6500 ARP/ VLAN issue

Unanswered Question
Nov 12th, 2009


I have a strange problem on a pair of 6500's. The configuration is such that both units are connected together via a port channel dot1q trunk that allows all vlans. One of the switches is running the inter vlan routing with the gateway addresses for clients on the vlan interface of the switch. This unit is also running DHCP for the voice vlan. The problem is that when a phone is plugged into a voice/data enabled port on the non routing switch it fails connectivity to most other devices including the gateway configured on the 6500. I have run a span on the phone port and can see all of the dhcp requests fine and this works, but i do not see the arp requests to the gateway. I then plugged a hub into the phone port along with an analyser and can see the phone making the arp request to teh gateway time after time with no response. I have checked the vlan id/ MAC addresses of packets and the arp/ MAC cache in both 6500's and this seems fine. Clearing the arp table on the routing 6500 allows connectivity. If i leave the phone connected for a while the connectivity seems to be OK, by which time the phone has given up looking for an ftp server. The interesting thing is that any phone connected directly to the routing switch works fine.

Many Thanks for any help.

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Edison Ortiz Thu, 11/12/2009 - 11:46

Verify the 6500 running the routing service is the spanning-tree root for all vlans..



gavin.jones Thu, 11/12/2009 - 12:16


Thanks for the response, i am on site and still failing to make any head way. Since the post i have now disabled the port channel and am just running a single dot1q trunk between the switches and i am still having the same problems. The routing switch is the root, but i have only just made this change, i presume i do not need a reboot. The routing switch thinks it is the root.

Thanks very much

Edison Ortiz Fri, 11/13/2009 - 08:51

Is the ARP information sent/received on the 6500 doing the routing from the 6500 not doing the routing?

I highly recommend opening a case with TAC. It seems there is something missing in the config or an issue with the IOS.

This type of troubleshooting can become lengthy in an online forum.

gavin.jones Fri, 11/13/2009 - 09:02


Yes everything shows OK in both MAC and ARP tables on the 6500's. I think the forwarding and VLAN/ARP is all good because if i plug a phone into a static non trunked port in either the data or voice vlan it just works. The problem i have is when the phone is in a voice vlan enabled port on the secondary 6500. Again primary routing 6500 works fine.

I agree, thanks for your time i will open a TAC case.


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