Help required - mac address table, virtual pc/ip addressing issue

mozmarr124 · ‎01-08-2014

Hi, hope someone out there can help?

This is the scenario

SW1 (WS-C2960G-48TC-L) port gig0/1 has a PC connected to it with ip address 10.182.8.6 and a Virtual IP address 10.182.8.107
SW2 (WS-C2960-24TT-L) port gig 0/1 has a PC connected to it with ip address 10.182.8.106

The system is designed so that if there is an issue with the PC connected to SW1, the PC on SW2 will take over the Virtual IP address and continue working.

We have a couple of other PC's in different subnets to the above PC's that use the Virtual IP (VIP) address to communicate with the PC that is 'on line'.

Unfortunately, the vendors software doesn't currently gratuitously refresh the arp to advertise the change of mac address for the VIP.

Is there anyway we can get the 2 dcnsw to 'refresh' on a regular time period to capture when the VIP changes to the other PC?

devils_advocate · ‎01-08-2014

Hi Stephen

Without meaning to sound rude, the software which uses a VIP is not very well designed if it is not capable of sending a Gratuitous ARP one the Active one fails.

The default ARP cache timeout is 4 hours so an ARP entry will remain in the table and once the timeout is up, the switch will send an ARP to check if the device is still alive and if not, remove the entry from the table.

You could look at reducing the ARP timeout on a per port basis:

#interface gi1/0/1

#arp timeout 60

This will change the ARP cache timeout to 60 seconds for that port but having not used this before, I am not 100% this will address your issue. I would not advise trying to change the global ARP cache timeout for a production switch as this will increase ARP traffic and could cause problems if reduced to a small value.