Etherchannel Configuration Guidelines; Configuring Etherchannels - Cisco Catalyst 2950 Software Configuration Manual

Chapter 25

Configuring EtherChannels

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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Configuring EtherChannels
You configure Layer 2 EtherChannels by configuring the Ethernet interfaces with the channel-group
interface configuration command, which creates the port-channel logical interface.
Note Layer 2 interfaces must be connected and functioning for IOS to create port-channel interfaces.
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Each EtherChannel can have up to eight compatibly configured Ethernet interfaces.
Configure all interfaces in an EtherChannel to operate at the same speeds and duplex modes.
Enable all interfaces in an EtherChannel. If you shut down an interface in an EtherChannel, it 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
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STP path cost for each VLAN
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– STP port priority for each VLAN
– STP Port Fast setting
If you configure SPAN on a port that is a member of the EtherChannel, it leaves the EtherChannel.
For EtherChannels:
– Assign all interfaces in the EtherChannel to the same VLAN, or configure them as trunks.
– If you configure an EtherChannel from trunk interfaces, verify that the trunking mode (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
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compatibly configured. Setting different STP path costs does not, by itself, make interfaces
incompatible for the formation of an EtherChannel.
Catalyst 2950 Desktop Switch Software Configuration Guide
Understanding EtherChannels
25-7