My Struggle With The Command “channel-protocol lacp”
A summary of my experience with the "channel-protocol lacp" command.
The Struggle
Studying EtherChannels on the CCNA 200-301, I didn’t get to see switch(conf-if)# channel-protocol lacp in action. Not only did seeing the lab in Boson strange & difficult, but it forces me to be more creative with weird lab scenarios if I can’t find them on the app. After all, the test is all about awkward scenarios.
While speaking to two CCIE friends, both of them hadn’t bundled that many ports. Rather, they said:
In reality, an engineer do not often need to bundle up to 8 links. Rather, they bundle 2 or 3 links running, for example, 1Gbps. This will sum to 3Gpbs, which should be enough depending on the network.
But, since LACP can support up to 16 links, with 8 only active at the same time. The scenario is possible, but they have no experience with it, hence they couldn’t help at that time.
It was a struggle for me. I didn’t know what to do. I had already exhausted all my questionnaire options. I took a break. Then I kept going back to it while doing keyword searches on specific outputs, then it began to make sense.
Weird Commands I Encountered
lacp system priorityvalue
lacp port-priorityvalue
Now, I don’t know if these commands will be on my exam coming up. But, I felt like this scenario was a great thing to learn from. The command is just an added knowledge.
When I thought about the scenario, a few things made it interesting to me.
Lesson Learnt While Searching For An Answer
An admin can mistakenly bundle more than 8 link to an ether channel running LACP. Then some users will begin to complain about the network. When you are called to troubleshoot, this is will be an interesting thing to check.
Among the 8 active links, 1 can fail at any time. Hence, setting up an extra link for redundancy will be necessary.
Let’s dive right into a practical of what I experience.
Practical
You have two switches (SW1 & SW2) both with the following characteristics:
Bundled with 10 links in between
Range of interfaces configured as Trunk
Range of interfaces configured with the channel-protocol lacp/pagp command to prevent it from either being forced into a bundle or use any other protocol than the protocol on specified in the channel-protocol command
Range of interfaces configured to negotiate EtherChannel protocol as LACP using the channel-group __ mode [active|passive]
Where The Switch Chooses a Port Replacement From
Among the 10 bundled port running LACP, 8 were active, while 2 were running as a Hot-standby. Meaning they are the replacement. So, the switch chooses a port from the ones running Hot-standby
This is an impressive action from LACP. Salute.
But what if we want, B to be selected as the most preferred port instead of A?
LACP System Priority
A switch with the lowest system priority can select the most preferred port. I noticed that this is where the global configuration command:
lacp system priorityvalue
comes to play. I almost confused this priority with the switch spanning-tree priority. But, I labbed it up, I was wrong. To view this detail the command below was used. Source
show lacp system-identifier
I realised that this command can’t run on a simulated device. It can only work on real gear. So I wasn’t able to find a screenshot. I turned on a device in the Cisco sandbox, but connecting to it kept failing.
If the system priorities are the same. Then other different circumstances will be chosen.
The Switch With The Lowest MAC Address
The LACP Port Priority
If an active port in the EtherChannels is down, the port with the lowest priority wins the challenge of becoming the most preferred. I noticed that this is where the interface configuration command:
lacp port-priorityvalue
comes to play. If all these are equal, the switch uses one last option.
The Port Number
Lesson Learnt After Finding an Answer
It wasn’t easy remembering that LACP can only support only 8 active ports. But after this lab, there’s no way I can ever forget this law. #labEveryday
I know now that, the LACP redundant ports are operating as a Hot-standby
I know now that, these redundant ports can be selected based on priorities.
Additional Reading
Spin up a real device on DevNet Sandbox then search for CML and click on reserve. SSH to this device to try out this lab.