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Catalyst 2940 Switch Software Configuration Guide
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Chapter25 Configuring EtherChannels Understanding EtherChannels
Figure25-2 Relationship of Physical Ports, Logical Port Channels, and Channel Groups
When a port joins an EtherChannel, the physical interface for that port is shut down. When the port
leaves the port-channel, its physical interface is brought up, and it has the same configuration as it had
before joining the EtherChannel.
Note Configuration changes made to the logical interface of an EtherChannel do not propa gate to all the
member ports of the channel.
Understanding the Port Aggregation Protocol and Link Aggregation Protocol
The Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP) facilitate the
automatic creation of EtherChannels by exchanging packets b etwe en Et her net int erfac es . PAgP is a
Cisco-proprietary protocol that can be run only on Cisco switc hes a nd on tho se switc hes lic en sed by
licensed vendors to support PAgP. LACP is defined in IEEE 802.3AD and allows Cisco switches to
manage Ethernet channels between switches that conform to the 802. 3A D pr otoc ol.
By using one of these protocols, a switch learns the identi ty of par tner s c apa ble of suppor t ing eit her
PAgP or LACP and learns the capabilities of each interface. It then dynamically groups similarly
configured interfaces into a single logical link (channel or aggregate port); these interfaces a re grouped
based on hardware, administrative, and port parameter constraints. For exam ple , PAgP groups the
interfaces with the same speed, duplex mode, native VLAN, VLAN range, and trunking status and type.
After grouping the links into an EtherChannel, PAgP adds the group to the span ning tre e as a sing le
switch port.

PAgP and LACP Modes

Table25-1 shows the use r-configurab le Et herC ha nnel m odes f or the channel-group interface
configuration command. Switch interfaces exchange PAgP packets only with partner interfaces
configured in the auto or desirable modes. Switch interfaces exchange LACP packets only with partner
interfaces configured in the active or passive modes. Interfaces configured in the on mode do not
exchange PAgP or LACP packets.
MODE
SYSTEM
RPS
STATUS
UTIL
DUPLX
SPEED
2
11
Catalyst 3550
2345678910
Logical
port-channel
Channel-group
binding
Logical
port-channel
45144
10/100/1000 ports
Physical
ports
GBIC module slots