
networks or subnets. If the switch (router) receives an ARP request for a host that is not on the same
interface as the ARP request sender, and if the router has all of its routes to the host through other
interfaces, it generates a proxy ARP packet giving its own local data link address. The host that sent
the ARP request then sends its packets to the router, which forwards them to the intended host.
Catalyst 3550 switches also use the Reverse Address Resolution Protocol (RARP), which function s the
same as ARP does, except that the RARP packets request an IP ad dress instead of a local MAC address.
Using RARP requires a RARP server on the same network segment as the router interface. Use the ip
rarp-server address interface configuration command to identify the server.
For more information on RARP, refer to the Cisco IOS Configuration Fundamentals Configuration
Guide for Release 12.1.
Command Purpose
Step1 configure terminal Enter global configuration mode.
Step2 no ip classless Disable classless routing behavior.
Step3 end Return to privileged EXEC mode.
Step4 show running-config Verify your entry.
Step5 copy running-config startup-config (Optional) Save your entry in the configuration file.