On Fri, Sep 17, 2010 at 12:17 PM, Darrell Root <[email protected]> wrote:
> > 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? > There are etherchannel modes that take this into account. If the switchport is etherchannel there won't be any issues since they will be treated as one port. Packets are forwarded and received based on an XOR operation in the addresses. If the ports are not etherchanneled they should not have the same mac address unless manually modified (not a good idea). There are etherchannel/trunking modes in most server OS's that will allow you to send on two ports but to only receive on one to keep from confusing the switch. Failover modes are also available. Also, most server OS's will not allow you to put the same IP address on an interface that is up and operational. If you were to have the same IP address on two NIC's with the same mac address connected to two non-stacked switches, there would be alot of issues. Each switch would have a CAM entry for the mac address on it's respective port that is connected directly to the server. If a frame were sent where the destination was on the other switch this would cause the receiving switch to expire it's entry for the directly connected NIC and begin sending traffic via the crosslink to the other switch. This change would soon be reversed though, as most computers constantly send control traffic such as mdns and all the smb stuff from M$. It would technically work despite the constantly changing CAM entries, if both server nics were able to receive any return traffic regardless of the source or application. It would cause high cpu on some switches depending on how much processing is done in software. It's pretty straighforward because the switches do not know there is a duplicate mac address until traffic is received from the other switch, hopefully that is minimal. Overall performance depends on what's in the rest of the network. For example: everything gets worse if there are a large number of downstream switches. Conclusion: Use etherchannel and stackable switches. _______________________________________________ cisco-nsp mailing list [email protected] https://puck.nether.net/mailman/listinfo/cisco-nsp archive at http://puck.nether.net/pipermail/cisco-nsp/
