[c-nsp] General switching question regarding load balancing host

[Matlock, Kenneth L]

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
matlockk at exempla.org


-----Original Message-----
From: cisco-nsp-bounces at puck.nether.net
[mailto:cisco-nsp-bounces at puck.nether.net] 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.
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