AL103 Revision 1.0
3.7 Port Aggregation (Trunking)
The AL103 supports trunking/port aggregation. Port aggregation and trunking is essentially a
method to treat multiple physical links as a single logical link. The benefit of trunking is the ability
to group multiple lower speed links into one higher speed link. For example, four full-duplex 100
Mbps links can be used as one single 800-Mbps link. This is very useful for switch to switch,
switch to server, and switch to router applications.
The AL103 considers a trunk as a single port entity regardless of the trunk composition. Two to
four ports can be grouped together as a single trunk link. The grouping of the ports in the trunk
must be from the top four ports or the bottom four ports of the device, i.e. port 0 to 3 or port 4 to 7.
In a multiple link trunk, the links within the trunk should have a balanced amount of traffic in order
to achieve maximum efficiency. One of the requirements for transmission is that the frames being
transmitted must be in order. Therefore, some sort of load balancing among the links of the trunk
must be deployed. The AL103 offers two methods of load balancing which can be selected in the
System Configuration Register I (register 00).
3.7.1 Load Balancing
The two load-balancing methods that AL103 uses to support trunking are port based and MAC
address based. Port based load balancing method is an explicit port assignment scheme. It requires
each individual port be assigned to a specific link (trunk port) in the trunk. If the port is not
assigned, the frame might be routed to the trunk randomly which may cause the frames to go out of
order. The port based load balancing trunk can be assigned as a 2-, 3-, or 4-port trunk.
During transmission of the frame, it will be routed from the source port to the assigned trunk port.
When a frame is received from any one of the trunk ports, it will be routed to the destination port
within the VLAN. In essence, the AL103 treats a trunk as any single port within the same VLAN. If
the ports traffic is evenly distributed among all the trunk ports, load balancing is achieved and the
aggregate bandwidth of the trunk can be as high as 800 Mbit/s (full-duplex).
The alternative is the MAC address based load balancing. When the AL103 receives a frame with a
trunk destination, it will automatically forward the frame to a port in the trunk based on the source,
destination, or the combination of the source and destination MAC address. The MAC address load
balancing decision is based on a proprietary algorithm. The MAC address based load-balancing
trunk also can be assigned as a 2-, 3-, or 4-port trunk.
3.7.2 Trunk Fail Over
If a link is lost in one of the trunk ports, frame loss will occur. The trunk fail-over feature in the
AL103 can prevent frame loss caused by link failure in a trunk port. When the trunk fail-over option
is enabled in register 2D, bit 9 (L2Fail), the AL103 will automatically shift the load from the port
with the lost link to the next available port. This option is available for MAC address based loading
trunk only. Once the port with a lost link recovers and links up, the AL103 will return to the original
trunk setting.
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Reference Only / Allayer Communications
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