PVST/RPVST and EtherChannel Explained

Administrators connect switches via multiple links to decrease the chance of network failure when the primary link fails. However, connecting switches with more than one link creates switching loops. A network never works with a switching loop. 

Cisco switches use a default protocol called PVST (Per VLAN Spanning Tree) / RPVST (Rapid Per VLAN Spanning Tree) to remove loops. It dynamically manages all links. If a switch has many paths to reach a destination, it chooses only one path and blocks all remaining paths to remove loops. If the selected path fails, it automatically enables the backup path.

However, there are two situations where administrators need a better option. These situations are the following.

  • Administrators want to configure multiple links between switches for load balancing.
  • Administrators want to speed up the PVST/RPVST convergency.

Load balancing

Load balancing optimizes the network and increases the data transfer speed. It equally distributes the load among all participants. Let's take an example. Suppose we have one connection between two switches. All data transfers will happen on this connection. But, if we have two connections, each connection will transfer only half of the total data. Since each connection has to transfer less data, it works faster. Besides speeding up the data transfer speed, load balancing provides many other advantages. However, load balancing and PVST/RPVST are opposite of each other. Load balancing needs more than one link to work, while PVST/RPVST keeps only one link to remove loops.

EtherChannel solves this problem. It combines multiple links into a single link. Since all links work as a single, PVST/RPVST does not block them. On the other hand, load balancing can use all links in the group to distribute the network traffic.

ehterchannel

PVST/RPVST convergency

PVST/RPVST starts all ports through various states. During these states, ports can not forward user frames. It causes network delay. For example, if the switch is running PVST, it cannot forward any traffic till the PVST reaches convergency. PVST needs fifty seconds to complete its operations and reach convergency. The switch can't forward user frames during these seconds. PVST repeats this process whenever there is a change in the network topology. 

For example, when a link connected to the port fails, PVST needs fifty seconds to select another port. A fifty-second delay for every network change badly affects the network performance. EtherChannel solves this problem. Since all links in an EtherChannel operate as a single link, PVST does not re-run its algorithm as long as a single link in the EtherChannel is functional.

EtherChannel Packet Tracer Example

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

packet tracer lab

Download the packet tracer lab.

This network has two switches. Both switches have two connections. These connections simulate load balancing. However, load balancing does not work until PVST is running. PVST treats these links as a loop and blocks one link to remove the loop.

stp blocked port

The show spanning-tree command displays all the running PVST instances on the switch. The output of this command displays the PVST status of all active ports. The following image shows the output of this command on both switches.

show spanning tree command

As the above output shows, PVST has blocked the G0/2 on S1 to remove the loop. It verifies the first situation. Remove one link and observe the time switches take to use the second link. 

Switches do not enable the backup link until PVST finishes its operation. It verifies the second situation. Connect the removed link again and configure EtherChannel on both switches.

EtherChannel configuration

Download the packet tracer lab with EtherChannel configuration.

After EtherChannel configuration, switches treat both links as a single link.

lab after etherchannel configuration

Run the show spanning-tree command on both switches again to verify all links appear as a single link to PVST/RPVST.

show spanning tree command

Since PVST/RPVST treats all links of the EtherChannel as a single, it does not change its database when a link from the EtherChannel goes down. 

link down

It will update the ports' status only when all links in the EtherChannel go down.

Conclusion

EtherChannel solves two main problems of PVST/RPVST. It allows us to create multiple connections for load balancing and redundancy. It also reduces the PVST/RPVST convergence time. PVST and RPVST are Cisco's STP variations. Cisco builds them on STP/RSTP. EtherChannel and PortFast features are available only on PVST and RPVST. They are not available on STP/RSTP. You can use these features only on Cisco switches.

ComputerNetworkingNotes CCNA Study Guide PVST/RPVST and EtherChannel Explained