Multiple IP Addresses Same MAC addresses

[dave cooper]

We have a Netapp storage device and we are having intermittent problems with it.

When I look at the ARP cache on our core switch I found that the storage guys have assigned multiple IP addresses to the same MAC.

Is this possible

Internet  172.16.14.23          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.16          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.17          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.18          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.19          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.28          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.29          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.30          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.31          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.24          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.25          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.26          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.27          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.37          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.38          219   02a0.9945.bfec  ARPA   Vlan400

Internet  172.16.14.39          219   02a0.9945.bfec  ARPA   Vlan400

[nkarthikeyan]

Hi,

ARP table from Core switch shows the mac-address of the device/interface from where it is learned if am not wrong. If you want to get the actual mac-address you need to check the respective access layer where it is connected. Also this would be the mac-address of the Access layer device connected interface. You can check once and confirm me. If not that we can try to find something else.

HTH

Regards

Karthik

[devils_advocate]

The ARP cache on the Core switch should show the Actual Layer 2 address of the host.

The CAM table on the Core switch will show the relevant interface that the MAC address was learned from. This could be a Trunk port to another switch or the actual outgoing interface if the host is plugged into the Core.

[nkarthikeyan]

Oops!! I got little confused. You are correct.....

We need to find out the actual interface where these IP's are binded from the access switch. from the core switch we cannot see that information.

Do you think VM might be the reason for this??? or a NATed IP address ??

Regards

Karthik

[devils_advocate]

I think it should be possible.

A packet either needs to be switched or routed depending on whether the Destination is on the same subnet as the source.

If it needs to be switched, the Source host will ARP for the Destination MAC address for the corresponding Destination IP address. Although in your setup, the Destination has several IP addresses for a Single MAC Address, the Source host is only trying to communicate with one of the IP Addresses, so as long as he gets a reply with the correct MAC address then he can send the frame. Once the packet is sent, the switch would use the CAM tables to figure out the route the frame needs to take to get to the host. Once it forwards the frame out the port towards the storage device, it will have a unique Destination IP address in the Layer 3 header so it should be able to figure out what network adapter it is intended for.

It would only be an issue if a Single MAC address is seen on multiple switchports.