Etherchannel Configuration Guidelines - Cisco WS-C3550-12G Software Configuration Manual

Configuring EtherChannel

EtherChannel Configuration Guidelines

If improperly configured, some EtherChannel interfaces are automatically disabled to avoid network
loops and other problems. Follow these guidelines to avoid configuration problems:
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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Each EtherChannel can have up to eight compatibly configured Ethernet interfaces.
Note Do not configure a GigaStack GBIC port as part of an EtherChannel.
Configure all interfaces in an EtherChannel to operate at the same speeds and duplex modes.
Enable all interfaces in an EtherChannel. An interface in an EtherChannel disabled by using the
shutdown interface configuration command is treated as a link failure, and its traffic is transferred
to one of the remaining interfaces in the EtherChannel.
When a group is first created, all ports follow the parameters set for the first port to be added to the
group. If you change the configuration of one of these parameters, you must also make the changes
to all ports in the group:
– Allowed-VLAN list
– STP path cost for each VLAN
– STP port priority for each VLAN
– STP Port Fast setting
An EtherChannel does not form if one of the interfaces is a Switch Port Analyzer (SPAN) destination
port.
A port that belongs to an EtherChannel port group cannot be configured as a secure port.
Before enabling 802.1X on the port, you must first remove it from the EtherChannel. If you try to
enable 802.1X on an EtherChannel or on an active port in an EtherChannel, an error message
appears, and 802.1X is not enabled. If you enable 802.1X on a not-yet active port of an
EtherChannel, the port does not join the EtherChannel.
For Layer 2 EtherChannels:
Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks.
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Interfaces with different native VLANs cannot form an EtherChannel.
– If you configure an EtherChannel from trunk interfaces, verify that the trunking mode (ISL or
802.1Q) is the same on all the trunks. Inconsistent trunk modes on EtherChannel interfaces can
have unexpected results.
– An EtherChannel supports the same allowed range of VLANs on all the interfaces in a trunking
Layer 2 EtherChannel. If the allowed range of VLANs is not the same, the interfaces do not
form an EtherChannel even when PAgP is set to the auto or desirable mode.
– Interfaces with different STP path costs can form an EtherChannel as long they are otherwise
compatibly configured. Setting different STP path costs does not, by itself, make interfaces
incompatible for the formation of an EtherChannel.
For Layer 3 EtherChannels, assign the Layer 3 address to the port-channel logical interface, not to
the physical interfaces in the channel.
Chapter 21 Configuring EtherChannel
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