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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 is unplugged accidentally in a static
trunk port, the administrator has to reconfigure it. Therefore, offering a tool with automatic recovery
capability is necessary for an administrator. LACP is a protocol that allows a switch able to know whether
its partner has the capability to co-setup a trunk between them.
Usually, if the 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 a logical link.
If item 1 is the case, higher costs occur, and the solution caused by the limitation of hardware
performance may not be feasible.
If item 2 is the case, extra costs are avoided and the network can be flexible according to the
demand of bandwidth because all equipment is already present. And what’s more, you can avoid worrying
about the interoperability issue. Applying LACP in your network, you will not only gain benefits below to
improve the performance of your network but also have these investments usable to future new products.
1. Public standardized specification
2. No interoperability issue
3. No change to IEEE 802.3 frame format, no change in software and management.
4. Increased bandwidth and availability
5. Load sharing and redundancy
6. Automatic
configuration
7. Rapid
configuration and reconfiguration
8. Deterministic
behavior
9. Low risk of duplication or mis-ordering
10. Support existing IEEE 802.3 MAC Clients
11. Backwards 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.