In my experience, you need to be VERY careful about the topology, as we've been bitten by this before. A server was set to load-balancing like that, and every other packet was causing the MAC to get re-learned by the switches off the other port.
Now normally it wasn't causing problems, but in this case when the traffic level got high enough that the MAC was flapping fast enough that we'd catch edge-cases where a packet comes in one port, and by the time the packet would get to forwarding that packet out, the MAC would have been learned along the port it just came in on. The switch was recognizing this and dropping the packet. So we'd see random packet loss to that device. Dropped it back to just 1 NIC (or 2 NICs off the same switch) and we were fine. Ken Matlock Network Analyst Exempla Healthcare (303) 467-4671 [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Jay Nakamura Sent: Friday, September 17, 2010 9:53 AM To: cisco-nsp Subject: [c-nsp] General switching question regarding load balancing host Some hosts/OS can do load balancing between NICs. If a host has two NICs connected to two switches,(the two switches are connected together) and load balancing between them, switch will see the same source MAC from two ports. How does a switch decide which ports to put in the forwarding table? Would it switch back and forth every time there is a packet? Is there any negative effect on the switch when that happens? Is this platform dependent? Sorry for the generalized question. _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/ _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
