Port Aggregation Protocol (PAgP)

Port Aggregation Protocol (PAgP)
The Port Aggregation Protocol (PAgP) is used to add more features to the EtherChannel technology.
This protocol is used to learn the capabilities of the neighbors’ EtherChannel ports. By
doing this, it allows the switches to connect via Fast EtherChannel automatically. PAgP has four
options when configuring the channel: on, off, desirable, and auto. The first two—on and
off—are self-explanatory. A desirable link wants to become a channel, whereas a link set to
auto doesn’t want to but will if it has to. A channel will form if one of the following combinations
is used:
 on-on
 on-desirable
 on-auto
 desirable-desirable
 desirable-auto
The PAgP protocol groups the ports that have the same neighbor device ID and neighbor
group capability into a channel. This channel is then added to the Spanning Tree Protocol as a
single bridge port.
For PAgP to work, all the ports must be configured with static, not dynamic, VLANs, and
all the ports must also be in the same VLAN or be configured as trunk ports. All ports must be
the same speed and duplex as well. In other words, all the ports must be configured the same
or PAgP will not work.
If an EtherChannel bundle is already working and you make a change on a port, all ports in
that bundle are changed to match the port. If you change the speed or duplex of one port, all
ports will then run that speed or duplex.