If improperly configured, some Fast
EtherChannel ports are disabled automatically to avoid network loops
and other problems. Use the following guidelines to avoid
configuration problems:
- Assign all ports in a channel to
the same VLAN, or configure the ports as trunk ports.
- If you configure the channel as a
trunk, configure the same trunk mode on all the ports in the
channel, on both ends of the link. Configuring ports in a
channel in different trunk modes can have unexpected results.
- Configure all ports in a channel
to operate at the same speed and duplex mode (full or half
duplex).
- If configuring a broadcast limit
on the ports, configure the broadcast limit as a percentage
limit for the channeled ports. With a packets-per-second
broadcast limit, unicast packets might get dropped for one
second when the broadcast limit is exceeded.
- If the channel is composed of
trunk ports, you must configure the same allowed VLAN range on
all the ports. When the allowed VLAN range is not the same for
all trunks in a channel, trunk ports on which a particular VLAN
is not allowed will drop the packets for that VLAN, while ports
on which the VLAN is allowed will transmit the traffic. If the
allowed VLAN range is not the same on all ports in the channel,
the ports do not form a channel when set to the auto or
desirable mode with the set
port channel command.
- Do not configure the ports in a
channel as dynamic VLAN ports. Doing so can adversely affect
switch performance.
- Make sure port security is
disabled on channeled ports. If you enable port security on a
channeled port, the port shuts down when it receives packets
with source addresses that do not match the secure address of
the port.
These rules are generally applicable
to most EtherChannel technology-capable modules; however, some
exceptions exist with later modules. For example, the Catalyst 6000
line cards do not constrain you to use even numbers of links. Three
link bundles can be created. Nor do the
ports have to be contiguous, or even on the same line card, as is
true with some Catalyst devices and line modules. The exceptions to
the EtherChannel rules for the Catalyst 6000 are as a result of the
newer chipsets on the line modules. These newer chips are not
present on all hardware. Be sure to check your hardware features
before attempting to create any of these other bundle types.
The EBC of earlier EtherChannel
technology-capable modules manages aggregated EtherChannel ports.
For example, the EBC manages traffic distribution across each
segment in the bundled link. The distribution mechanism is described
later in this section.
When selecting ports to group for an
EtherChannel process, you must select ports that belong to the same
EBC. On a 24-port EtherChannel technology-capable module, there are
three groups of eight ports. On a 12-port EtherChannel
technology-capable module, there are three groups of four ports. 
For example, in a 12-port module, you
can create up to two dual-segment EtherChannel configurations within
each group, as illustrated in Example A of Figure .
Or, you can create one dual-segment EtherChannel configuration
within each group, as in Example B of Figure .
Example C illustrates a four-segment and a two-segment EtherChannel
configuration.
You must avoid some EtherChannel
configurations on early Catalyst 5000 equipment. Example D of Figure
illustrates an invalid two-segment EtherChannel configuration using
Ports 3 and 4 of a group. The EBC must start its bundling with the
first ports of a group. This does not mean that you have to use the
first group. In contrast, a valid dual-segment EtherChannel
configuration can use Ports 5 and 6 with no EtherChannel segment on
the first group.
Example E illustrates another invalid
configuration. In this example, two EtherChannel segments are
formed. One is a dual-segment EtherChannel configuration, the other
is a four-segment EtherChannel configuration. The dual-segment
EtherChannel configuration is valid. The four-segment EtherChannel
configuration, however, violates the rule that all ports must belong
to the same group. This EtherChannel configuration uses two ports
from the first group and two ports from the second group.
Example F shows an invalid
configuration where an EtherChannel configuration is formed with
discontiguous segments. You must use adjacent ports to form an
EtherChannel configuration.
Finally, Example G shows an invalid
EtherChannel configuration because it does not use the first ports
on the module to start the EtherChannel process. You cannot start
the EtherChannel process with middle ports on the line module.
All the examples in Figure
apply to the 24-port modules too. The only difference between a 12-
and 24-port module is the number of EtherChannel segments that can
be formed within a group. The 12-port module allows only two
EtherChannel segments in a group, whereas the 24-port module
supports up to four EtherChannel segments per group.
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