[c-nsp] General switching question regarding load balancing

[Darrell Root]

Jay wrote:
> 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?

I remember a case where one 4507 switch had 2 ports which were "switchport mode access".  But
there was one host out those two gig ports which was configured as a 2-port etherchannel on the
host side.

The 4507 was constantly updating it's mac address table:  source mac xxxx.xxxx.xxxx is now out
gig 3/1, wait now it's out gig 3/9, wait now its out gig 3/1, wait now it's out gig 3/9.  CPU on the
4507 went way up in the Cat4k Mgmt LoPri process.

Of course we just saw the high switch cpu.  It was hard to figure out that it was two access ports
going to one host which was a 2-gig etherchannel and took a TAC case in that circumstance.

I'm assuming plugging into 2 different switches will result in similar behavior, because somewhere
in your switch mesh you'll have a switch which has to constantly update a particular mac entry between
two different target ports. 

BTW, if anyone knows an easy way to identify the above problem (particular mac address constantly
cycling between two destination ports resulting in high cpu) I'd appreciate it.

Darrell