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Aggregated Ethernet
Aggregated Ethernet (AE) enables you to group multiple Ethernet interfaces to form a
single physical link at the MAC layer, allowing you to increase your speed beyond what
a single link can provide, as well as increasing redundancy. AE has had several names
over the years. Officially, it is a standard in 802.1AX; however, many people still refer
to it as 802.3ad because it appeared in section 41 of that standard. Other names that
have been used by various vendors include NIC teaming, port channel, port teaming,
port trunking, link bundling, and EtherChannel. In this chapter, however, we will refer
to the link as AE.
How does a standard get its name, such as 802.1AX or 802.3ad? Well,
the 802 refers to an actual working group for the standard. The letters
themselves have no meaning; letters are just incremented starting from
a and going to z. So, in order: a, aa, ab, b, bb, and so on. The capitalization
indicates a standalone document, whereas lowercase denotes a
supplement to an existing document. At least, that was the idea when
the naming convention was created.
Historically, link aggregation is configured statically, meaning that the local device
bonds the physical interfaces together. This meant that there was no way to link path
failures (if there was an intermediate device) or to know whether the other end of the
link was also bundling the interface. For that reason, the industry-standard Link Aggregation
Control Protocol (LACP) was created. Essentially, LACP sends messages to
the remote end of the link to determine bundle parameters as well as link availability.
This creates a “bundle” with multiple physical “members.”
LACP is not necessary in order to create an AE link. You can create the configuration
statically on each link, essentially telling the system to bundle several links together
and be on its way. However, there are some problems with static configuration:
• There is no keepalive mechanism to determine whether the entire path is up if there
is an intermediate device between the bundle.
• There is no way to ensure that the port physically connects to the correct device
(see Figure 11-3, shown previously).
• There is no way to ensure that the configuration is correct on both sides.
LACP solves this without any change to the actual Ethernet frame format used for
transport across the bundle. Essentially, LACP periodically advertises the aggregation
capabilities and state information across each member link. The partner (the other end
of the link) compares its own configuration to what was received, and decides whether
the link should be part of the bundle. Also, if a link was already determined to be part
of the same bundle, and LACP messages are not received for a period of time, that link
can be brought down.
These messages are sent on a link-by-link basis, asynchronously. No synchronization
is necessary for the bundle.
On each aggregated link, a partner and an actor are created. The actor is the device that
takes the action, and the partner is the device sending the message. An actor compares
its local configuration with the received partner message. This messaging happens in
both directions; as a result, a given link is always an actor or a partner—the name is
relative to a given direction. This means, however, that an additional check can be
made. In confusing terms, we can say that the partner’s partner information is a view
of the actor’s current configuration. Thus, the partner’s partner information can be
compared with its own actor state to make sure the partner received the correct information
from the actor. In simpler terms, the received state is sent back to the sender in
order to verify that the sender information was received.
In order to make sure aggregated bundles are formed only with links that interconnect
the same devices, some type of identifier must be used to identify the bundle. LACP
uses a system ID to identify messages from a device, and only messages received from
the partner with the same system ID will be considered for link aggregation.
你的交换上也需要把相应的port做link aggregation。绑定成一个interface。
你是啥交换机，配置贴上来？ 802.1ad（汇聚）支持否？ |
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发表于 2011-02-10 16:35
发表于 2011-02-11 16:15