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Chapter 7 Adminis tering the Switch Managing the ARP Table

Managing the ARP Ta ble

To commun icate w ith a de vice (o ver Eth ernet, for e xamp le), the softw are fi rst mu st dete rmine the 48-bi t
MA C or the local data link address of that device. The process of determining the local data link address
from a n I P a ddress is c al led address resolution.
The Addr ess Resoluti on Protoco l (ARP ) associates a host IP address wit h the corres pondi ng media or
MAC addresses and the VLAN ID. Taking an IP address as input, ARP determines the associated MAC
addres s. Once a MA C ad dress is determ ined , the IP-MAC address asso ciatio n is store d in an ARP c ache
for ra pid retri ev al. Then the IP data gram is enca psula ted in a link-la yer fr ame and sen t ov er the netw ork.
Encapsulation of IP datagrams and ARP requests and replies on IEEE 802 networks other than Ethernet
is specified by the Subnet work Access Protocol (SNAP) . By default, stand ard Ethe rnet- style A RP
encaps ul ation (r epres ented by the arpa keyword) is enable d on the IP inte rface.
ARP entries added m anually to the table do not age and must be manually removed.
For CLI procedur es, refer to the Cisco IOS Rele ase 12.1 doc umen tation on Cisco. com.

Switch Software Releases

The switch software is regularly updated with new features and bug fi xes, 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 av ailab le thr ough author ized rese llers. Cis co also s upplie s a TFTP serv er that yo u can download
from Cisco.c om.
Before up gradi n g a swi tch, first find out the version of t he s oftware that the sw it ch is ru nning . You can
do this by selecting Reports > Inventor y or by using the s how ver sio n 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 requir ements
•Switch upg rade gui delines an d procedur es