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Advanced ARP Troubleshooting Troubleshooting ARP
page 11-6 OmniSwitch Troubleshooting Guide September 2005
Advanced ARP Troubleshooting
If the MAC addresses is already learned on the port and the ARP is not getting resolved then we can
further troubleshoot on the switch to see if the ARP request is reaching the switch and switch is respond-
ing back.
To troubleshoot the ARP packets we need to use the diagnostic CLI commands. Precaution must be taken
when using these commands as it might dump a lot of information on the screen.
The command to use is as follows:
-> debug ip packet start ip-address 10.10.42.159 start
-> 16 R 16/2 080020a8f08a->(ffffffffffff) ARP Request 10.10.42.159->10.10.42.1
16 S 16/2 00d0957962d1->080020a8f08a ARP Reply 10.10.42.1->10.10.42.159
16 R 16/2 080020a8f08a->00d0957962d1 IP 10.10.42.159->10.10.42.1 ICMP 8,0 seq=0.
16 S 16/2 00d0957962d1->080020a8f08a IP 10.10.42.1->10.10.42.159 ICMP 0,0 seq=0.
-> debug ip packet stop
The above capture shows that a request for ARP came in on slot 16 port 2 for ip address 10.10.42.1. The
reply was sent by the switch to 10.10.42.159 at MAC address 08:00:20:a8:f0:8a.
This confirms that the switch is replying back to the ARP. Now the ARP cache of the workstation should
also show the correct ARP entry for the switch. If not, then a sniffer should be placed between the switch
and the workstation to look at the packets to analyze if the packets are corrupted or either one of the
devices is not responding in the correct format.
If the debug ip packet command does not show any output when specified with IP address then other
variations like traffic from that NI can be analyzed using the command:
-> debug ip packet start board ni 16
-> debug ip packet stop
This command will show all the packets coming in NI 16, so the output will be a little confusing and pack-
ets will have to be filtered to gather the required information.
If none of the above commands show any traffic coming in from the IP address for the device then it will
point towards the physical layer issue. Workstation as well as the physical port to which the workstation is
connected needs to be examined for further analysis.