Etherchannel

Today we are going to talk about etherchannel. As you know redundancy is huge when it comes to networking, and building a network that can stay online through many failures. Eliminating a single point of failure is one of the best ways to provide redundancy and etherchannel does just that. Etherchannel is a technology in switches that allows you to group and combine multiple physical cables into one logical link. You can turn 3,4,5,6 redundant cables to act as one logically, the switch will interpret it as one and even STP will interpret it as one cable. The max amount of cables that you can bundle is 16 but not all can be active. In LACP only 8 can be active and 8 can be configured on standby. This provides redundancy because if any of those cables were to fail, the link between the switches would continue to operate as if nothing happened, providing a very high availability without any real drawbacks.

There are some rules for this to work though. In order for this to work, the cables that are being grouped together must be redundant meaning that there are two or more cables that connect between the same pair of switches. So say Switch 1 connects to Switch 2 with 3 cables, since there are 2 redundant cables they can all be grouped into one. When they are grouped into one, they are called a port-channel. This port-channel will be the new interface, where you can edit all of the bundled logical links at once. When you look at a STP when etherchannel is enabled it will list the port-channel instead of the individual links. There are more rules that must be followed in order for cables to be logically bundled. In order for etherchannel to work all the redundant cables need to have the same speed, duplex, switchport mode, and the same VLAN configuration. Without these settings matching an etherchannel will fail to form.

Per usual there’s an industry standard which is LACP (Link Aggregation Control Protocol) and a Cisco proprietary PaGP (Port Aggregation Protocol). There is also an etherchannel mode that can be configured statically.

LACP- LACP is the standard protocol and can be used universally on switch devices. This protocol has two modes called “active” and “passive.” When an etherchannel is formed with the mode active, the switch will actively or intently try to form an etherchannel with the neighboring links in the same port channel group. Every port channel is assigned with a unique number. When an etherchannel is configured in passive mode, that means that the ports on that switch will actively wait for the switch in the same port-channel group to initiate an etherchannel connection. The passive mode will not initiate, meaning if both switches are configured with LACP in passive mode, an etherchannel bundle will never form. The only combination that will form an etherchannel is “active + passive” or “active + active.” 

PaGP- PaGP is the Cisco proprietary version of etherchannel. It basically works the same way as the industry standard LACP, but instead of it being called active it is called “desirable”. And instead of it being called passive it is called “auto.” Even though there are different names they essentially do the same thing, desirable mode is the mode where if configure will try to form an etherchannel bundle with the identical links on the other end. If the port-channel was configured with the mode auto, then the interfaces it was configured on will not actively seek to form an etherchannel bundle but will wait patiently until the other side initiates. When the other side initiates via advertisements, it will then form an etherchannel. If both sides are configured with auto, then no etherchannel bundle will form. The only combinations that will form an etherchannel bundle are “desirable + auto” and “desirable + desirable.” 

Static- The last option for etherchannel is a statically assigned etherchannel. It functions the same way, but this time there is no mode that will patiently wait for the other side to initiate. The only mode that exists in a static etherchannel is on, therefore the only option for an etherchannel bundle is “on + on.” 

Load Balancing- One more feature to mention about etherchannel is its ability to load balance between which specific cable the data will go through for a specific session. Traffic can be load balanced by a form of 6 criteria, I am going to list it out here: 

1. Source MAC
2. Destination MAC
3. Source and destination MAC
4. Source IP 
5. Destination IP 
6. Source and destination IP 

When you are load balancing with an etherchannel, you are only allowed to pick only one of those six options. The most recent configuration will take precedence. The protocol whether PaGP or LACP will take whatever criteria you choose and perform a calculation that provides a hash. The result of this hash will determine which cable will be assigned for the traffic flow.