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ARP Problems 2848, 5308, 2650

Jon Fayers
Occasional Advisor

ARP Problems 2848, 5308, 2650

I have a strange problem with a few servers on my network. The servers in question are isolated at present on their own 2848 with a 1Gb uplink to a 2650 which has the rest of my servers attached to it plus a connection to my Firewall/Router Cisco 515.
If ping from one of the servers on the 2848 to another server on the 2848 it occassionally ( well the vast majority of the time) performs one ping and then times out. Further investigation lead me to look at the ARP table on the 2848 where I discovered that the server I am trying to ping has the the MAC address of the router!!??. This means that I cannot ping this server even though its on the same switch.
If I remove the uplink to the 2650 normality os returned. I have already swapped out the 2650 (was a Dell Switch same problem)

I have managed to circumvent the problem on the server (W2K3) by adding a static arp entry - Is there a way to add static arp entires to the switch. This wouldn't be a preferred solution but might help for the time being.

I was getting some "blocked by LACP" messages and disabled lacp on all the ports but it didn't make any difference.
I should also mention that the 2650 has 3 VLANs on it whereas the 2848 only has the Default Vlan (out of the box)

Anyone any ideas as to why the Mac Addresses I am getting in the Arp table are incorrect?
Kell van Daal
Respected Contributor

Re: ARP Problems 2848, 5308, 2650

Adding static arp entries to the switch won't help. The static entry needs to be on the node that needs to communicate.
In your case that is the server and not the switch.
If the switch was routing between the nodes (your servers), then it would help.
But since you have only one VLAN on the 2848, this isn't the case.

About the problem.

I have 2 situations that could be the cause of the problem:

- I once saw two Cisco routers having the exact same MAC-address. This could cause it.
But I reckon you allready investigated it.

- Does the Cisco Pix have proxy-arp enabled?
If yes, disable it if you don't need it. And as an extra precaution, check your subnetmasks. Proxy arp can do exactly what you describe here. Respond with it's own MAC-address when a client broadcasts to find a MAC address belonging to an IP address.
If I'm not mistaken, proxy arp is default enabled on all interfaces. To disable it, use:
sysopt noproxyarp inside
inside being an example interface name.

Hope this helps