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Port Trunking and Load Balancing in Blade Switches
Circumstances can occur where one address in the source/destination pair is a constant. For
example, the destination might be a server or, even more likely, a router. In that case, if both
the source address and destination address option is selected, you will still see statistical load
balancing, because the source address is always different.
Default Settings for Load Balancing
The default four the interconnect switch is to use the source MAC address-based load
balancing. This means that all packets the switch receives on a non-trunk port with the same
source MAC address (SA), and that are destined to MAC addresses on the other side of the
port trunk, will use the same link in the port trunk. Source-based forwarding should be used
when many stations attached to the switch are sending to a few stations (such as a single
router) on the other side of the port trunk. This better distributes traffic across all links in the
port trunk.
Also, switches maintain a notion of a "primary" port on which to transmit traffic such as
Spanning Tree Protocol, multicasts, and unknown unicasts. The properties of this primary
port determine the properties of how the port trunk works with features like Spanning Tree,
VLAN, multicasting, and so on.
By default, XConnects between Switch A and Switch B in the chassis form a port trunk
“XConnect’ with two links, as shown in the following figure.
With source-MAC address forwarding, when packets are forwarded to a port trunk, they are
distributed across the ports in the port trunk based on the source-MAC address (SA) of the
incoming packet. Therefore, to provide load balancing, packets from different hosts use
different ports in the port trunk, but packets from the same host use the same port in the port
trunk.
With destination-MAC address forwarding, when packets are forwarded to a port trunk, they
are distributed across the ports in the port trunk based on the destination host's MAC address
(DA) of the incoming packet. Therefore, packets to the same destination are forwarded over
the same port, and packets to a different destination are sent on a different port in the port
trunk.
G-2 HP ProLiant BL e-Class C-GbE Interconnect Switch User Guide
HP CONFIDENTIAL Codename: DeLorean Part Number: 263682-002 Last Saved On: 2/5/03 10:51 AM