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The Port Aggregation Protocol (PAgP)
adds additional features to Fast EtherChannel technology. PAgP aids
in the automatic creation of Fast EtherChannel links. PAgP packets
are sent between Fast EtherChannel technology-capable ports. The
protocol learns the neighbors and their group capability dynamically
and informs the neighbors of its local group capability. After the
protocol determines correctly paired, bidirectional, point-to-point
links, the protocol groups the ports that have the same neighbor
device ID and neighbor group capability into a channel. Then the
channel is added to Spanning Tree as a single bridge port.
Some restrictions have been
deliberately introduced into PagP, which will not form a bundle on
ports that are configured for dynamic VLANs. PAgP requires that all
ports in the channel belong to the same VLAN or are configured as
trunk ports.
Because dynamic VLANs can force the
change of a port into a different VLAN, dynamic VLANs are not
included in Fast EtherChannel participation. When a bundle already
exists and a VLAN of a port is modified, all ports in the bundle are
modified to match that VLAN. PAgP does not group ports that operate
at different speeds or port duplex. If speed and duplex are changed
when a bundle exists, PAgP changes the port speed and duplex for all
ports in the bundle.
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