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Port Trunking and Load Balancing in Blade Switches
Introduction
IEEE 802.3ad and EtherChannel compatible port trunks allow multiple physical Ethernet links to be combined into one logical channel/trunk. This allows load sharing of traffic among the links in the port trunk as well as redundancy in the event that one or more links in the port trunk should fail. Port trunks can be used to interconnect
A port trunk aggregates the bandwidth of up to eight compatibly configured ports into a single logical link. Blade switches support a maximum of six port trunks. All Ethernet ports support port trunks with no requirement that the ports be contiguous, but do require that they must be the same speed.
NOTE: Dynamic Link Aggregation Control Protocol (LACP) is not supported.
Load Balancing: Determining which Link to Send Traffic Across
The
(SA XOR DA) in the frame to be forwarded across the port trunk.
A port trunk distributes frames across the links by reducing the last three lower order bits of the binary pattern formed from the MAC addresses in the frame to a numerical value. In addition, the port trunk calculates the modulus of that numerical value against the number of available links in that port trunk, to determine which one of the links to send traffic across. IEEE 802.3ad/Port trunk frame distribution policies are based on hashing algorithms that use formulas mentioned below with examples. The algorithm is deterministic; given the same addresses and session information, you always hash to the same port in the port trunk, preventing
The selected mode applies to all port trunks configured on the switch. Use the option that provides the greatest variety in your configuration.
For example, if the traffic on a port trunk is going only to a single MAC address, using the destination MAC address always chooses the same link in the port trunk; using the source addresses or IP addresses might result in better load balancing.
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