Command:

debug arp


Mode:

Router#


Syntax:

debug arp

no debug arp

Syntax Description:

This command has no arguments or keywords.

Command Description:

 

To display information on Address Resolution Protocol (ARP) transactions, use the debug arp

privileged EXEC command. The no form of this command disables debugging output.

Use this command when some nodes on a TCP/IP network are responding, but others are not. It shows

whether the router is sending ARP packets and whether it is receiving ARP packets.


Example:

The following is sample output from the debug arp command:

Router# debug arp

IP ARP: sent req src 172.16.22.7 0000.0c01.e117, dst 172.16.22.96 0000.0000.0000

IP ARP: rcvd rep src 172.16.22.96 0800.2010.b908, dst 172.16.22.7

IP ARP: rcvd req src 172.16.6.10 0000.0c00.6fa2, dst 172.16.6.62

IP ARP: rep filtered src 172.16.22.7 aa92.1b36.a456, dst 255.255.255.255 ffff.ffff.ffff

IP ARP: rep filtered src 172.16.9.7 0000.0c00.6b31, dst 172.16.22.7 0800.2010.b908

In the output, each line of output represents an ARP packet that the router sent or received. Explanations

for the individual lines of output follow.

The first line indicates that the router at IP address 172.16.22.7 and MAC address 0000.0c01.e117 sent

an ARP request for the MAC address of the host at 172.16.22.96. The series of zeros (0000.0000.0000)

following this address indicate that the router is currently unaware of the MAC address.

IP ARP: sent req src 172.16.22.7 0000.0c01.e117, dst 172.16.22.96 0000.0000.0000

The second line indicates that the router at IP address 172.16.22.7 receives a reply from the host at

172.16.22.96 indicating that its MAC address is 0800.2010.b908:

IP ARP: rcvd rep src 172.16.22.96 0800.2010.b908, dst 172.16.22.7

The third line indicates that the router receives an ARP request from the host at 172.16.6.10 requesting

the MAC address for the host at 172.16.6.62:

IP ARP: rcvd req src 172.16.6.10 0000.0c00.6fa2, dst 172.16.6.62

The fourth line indicates that another host on the network attempted to send the router an ARP reply for

its own address. The router ignores meaningless replies. Usually, meaningless replies happen if a bridge

is being run in parallel with the router and is allowing ARP to be bridged. This condition indicates a

network misconfiguration.

IP ARP: rep filtered src 172.16.22.7 aa92.1b36.a456, dst 255.255.255.255 ffff.ffff.ffff

The fifth line indicates that another host on the network attempted to inform the router that it is on

network 172.16.9.7, but the router does not know that the network is attached to a different router

interface. The remote host (probably a PC or an X terminal) is misconfigured. If the router were to install

this entry, it would deny service to the real machine on the proper cable.

IP ARP: rep filtered src 172.16.9.7 0000.0c00.6b31, dst 172.16.22.7 0800.2010.b908


Misconceptions:

           None.


Related Commands:

            None. 


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World Wide Education

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