today's lesson:

today's lesson:

- some machines shut down their NIC for having a flapping connection.
IBM AS/400 shut it's NIC after being forced down...
case: IBM's NIC connects to switch as 10 mbps half duplex, switch are set to 100. after 10 minutes or so, the interface goes down and up by itself. after 3 consecutive downs and ups, it stopped.

- do not change configuration on live production network.

- do not ever clear arp table on live production network, unless on life-death situation.

- do not trust line status and protocol status on cisco switch, NIC connected to it can be unreachable while switch reports protocol OK.

- mitigate network problem bottom-up, don't guess, ask, prove it...

- double gateway scenario never dies.... possible to create a routing loop! (with L3 switches and VLAN interface)