spanning tree and etherchannel

Unanswered Question
Feb 20th, 2008

Hi all, I know with etherchannel it treats the number of ports as 1 link, how is this done, does the switch use 1 mac address between all the ports ?

I have this problem too.
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shivlu jain Wed, 02/20/2008 - 02:06

An EtherChannel is an aggregated set of physical ports presented as a single logical port. The goal of EtherChannel is to provide greater bandwidth and availability than that of a single port.

Spanning Tree Protocol (STP) sees an EtherChannel as a single port. This presents a danger of the creation of forwarding loops if channeling ports are not consistent on both sides of the channel.

If switch A has two separate physical links that are not in a channel, and switch B considers those same links to be part of the channel, switch B sends a broadcast or unknown unicast packet to switch A. Since the links are not bundled together as a channel on switch A, the packet is forwarded back to switch B.This causes packet duplication and changes the forwarding table on switch B to point in the wrong direction



Istvan_Rabai Wed, 02/20/2008 - 10:14

Hi Carl,

For the Etherchannel the switch creates a logical interface called port-channel interface.

This interface is seen as one interface for protocols such as STP and routing protocols.

For example, if you use the command:

"channel-group 11 mode on"

then the port-channel will be interface Po11.

You can check it in the running-config as well. You can configure it with the "interface Po11" command, like other interfaces, you can even give it an ip address, if the Etherchannel is configured as a layer 3 interface.



lamav Wed, 02/20/2008 - 10:24

Istvan, I love your posts...they are always so informative and complete...awesome, man...


Istvan_Rabai Wed, 02/20/2008 - 10:33

Thank you Victor,

I saw one or two of your posts where you gave similarly detailed and good explanations on STP.

If we can help people, then let's help them, right?



Francois Tallet Wed, 02/20/2008 - 11:22

Is this an STP question? Because STP does not really use the address of the ports. Sure BPDUs are send with the mac address of the port as a source, but the payload of the BPDU only uses a mac address from the source bridge.

Else if the channel is an L2 port, source addresses are based on the sender address (transparent bridging means that the frame is in theory not changed by the bridging entity). If the channel is an L3 port, then you will have a single source mac address for all the traffic it generates.



carl_townshend Thu, 02/21/2008 - 02:23

am i right in saying if I turned this port into a layer 3 port by using the no switchport command, would this disable spanning tree on that port ?


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