Command Reference

debug arp

Use the debug arp EXEC command to display information on Address Resolution Protocol (ARP) transactions. The no form of this command disables debugging output.
[no] debug arp

Usage Guidelines

Use this command when some nodes on a TCP/IP network are responding, but others are not. It shows whether the router is sending ARPs and whether it is receiving ARPs.

Sample Display

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.

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 someone is running a bridge 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