Port Aggregation Protocol (PAgP) Explained

Port Aggregation Protocol (PAgP) is a Cisco proprietary protocol for EtherChannel Configurations. It dynamically adds and manages interfaces in an EtherChannel. This tutorial explains how to configure, verify, and manage it on Cisco switches.

PAgP EtherChannel Configuration

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

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

It needs two arguments: an EtherChannel number and mode type. A switch supports up to six EtherChannels. Hence, you can use any number from 1 to 6 for the EtherChannel. There are five modes you can use with this command. The following table lists them.

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 auto mode, it listens for PAgP messages from the other side. If it receives PAgP messages, it enables the PAgP and adds the interface to it. 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, or download the following prebuilt lab and open it in Packet Tracer.

Download the Packet Tracer lab

If you add more than one link between two switches, the switches automatically disable all additional links to remove loops. Switches use the STP protocol for this.

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

As shown in the output above, STP blocked F0/2, F0/3, and F0/4 ports to prevent loops. Since STP has blocked these ports, the switch cannot 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 and 6. The channel number must be the same across all interfaces you want to add to a channel group. You can add up to eight ports to a single EtherChannel.

You must configure one side of a link in the desirable mode. You can configure the other side in desirable or auto mode.

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.

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

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.

As shown in the output above, 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. You can use the show etherchannel port-channel command to view detailed information about the EtherChannel.

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

As shown in the output above, this pool has four ports. The output also lists each port's state in the pool. The P indicates that the port has passed all necessary configuration checks.

Conclusion

Port Aggregation Protocol (PAgP) is an EtherChannel protocol. It is a Cisco proprietary 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 the other side, it matches the message's parameters against the port's parameters. If both match, it enables PAgP on the port. A switch can have six EtherChannels. An EtherChannel can have up to eight ports.