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3-6. Link Aggregation
Basically, Link Aggregation is to aggregate the bandwidth of more than one
port to an assigned logical link. This highly increases total bandwidth to the targeted
device. There is more than one Link Aggregation technology in many vendors’
switch products already, which may cause the problem of interoperability. This is the
reason why now we have 802.3ad Link Aggregation Control Protocol (LACP).
Why 802.3ad (LACP)?
Network is varying. For example, if a port malfunctioned or unplugged
accidentally in a static trunk port, administrator has to reconfigure it, or the network
will get in trouble. Therefore, offering a tool with automatic recovery capability is
necessary for an administrator. LACP is a protocol that allows a switch to be able to
know whether its partner has the capability to co-setup a trunk between them.
Usually, if administrator wishes to increase the bandwidth of a specific link,
he may:
1. Buy new network equipments with higher throughput, or
2. Aggregate the bandwidth of more than one port to become a local link.
If item 1 is the case, you will have to pay much more for the equipments; and
the results may not be scalable if the unsatisfied performance is caused by
hardware limitations.
If item 2 is the case, no extra cost is required, and the demand of bandwidth
can be flexible because all that needs is to reconfigure equipments that are there
already. And what’s more,
no worries necessary regarding the interoperability issue.
Applying LACP in your network, you will not only gain benefits as listed below to
improve the performance of your network but also have these investments reusable
to future new network bandwidth planning.
Public standardized specification
1. No interoperability issue
2. No change to IEEE 802.3 frame format, no change necessary for
software and management.
3. Increased bandwidth and availability
4. Load sharing and redundancy
5. Automatic configuration
6. Rapid configuration and reconfiguration
7. Determinated behavior
8. Low risk of duplication or mis-ordering
9. Support existing IEEE 802.3 MAC Clients
10. Backward compatibility with aggregation-unaware devices
There are also some constraints when applying LACP.
1. LACP does not support inter-switch bandwidth aggregation.
2. The ports aggregated must operate in full-duplex mode.
3. The ports in the same Link Aggregation Group must have the same
speed, for example, all with 100Mbps or all 1000Mbps. You cannot
aggregate a 1000Mbps and two 100Mbps for a 1.2Gbps trunk port.