Another mechanism for redundant links in
a Spanning-Tree environment is called Fast EtherChannel technology.
Fast EtherChannel technology builds upon standards-based 802.3
full-duplex Fast Ethernet. This technology allows parallel links to
be treated by Spanning Tree as one physical link.
Fast EtherChannel technology offers
bandwidth scalability within the campus by providing full-duplex
bandwidth of 200 to 800 Mbps. The implementation of Fast
EtherChannel technology, in addition to providing high bandwidth,
also provides load sharing and redundancy. This technology provides
load balancing and management of each link by distributing Unicast,
multicast, and broadcast traffic across the links in the channel. In
addition, Fast EtherChannel technology provides redundancy in the
event of link failure. If a link is lost in a Fast EtherChannel,
traffic is rerouted to one of the other links in less than milliseconds, and the convergence is transparent to the user.
Fast EtherChannel and Gigabit
EtherChannel use a load distribution algorithm based on the
destination MAC address. 
A bundle is a group of links managed
by the Fast EtherChannel process. Fast EtherChannel technology
provides statistical load distribution of connections over multiple
links in a bundle. If one link in the bundle fails, the Ethernet
Bundle Controller (EBC) informs the Enhanced Address Recognition
Logic (EARL) of the failure, and the EARL ages out all addresses
learned on that link. This condition allows the EBC and the EARL to
recalculate in hardware the source-destination address pair on a
different link. When a source address queries and the destination
responds, the address is relearned on a different link in the
bundle.
Fail-over time is the time it takes
for the new address to be relearned. Assuming that one packet sent
by the source results in an instant response, fail-over takes place as quickly as 10 microseconds. Fail-over may take longer because
of the windowing that the particular application uses to send
packets before it expects an acknowledgment. Even then, relearning
should not take more than a few milliseconds, and as a result, no
application or session timeout will be seen due to link
failure.
When one link fails, all traffic is
relearned from that link and carried on other links in the Fast
EtherChannel. However, when the link comes back, traffic will not
necessarily be immediately relearned on the reinstated link. For
example, suppose link 1 fails and is relearned on link 2. Now link 1
is reconnected with all other active links. Traffic previously on
that link never reverts back. Traffic stays on link 2 (or the link
the traffic was relearned on after link 1 failure) until it is aged
naturally out of the content-addressable memory (CAM) table, in
which case, when that address is relearned, it is learned again on
link 1.
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