Solved CCNA - Troubleshooting: Trunking & Etherchanneling

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Hello,

I'm currently going through the CCNA troubleshooting pt 3 video discussing etherchanneling and trunking. In the video it is discussed that etherchanneling doesn't work for trunks when you have 3 switches set-up in a triangle (timestamp 46:15 - 46:45). I'm curious as to why it wouldn't work? It is my understanding you can have up-to 8 ports grouped into various etherchannels, so what would prevent me from grouping up 4 ports (2 trunking channel-groups of 2 ports)? I would appreciate any explanation, or if anyone could point me to appropriate (hopefully online) reading.

Thanks,

Andrew

@andrew-dorey My guess is Spanning Tree Protocol.

STP will block traffic to one of the channels and keep it from staying up.

EDIT:
so I just tried it on GNS3 and it worked just fine. Although one of the channels is in blocking. I will watch the video to see exactly what is meant by "not going to work" maybe there's a context we are missing. Maybe he was referring to the fact that no traffic is going to be passing over 1 channel because it's in blocking.

my topology

SW1

SW2

SW3

my config

sw1
interface range eth 0/0-3
switchport trunk enc dot
switchport mode trunk
channel-group 1 mode on

interface range eth 1/0-3
switchport trunk enc dot
switchport mode trunk
channel-group 2 mode on

sw2
interface range eth 1/0-3
switchport trunk enc dot
switchport mode trunk
channel-group 1 mode on

interface range eth 2/0-3
switchport trunk enc dot
switchport mode trunk
channel-group 2 mode on

sw3
interface range eth 0/0-3
switchport trunk enc dot
switchport mode trunk
channel-group 1 mode on

interface range eth 2/0-3
switchport trunk enc dot
switchport mode trunk
channel-group 2 mode on

I watched the video and what Don is referring to is Load Balancing.

What Don did first was Load Balance by just using different Roots for different VLANS. This way VLANs would be restricted to a chosen path. This is what he called "fake load balancing". Etherchannel does actual load balancing across it's links. Depending on your hardware you can load balance according to Source/destination MAC or IP.

When you try to load balance on the topology above using Etherchannels it doesn't really work the same because as I mentioned before, STP puts one channel into blocking so some traffic is going to have to take the long way around the network anyway.

For example: if SW3 has PC3 on VLAN1 and it sends a message to PC2 on VLAN1 attached to SW2, it has to go through SW1 (assuming SW1 is the root bridge) because Port-channel 2 is in blocking.

@andrew-dorey,

@Daniel-Espinal is correct for part of the answer, good job!

Remember that STP between multiple switches will allow only a single path through the switches and all traffic must go through the root bridge.

The other part to this answer though is not that you can't implement EtherChannel between each of the two switches but between all three. So do not look at each link but as the entire topology in a triangle in which we're attempting to do EtherChannel not between TWO devices but 3. If you wanted to try this it can be done but with creating some L2 Tunneling between the switches but that's probably more CCIE switching configuration rather than what you're looking to do!

![alt text] (https://goo.gl/photos/CsXAFZrjTnhE9HNn9) I don't know why the image doesn't appear but it worked when I clicked on it.

@Daniel-Espinal thanks for the detailed reply.

That makes sense with STP blocking ports... looks like I need to review it some more.

Thanks,

Andrew

@andrew-dorey,

Make sure you take a look at the diagram I created as well. Like I said, Daniel got a part of it but it's a matter of understanding why it won't work when you're trying to do EtherChannel among 3 switches and not just multiple links between 3 switches.

@Ronnie-Wong it's true what they say about a picture being worth a thousand words. It never even occurred to me to use Etherchannel in that way. That's such an interesting concept. Now I am curious as to the specifics of why it wouldn't work. I mean because of STP, traffic destined to a non root bridge from a non root bridge would be lost about 50% of the time. But is the possibility of creating loops the only reason why this wouldn't work? How about a topology like this

There are no loops because Routers don't forward broadcasts. Routing would be complicated I admit.

I'm going to play around with this topology to see how it would fail and what I'd need to make it work.

Very exciting!!.

going through setting it up I see all the reasons why it wouldn't work.

Great learning experience. You mentioned a tunnel. I have yet to try a GRE set up. I have no clue how I'd go about doing it across 2 paths but learning is half the fun.

@Daniel-Espinal said in CCNA - Troubleshooting: Trunking & Etherchanneling:

going through setting it up I see all the reasons why it wouldn't work.

Great learning experience. You mentioned a tunnel. I have yet to try a GRE set up. I have no clue how I'd go about doing it across 2 paths but learning is half the fun.

This is where you would setup some layer 2 tunneling that would be complicated but in your diagram that would have tunnel exiting traffic leaving SW1 to be tunneled through SW3 to exit out to a connection between SW3 and SW2. Then you would have to do the same for each switch and each direction. This is complicated in the extreme and thereby rarely done because you're essential creating a band-aid so that you can implement EtherChannel when the other two options that Don mentioned would actually be preferable.