Port Aggregation Protocol (PAgP) Explained

Port Aggregation Protocol (PAgP) is a Cisco proprietary for EtherChannel Configurations. An EtherChannel is a group of links that work together as a single link. There are two ways to configure an EtherChannel: Static and Dynamic. In the static method, we manually configure an EtherChannel. In the Dynamic configuration, we configure an EtherChannel protocol, and then the protocol configure and manage the EtherChannel. PAgP is a EtherChannel protocol.

Commands/Steps for the PAgP EtherChannel Configuration

The following interface configuration mode command creates a new EtherChannel if an EtherChannel with the specified number does not exist and adds the interface to the EtherChanel. If an EtherChannel with the specified number exists, it adds the interface to the EtherChannel.

Switch(config-if)#channel-group [number] mode [mode type]

The above command needs two arguments: number and mode type. 

A switch supports up to six EtherChannel. Hence, you can use any number from 1 to 6 here. 

There are five modes you can use with this command. The following table lists them.

Mode Type EtherChannel Type Protocol Description
on static/manual Enable manual EtherChannel unconditionally
desirable dynamic PAgP Enable PAgP EtherChannel and send PAgP messages
auto dynamic PAgP Enable PAgP EtherChannel if PAgP message receives
active dynamic LACP Enable LACP EtherChannel and send LACP message
passive dynamic LACP EnableLACP EtherChannel if LACP message receives

For PAgP EtherChannel configuration, you have two modes: desirable and auto. If you configure the desirable mode, the switch enables PAgP EtherChannel and adds the interface. If you configure the auto mode, it listens for PAgP messages from another side. If it receives PAgP messages, it enables the PAgP EtherChannel and adds the interface.

You must run the above command on all interfaces you want to add to the EtherChannel.

EtherChannel Packet Tracer Example

Create a packet tracer lab, as shown in the following image.

packet tracer lab

Download the Packet Tracer lab

STP is the default on all Cisco switches. It automatically finds loops and block ports that cause the loops. 

stp disabled ports

We can verify it by viewing the STP port states. The show spanning-tree command displays the STP port states.

show spanning tree command

As we can see in the above output, STP blocked F0/2, F0/3, and F0/4 ports to remove loops. Since STP has blocked these ports, the switch can't use them to forward frames.

Access the interface configuration mode of F0/1, F0/2, F0/3, and F0/4 interfaces and run the following command.

Switch(config-if)#channel-group 1 mode desirable

You can use any channel number between 1 to 6. The channel number must be the same on all interfaces you want to put in a channel group. You can add up to eight ports to a single EtherChannel. 

pagp configuration

As mentioned earlier, you must configure one side of a link in the desirable mode. You can configure another side in the desirable or auto mode.

mode comparision

Access the interface configuration mode of F0/1, F0/2, F0/3, and F0/4 interfaces on the second switch and run the following command.

pagp configuration

Download the Packet Tracer lab with PAgP configuration

Verifying PAgP

There are many ways to verify the PAgP configuration. After the PAgP configuration, STP treats all links in the pool as a single link. You can use the show spanning-tree command to verify this.

show spanning tree

As we can see in the above output, there is only one interface instead of four. It is a pool of all four interfaces. STP treats this pool as a single interface. If any interface from this pool fails, it does not trigger STP. STP reruns its algorithm only when all interfaces inside the pool fail.

We can use the show etherchannel port-channel command to view detailed information about the EtherChannel.

etherchannel show command

We can also use the show etherchannel summary command to view the ports and their states in each etherchannel.

etherchannel summary

As we can see in the above output, this pool has four ports. The output also lists each port's state in the pool. The P means the port has passed all the necessary configuration checks.

This tutorial is the third part of the tutorial "EhterChannel Concepts and Configurations Explained.". Other parts of this series are the following.

Chapter 01 EtherChannel Basic Concepts Explained

Chapter 02 EtherChannel Manual Configuration

Chapter 04 Link Aggregation Control Protocol (LACP) Explained

Chapter 05 EtherChannel Load Distribution Explained

Conclusion

Port Aggregation Protocol (PAgP) is an EtherChannel protocol. It is a Cisco's propritey protocol. It works only on Cisco switches. It allows us to add up to eight ports in an EtherChannel. It supports two modes: desirable and auto. In desirable mode, it unconditionally enables the EtherChannel on the port and sends PAgP messages from the port. In auto mode, it listens for PAgP messages. If it receives PAgP messages from another side, it matches the message's parameters with the port's parameters. If both match, it enables the PAgP on the port. A switch can have six EtherChannels. An EtherChannel can have up to eight ports in it.

ComputerNetworkingNotes CCNA Study Guide Port Aggregation Protocol (PAgP) Explained