Notes on ARP, Switch Data Collection, and End Hosts

NetMRI uses ARP cache refresh to control LAN switches from which switch-forwarding data is collected.

When ARP cache refresh is enabled for a LAN switch, before further collection of switch-forwarding data, NetMRI pings all the IP addresses that are known to exist as members of the device's switch-forwarding table, by relating previously gathered switch-forwarding data MAC addresses to determine their IP addresses. Since this requires a prior SPM poll, the first poll of any switch by SPM after the device is added to the license will not perform an ARP cache refresh.

For subsequent polling events, NetMRI performs ARP cache refresh by sending a UDP packet on Port 80 to each IP address in the switch's forwarding table, without waiting for return messages. This has the effect of re-populating the switches' forwarding table. NetMRI sends nothing to the switches themselves, the refresh is performed against all the end hosts and other devices connected to each LAN or distribution switch.

The appliance also tracks the time period required to collect switch-forwarding data and compares the result to the last ARP refresh. Once a timeout period is exceeded, NetMRI will re-perform the refresh on the switch, and repeat as necessary as data is collected from the device.

The appliance automatically performs an ARP cache refresh immediately before a new switch port polling session.