Cisco Systems 3550 Understanding the Port Aggregation Protocol

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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter21 Configuring EtherChannel Understanding EtherChannel
Figure21-2 Relationship of Physical Ports, Logical Port Channels, and Channel Groups
After you configure an EtherChannel, configuration change s applied to the port-channel interface apply
to all the physical interfaces assigned to the port-channel interface. Configuration changes applied to the
physical interface affect only the interface where you apply the configurat ion. To change the parameters
of all ports in an EtherChannel, apply configuration c omm ands t o th e port- ch anne l i nte rfa ce, for
example, Spanning Tree Protocol (STP) commands or commands to confi gur e a La yer 2 Et herC ha nne l
as a trunk.
Understanding the Port Aggregation Protocol
The Port Aggregation Protocol (PAgP) facilitates the automatic creation of EtherChannels by
exchanging packets between Ethernet interfaces. By using PAgP, the switch learns the identity of
partners capable of supporting PAgP and learns the capabilities of each interface. It then dynamically
groups similarly configured interfaces into a single logical l ink ( c hanne l or agg re gate por t); thes e
interfaces are grouped based on hardware, administrativ e, a nd po r t pa rameter constraints. For example,
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 spanning tr ee
as a single switch port.
MODE
SYSTEM
RPS
STATUS
UTIL
DUPLX
SPEED
2
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Catalyst 3550
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Logical
port-channel
Channel-group
binding
Logical
port-channel
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10/100/1000 ports
Physical
ports
GBIC module slots