etherchannel

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Edison Ortiz Thu, 08/23/2007 - 20:05

> Can i configure spanning-tree in the etherchannel?

Spanning-Tree is enabled by default on Layer2 ports.

What kind of configuration are you looking for ?

> do i need to configure dot1q in this case for the trunking?

If multiple Vlans will reside on each switch and you need them to communicate switch-to-switch, sure - just go under the port-channel interface and enable dot1q.

glen.grant Sat, 08/25/2007 - 18:55

If you have no vlans configured then only spanning tree for vlan 1 will run . Think you are getting etherchannel and trunking confused.Dot1q has nothing to do with etherchannel that is a trunking function.

lamav Sun, 08/26/2007 - 19:08

Hi, Ill try not to be redundnt regarding other people's contributions...but here goes.

I think you do have trunking and etherchannel confused, as someone else pointed out.

The point of etherchannel is to create one LOGICAL connection (bundle) out of multiple PHYSICAL connections so that STP does not see the multiple physical connections and block them. In other words, if you configure, say, 3 dot1q or ISL trunks between switches to maximize bandwidth capabilites, you can bundle those 3 physical trunks into one logical trunk and fool the switch and STP into thinking that only one connection exists.

Now, you can add STP configuration commands under the port-channel interface, which is automatically created when you use the

channel-group x mode yyy" command.

Something like:

interface port-channel 1

spanning-tree guard loop

hi! that's to say if i've etherchannel which group eg. 2 physical link into 1 logical link between 2 switches, STP will still be running. Just that no port will be blocked in this case.

If i've 2 logical etherchannel group (2 physical link) each, then one of the etherchannel group will be blocked.

I will not need to have any encapsulation for the trunking if i did not configure any vlan for the cases above. Am i right to say that?

lamav Mon, 08/27/2007 - 04:23

"hi! that's to say if i've etherchannel which group eg. 2 physical link into 1 logical link between 2 switches, STP will still be running. Just that no port will be blocked in this case.

If i've 2 logical etherchannel group (2 physical link) each, then one of the etherchannel group will be blocked."

Correct.

"I will not need to have any encapsulation for the trunking if i did not configure any vlan for the cases above. Am i right to say that?"

No. If you connect 2 switches together, you are going to trunk them. In that case its either dot1Q or ISL.

jorgenolla Mon, 08/27/2007 - 04:52

You really don't need to configure more than the etherchannel. The 3550s are in dynamic desirable mode, meaning they will trunk automatically when you connect them. The default trunking protocol is ISL.

The EtherChannel counts as only one port. So if you have only one etherchannel running trunk, there will be no STP instance running.

If you connect another trunk, whether it be a single link, or an etherchannel link, stp will come up automatically.

Regards

lamav Mon, 08/27/2007 - 05:08

Jorge:

Once again, you are giving out misinformation. Kindly be careful about what you write because you can really screw someone up.

First, configuring a trunk and configuring an etherchannel are 2 different things. A trunk is not magically created simply becuase you have configured a port to be part of an etherchannel. Configuring an etherchannel simply means that you have logically bundled a group of ports to augment the bandwidth availability. That etherchannel can be a L2 or L3 etherchannel.

Second, as long as STP is configured on the switch it is ALWAYS running. The STP algorithm 'discovers" which ports are access ports, which ones are trunks and if any redundant L2 links exist that may cause a switching loop.

Each port goes through the different STP port states: blocking, listening, learning and then forwarding (if that port does not create a possible loop, otherwise it will remain blocked for whichever vlans are redundant [pvst+]).

Lastly, STP doesnt simply "come up" when a redundant link is installed. It is always there if you have it configured on the switch. If installing that redundant L2 link creates a redundant path, and therefore a potential L2 loop, STP will first reconverge after the topology change, and then that port will be blocked by STP. And if you are running pvst+, which I believe is Cisco's default (check that out), it will only block traffic on those vlans which are part of the redundant topology.

jorgenolla Mon, 08/27/2007 - 05:31

To my friend Lamav :-)

DTP and STP are always running if not disabled by the administrator, I am with you there my friend.

Here are some clarifications:

1)"A trunk is not magically created simply becuase you have configured a port to be part of an etherchannel"

If DTP on the ports are set to dynamic desirable, in both the switch and the neighboring switch, which in the case of the 3550s; DTP will automatically set the link as trunk, whether it be a single link, or a L2 etherchannel.

2) "STP doesnt simply "come up" when a redundant link is installed. It is always there if you have it configured on the switch"

Once again I didn't say "STP", I said "STP instance" which are two different things. If there are no loops in the topology, there are no instances of STP running! Once a loop is created, you will see an STP Instace for each VLAN.

Best Regards

P.S. Can someone break this argument by clarifying where lamav and I are not in agreement.

lamav Mon, 08/27/2007 - 05:43

Jorge!

We do not need a referee! LOL :-D

This is all good. Its all part of healthy discussion and debate..

Peace

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