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CCNA RnS Article #36: Improve STP with EtherChannel (L2 Static)

November 06, 2023 , ,

CCNA RnS Article #36: Layer 2 EtherChannel Configuration

In this article, we are going to see how layer 2 (L2) EtherChannel configuration is performed. EtherChannel is one of the features that make STP/RSTP better. Multiple physical links between two switches are treated as individual links and to avoid a loop one of the links works and the rest are put into a blocking state. However, if an EtherChannel is configured these physical links are known as EtherChannel which is a logical interface.

For an example you can see there are two different Port-Channels ( EtherChannel) Po1 and Po2 in the below sample diagram to clarify where they actually configured. 

Etherchannel @thenetworkdna.com
Fig 1.1- EtherChannel

An Ethernet Switch considers an EtherChannel as a single link and it remains up until all physical links go down. Any physical link failure in EtherChannel does not cause STP convergence. EtherChannel in a network provides two major benefits – One, multiple links bandwidth is combined and there is more capacity available on EtherChannel to carry traffic compared to a single physical link. 

Traffic is load-balanced across multiple links. Two, high availability as there are multiple links are bundled together, and in case of one link failure others can take care of traffic forwarding – without going through STP convergence.

There are two ways an EtherChannel is configured –

  • Manual EtherChannel Configuration
  • Dynamic EtherChannel Configuration

⭐ Manual EtherChannel Configuration 👇

The channel-group <number> mode on command is used to configure manual EtherChannel. This command needs to be entered for each physical link. All physical links that need to be part of the EtherChannel should use the same “<number>” argument in the command.

Figure 1: Multiple links without EtherChannel

As per Figure 1, there are 3 physical links between Switch 0 and Switch 1. The output from Switch 0 shows that only one interface Fa0/1 is in the forwarding state (Root) rest all are in the blocking state. The state of these ports is alternate which means in case of root port failure one of the links will be transitioned to forwarding and be a root port. Any link failure will cause STP to converge.

Till now it is clear, that there is no benefit of having multiple links between switches if we are not able to use them effectively. Although we have 100x 3 = 300Mbps of connections available but only 100 Mbps is usable as other physical ports are blocking. Ethernet solves this problem. Let’s see this using our example.  

⭐Related : Switching basics: Etherchannel Guard Misconfig Command



Figure 2: Alternate Blocking State for redundant link

 

Figure 3: Static EtherChannel Configuration Example

Figure 3 shows interface fa0/1 – 3 are part of an EtherChannel 1. Once the EtherChannel is formed between two switches the STP output confirms all ports are part of EtherChannel. This port channel is layer 2 and in Use –

Figure 4: EtherChannel Summary

As per Figure 5, only one interface (logical – port-channel 1) is showing up and is forwarding. All physical links are grouped into a logical interface and STP does not see it as a problem. Till there is a single link in the UP state, this port channel will keep working without any STP convergence.

Please don’t get confused with terms – EtherChannel, Channel-Group, and Port-group all are synonyms.

Hope you got the idea that a network Ether Channel increases the total bandwidth between segments and improves STP function. We’ll see you will part 2, where we’ll cover how dynamic EtherChannel is formed. Hope you find this informative.

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