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Chapter 27 Configuring Eth erChannels Understanding EtherChannels
Figure 27-2 Relationship of Physical Ports, Logical Port Channels, and Channel Groups
After y ou co nf igu re an Eth erC hann el , configurati on chan ges appli ed to th e port -ch anne l inter fa ce ap ply
to all the physical interfaces assigned to the port-channel interface. Configuration changes applied to the
physical interface af fect only the interface where you apply the configuration. To change the parameters
of all por ts in an Et he rChann el , a pply c onfigura tio n co mma nd s t o the po rt-c hanne l in terfa ce, fo r
example, spanning- tree c omman ds or comm ands to configu re a Layer 2 EtherCha nnel as a t runk.

Understanding the Port Aggregation Protocol

The Port Aggregation Protocol (PAgP) facilitates the automatic creation o f 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 co nfigured in terfaces into a singl e logica l link (channel or aggregate port); the se
interfaces a re grouped ba sed on hardware, administrative, and port parameter constra ints. 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 tree
as a si ngle swit ch por t.
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