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Publication date: March, 2007
Revision A1
45
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 trouble. 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 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 the item 1 is the case, you will pay much more cost beyond your budget,
and the solution caused by the limitation of hardware performance may not be
scalable.
If the item 2 is the case, now you do not have to pay much more extra cost
and can keep flexible according to the demand of bandwidth because all
equipments are there already. 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.