Introduction to Port channels (Etherchannel)

Introduction to Port channels (Etherchannel)

One of the key feature of today’s networks is to be highly available. With increasing demand from our applications, network is supposed to provide more capacity on the links. Port channel is combining more than one physical links together. Logically these links appear as a single link. It can be seen below. 

Port channel is also known as Etherchannel. Port channel is logical port on network switch which bundles underlying physical links. It is nothing but few configuration commands on network switch.  Different platform has different flexibility for maximum number of physical links a port channel can combine. For example In Cisco Nexus 5000 series switches you can bundle maximum 16 links together.  Please note that there is compatibility requirement in order to successfully bundle the links. In simple terms all the links should be of same gender and capabilities. For example speed, duplex, flow control, port mode, VLANs,  MTU, and media type should match in order to successfully bundle links together. Different products choose different set of parameters for compatibility check. These parameters are shared by each switch when they are trying to bring port channel up. There is standard protocol to negotiate these parameters which is called LACP (Link Aggregation Control Protocol , Standard 802.1AX currently).

Port channel modes
Should a switch initiate port channel formation request to other switch or it should passively listen and wait for others to initiate? Should your switch even negotiate port channel with other switch? You need flexibility to achieve all of those scenarios. There are three modes of port channels.
Active: Port channel in active mode will attempt to initiate port channel bundling with other switch. If other switch responses with request and compatibility criteria is met  then port channel will be formed.
Passive: Switch silently waits for others to initiate the bundling request. When request is receive this switch in passive mode will reply back to form the port channel.
ON: This mode of port channel is always in bundle mode.

How to configure on Port channel on cisco Switches

Following config shows that we are bundling two ports g2/0/0 and g4/0/0 into port channel. This port channel will be a new logical interface on our switch.

Please also note that port channel mode in above configlet is Active which means this switch will initiate the negotiation of LACP. There are many tweaks and additional config commands for specific use cases. You can refer to cisco documentation for details.

Advantages of Port channel

Increased Bandwidth/Capacity: If you combine 2 physical links of 10 Gbps you get bandwidth of 20 Gbps on that port channel. Mostly uplinks are combined in this order to have more capacity on upstream connectivity.
High Availability: Switches will always be able to forward traffic on port channel as long as there is at least one active link on it. If you have 4 physical links combined in a port channel, you would always have connectivity even if 3 links are down. Ofcourse you will have lower bandwidth if your links are down.

Problem with Port channels:
Port channels require you to have physical links on same switch in order to bundle them together however for datacenter networks and many other uses cases you want to bundle links which are connected to two different switches. Different vendors have different implementation for this type of requirement, Cisco uses virtual Port channel (Multi Etherchannel) technology to achieve this.