incomplete ARP on Cisco 6500 switch

gflorescu · ‎06-23-2009

I have a Cisco 6513 switch that performs inter-vlan routing.

I have a series of Cisco 3750 switches and a 3560 switch connected to the 6513 via trunk links. (4 3750 switches and 1 3560)

I am having trouble with ping/telnet from a desktop PC located behind one of the 3750s. I cannot ping or telnet to any of the 3750s or the 3560.

The only switch I can ping or telnet into is the 1st 3750 switch that's directly connected to the 6500 switch.

About the only way I can get it to work is if I telnet into the 1st 3750 and then telnet into another switch from there. At that point, if I initiate a ping out from one of the "trouble" switches, then I can ping/telnet into the switch from anywhere. This will last for a little while, then stop working again.

I changed the ARP timeout value on the 6500 switch to be 14400. That seemed to help for a little while but now it's not working again.

I think there's a problem with the 6500 switch learning the ARP from the downstream switches because if I do a "show mac-address-table" and look for the mac address of the 3750 switches, I can't find them in there.

smothuku · ‎06-23-2009

Hi ,

Can you paste the cofig of devices...

do clear mac-address-table on 6500 and check whether you will be able to login to devices or not..

Cheers :) MSK

Giuseppe Larosa · ‎06-23-2009

Hello Gilbert,

this could be caused by Proxy ARP enabled on some device.

you can check this with

sh ip interface type x/y

look for the proxy ARP line.

the fact that changing the ARP timeout helped for a while points to ARP activity.

Another aspect to investigate is the real extent of the L2 management vlan: in order to get complete ARP replies the vlan (as L2 broadcast domain) has to be permitted on all trunk ports on on both ends of each link on all switches.

Hope to help

Giuseppe

gflorescu · ‎07-24-2009

I figured it out. It was due to VTP pruning that was turned on the 6500 switch. As soon as I turned it off, it started working like a charm.