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Understanding How EtherChannel Works
EtherChannel Overview
Fast EtherChannel and Gigabit EtherChannel port bundles allow you to group multiple Fast or Gigabit
Ethernet ports into a single logical transmission path between the switch and a router, host, or another
switch.
Depending on your hardware, you can form an EtherChannel with up to eight compatibly configured Fast
or Gigabit Ethernet ports on the switch. In addition, on the Catalyst 4000 family switches, you can
configure an EtherChannel using ports from multiple modules. All ports in an EtherChannel must be the
same speed.
The switch distributes frames across the ports in an EtherChannel according to the source and destination
Media Access Control (MAC) addresses. If a port within an EtherChannel fails, traffic previously carried
over the failed port switches to the remaining ports within the EtherChannel. A trap is sent upon a failure
identifying the switch, the EtherChannel, and the failed link.
Both Fast and Gigabit EtherChannel bundles can be configured as trunk links. After a channel has been
formed, configuring any port in the channel as a trunk applies the configuration to all ports in the
channel. Identically configured trunk ports can be configured as an EtherChannel. For more information,
see the
"Configuring VLAN Trunks on Fast Ethernet and Gigabit Ethernet Ports."
Understanding Administrative Groups and EtherChannel IDs
Configuring an EtherChannel creates an administrative group, designated by an integer between 1 and
1024 inclusive, to which the EtherChannel belongs. You can assign an administrative group number
manually or let the system software assign the next available administrative group number automatically.
Forming an EtherChannel without specifying an administrative group number creates a new
automatically numbered administrative group consisting of the ports you configure as an EtherChannel.
An administrative group can contain a maximum of eight ports.
You can define an EtherChannel administrative group without forming an EtherChannel. Only ports
belonging to the same administrative group can form a single EtherChannel together.
In addition to the administrative group number, each EtherChannel is automatically assigned a unique
EtherChannel ID. Use the show channel group admin_group command to display the EtherChannel ID.
EtherChannel administrative group numbers are stored in NVRAM and remain the same after the switch
is reset or power cycled. EtherChannel IDs are not saved in NVRAM. The ID can change if the
EtherChannel is torn down and renegotiated, or if the switch is reset or power cycled.
Understanding the Port Aggregation Protocol
The Port Aggregation Protocol (PAgP) facilitates the automatic creation of Fast EtherChannel and
Gigabit EtherChannel links by exchanging packets between channel-capable ports. The protocol learns
the capabilities of port groups dynamically and informs the neighboring ports.
After PAgP identifies correctly paired channel-capable links, it groups the ports into a channel. The
channel is then added to the spanning tree as a single bridge port. A given outbound broadcast or
multicast packet is transmitted out one port in the channel only, not out every port in the channel. In
addition, outbound broadcast and multicast packets transmitted on one port in a channel are blocked
from returning on any other port of the channel.
Software Configuration Guide—Catalyst 4000 Family, Catalyst 2948G, Catalyst 2980G, Releases 6.3 and 6.4
6-2
"EtherChannel Configuration Guidelines and Restrictions" section on page 6-4
Chapter 6
Configuring Fast EtherChannel and Gigabit EtherChannel
and
Chapter 11,
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