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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 7 Administering the Switch
Managing the ARP Table
Managing the ARP Table
To communicate with a device (over Ethernet, for example), the software first must determine the 48-bit
MAC or the local data link address of that device. The process of determining the local data link address
from an IP address is called address resolution.
The Address Resolution Protocol (ARP) associates a host IP address with the corresponding media or
MAC addresses and the VLAN ID. Taking an IP address as input, ARP determines the associated MAC
address. Once a MAC address is determined, the IP-MAC address association is stored 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 and replies on IEEE 802 networks other than Ethernet
is specified by the Subnetwork Access Protocol (SNAP). By default, standard Ethernet-style ARP
encapsulation (represented by the arpa keyword) is enabled on the IP interface.
ARP entries added manually to the table do not age and must be manually removed.
For CLI procedures, refer to the Cisco IOS Release 12.1 documentation on Cisco.com.
Switch Software Releases
The switch software is regularly updated with new features and bug fixes, and you might want to upgrade
your Catalyst 2950 switch with the latest software release. New software releases are posted on Cisco.com
and are available through authorized resellers. Cisco also supplies a TFTP server that you can download
from Cisco.com.
Before upgrading a switch, first find out the version of the software that the switch is running. You can
do this by selecting Reports > Inventory or by using the show version user EXEC command.
Knowing the software version is important, especially for:
• Compatibility reasons (for example, for switch clusters)
• LRE and non-LRE Catalyst 2950 switches, which do not share the same software image. The LRE-only
image cannot be installed on non-LRE switches. The non-LRE image does not include LRE functionality
and should not be installed on LRE switches.
Refer to the release notes
(http://www.cisco.com/univercd/cc/td/doc/product/lan/cat2950/12111YJ/ol236205.htm) for
• Switch requirements
• Switch upgrade guidelines and procedures