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Community Member

CAT 6500 does sequential arps

I have a cat 6500 running IOS version 12.1(12c)E4 and every 5 hours 18 minutes and 20 seconds I see it ARPing sequentially starting from x.x.x.1 to x.x.x.254. The reason I care is because right after that I have some Windows 2003 servers that reboot. So far I have only sniffed 1 vlan so I don't know if it happens on other vlans. Is there something in the 6500's config that tries to refresh its APR cache at that interval or is it doing it on behalf of something? The source MAC address is the CATs so I wouldn't think it is on behalf of something.

Thanks for any suggestions,



Re: CAT 6500 does sequential arps

The cat may be doing the arps but it may be doing them on behalf of a packet/s it is trying to forward for another host.


Re: CAT 6500 does sequential arps

I would check to see if you don't have a station that might be infected trying to go to many different addresses . You can look at this by turning on ip cache flow on your interfaces and then looking at the flows to see if anything looks strange .

Community Member

Re: CAT 6500 does sequential arps

This is normal behavior 60 seconds before the ARP aging time expires. The switch will send a gratuitous ARP before the host ages to assure that it is still alive.

You can do a show interface and it will tell you what the aging time is. By default, should be 4hrs.

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