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why don't my server IP addresses appear in ARP table

I've got four servers behind two 2960 switches (server farm - no workstations or PC's). Each server has three IP addresses - total 12 IP addresses. Switch A has four IP addresses in its ARP table - itself, the other switch, the gateway, and one server IP address. Switch B has only three IP addresses, itself, the other switch, and the gateway.

Why do the other IP addresses not appear in the ARP table?

Cisco Employee

Re: why don't my server IP addresses appear in ARP table

Hi Friend,

Can you try to ping that server from the switch B and see if the ARP entry is created?



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Re: why don't my server IP addresses appear in ARP table


I believe that Ankur is on the right track. The ARP table in end stations, like servers, reflects what addresses on the local subnet the end station has communicated with recently (for Windows boxes it is frequently within the last 5 minutes). We would generally expect to find the default gateway in the ARP cache since the server will frequently communicate with the gateway. I am not surprised to have another server since the servers may need to communicate with another server from time to time. But I would not expect to find all the addresses in the subnet in the ARP cache since the server is not likely to communicate with all other devices on a frequent basis.

If the server attempts to communicate (like ping) with other server, then the other servers address should be in the ARP table for a while and then be removed.




Re: why don't my server IP addresses appear in ARP table


The ARP cache of layer 2 switches like the 2960 is used by the management interface of the switch. You will only see the IPs that the switch itself communicates with for management purposes (telnet,SNMP).

The server traffic that passes through the switch is forwarded based on the layer 2 ethernet MAC address of the frames, with the switch not needing to know the IP to MAC mapping. If you show mac-address address you will see the MAC forwarding table which will include the connected servers.

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