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Easy Doubts about etherchannels

Hi all ¡¡¡

I have some doubts about etherchannel-->

1º)Etherchannel and channel-group its the same?

2º)Why is best to use etherchannel negotiation protocols(lacp and PAgP) to etablish a etherchannel in mode ON... Where is the advanjate?In all the cases i need etablish the number of channel group etc...

7 REPLIES
Cisco Employee

Re: Easy Doubts about etherchannels

Hello,

Etherchannel and channel-group its the same?

Essentially, yes. More precisely, the EtherChannel is the name of the particular technology to bundle several Ethernet links into a single bundle and load-balance the traffic. The channel-group is a Cisco CLI command to assign a particular physical port to an EtherChannel bundle.

Why is best to use etherchannel negotiation protocols(lacp and PAgP) to 
etablish a etherchannel in mode ON... Where is the advanjate?

An EtherChannel consists of several physical links interconnecting two switches. By their very nature, they are forming a loop. However, the STP considers the entire EtherChannel group to be a single port and does not block or unblock individual member ports. If ports are bundled into an EtherChannel on one end and still unbundled on the other, heavy switching loops and MAC table instabilities can occur. Also, if by any accident some of the links from a single EtherChannel on switch A are connected to switch B and some other to switch C (a split bundle), similar problems may be observed.

Therefore, it is of utmost importance that the group of ports is considered a bundle only if both switches agree to group the respective ports into a bundle. This is the purpose of the PAgP and LACP protocols. Both are signalling protocols to negotiate the creation of an EtherChannel and perform necessary sanity checks before allowing a port to join the EtherChannel group. Only if the PAgP/LACP negotiation is successfull between two switches, the ports are bundled into an EtherChannel.

Whenever possible, one should avoid using the on mode of EtherChannel without any negotiating protocol. The on mode does not perform any similar inter-switch consistency checks and makes the entire port group very vulnerable to misconfigurations and miswirings.

Best regards,

Peter

New Member

Re: Easy Doubts about etherchannels

Thanks fot the replyes '¡¡¡¡ Ok, using LACP or PAgP is basically to force the 2 end points to negotiate...I thoug there would be more advantajes...

Finally, i have make a easy practice in packet tracer and i think(i'm sure) the program have some bugs about etherchannel.

The practice its very easy .consist of 3 switches.  The switch B conects with the switch A and C. They have 2 etherchannels, one with LACP and the Other with PAgP.

If i put the command "show etherchannel" in switch B it talks about 2 LACP etherchanels ¡¡¡¡¡¡¡¡¡

I have perceive too, if i try to etablish a etherchannel in packet tracer with one switch in mode Passive and the other in mode active it not works ¡¡¡¡

Look please and sorry , i speak very bad english ¡¡¡¡¡¡¡¡

Cisco Employee

Re: Easy Doubts about etherchannels

Hi,

Thanks fot the replyes '¡¡¡¡ Ok, using LACP or PAgP is basically to 
force the 2 end points to negotiate...I thoug there would be more 
advantajes...

Basically, yes, the LACP and PAgP allow for switches to negotiate which ports are suitable for bundling and into which channel the ports belong. You seem to be somewhat downplaying the importance of it. I, on the other hand, cannot stress enough how important it is to have this negotiation take place before the ports are included into an active EtherChannel. A simple mistake in configuration or wiring can bring down your network. These protocols can prevent just that. That is a huge advantage.

Regarding the Packet Tracer, I would not rely too much on its EtherChannel simulation. I haven't tested that but the Packet Tracer has been created with special focus on CCNA and CCNA Security courses. The EtherChannels are CCNP stuff and the Packet Tracer does not aim that far.

Best regards,

Peter

New Member

Re: Easy Doubts about etherchannels

Thanks Peter.

I would have liked to test the pkt to confirm if it is a bug but, i'm sure packet tracer no works fine...

I thougth an advantaje tu use dinamic negotiation to etherchannels. In some cases you need some configurations (vlans, speeds etc) and this negotiation can chek it...

Thanks and regards.

New Member

Re: Easy Doubts about etherchannels

Hi ¡¡

One last question.

What is the purpouse of the command "channel-protocol lacp/PAgP". Its necessary insert him? I can create the channel group using directly mode auto, active etc without using this command...

Thanks ¡¡¡

Cisco Employee

Re: Easy Doubts about etherchannels

Hello,

The documentation about the channel-protocol command is very terse and lacks details. To my best experience, this command is purely cosmetical and its only result is that it only allows using the channel-group commands that correspond to the protocol specified by the channel-protocol. In particular:

  • channel-protocol lacp allows using only channel-group active/passive . Keywords desirable and auto are disallowed because they refer to PAgP protocol
  • channel-protocol pagp allows using only channel-group desirable/auto . Keywords active and passive are disallowed because they refer to LACP protocol

In my opinion, this command was simply introduced to help people that have trouble remember which of the keywords active/passive/desirable/auto refer to which protocol. It has no other meaning. The protocol itself is not negotiable, i.e. you cannot create an EtherChannel that speaks both protocols.

Best regards,

Peter

New Member

Re: Easy Doubts about etherchannels

Thanks Peter,

best regards ¡¡¡¡

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