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We have a large area MAN provided by a local cable company. Each location has a cable modem that connects directly back to a central location, i.e. it doesn't cross the internet, it is a private connection. Each out building shows shows up at the central location though a trunk fiber and each building is a different vlan.

The issue is we are in the process of upgrading and merging our networks (there were two admin networks now we have one net admin for everything). We switched to a Cisco 4500 and have ip address on each vlan. The issues seems to be that PCs are doing an arp request for the router (4500)'s ip (a "Who Has?") but they get no response. Strange thing is that DHCP is working fine. Also there are a couple of PCs that are working just fine.

If I create a static arp entry (arp -s ) then that pc will work. If I then do a repair on the connection the static entry goes away and a working dynamic entry shows up.

I've called the ISP and they say all of their gear is 100% and because some PC work it is our fault.


Thanks, Kerry

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Can you check if the router's actually getting that ARP request using a capture? – Shane Madden Apr 26 '12 at 1:29
Yes it is, and it is responding, but the responses aren't getting back to the sites. – Kerry Hatcher Apr 27 '12 at 18:28
What results to do you get from doing a traceroute to the gateway IP from the PC and then do one from PC thats working. See if there a different hops. – onxx Apr 30 '12 at 21:25
up vote 0 down vote accepted

Turns out it was the ISP GASP. There was a misconfiguration on some of their gear that caused a limit of 15 MACs in their ARP table. After they fixed their gear everything ended up just fine.

Thanks, Kerry

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