3750 stack Etherchannel

Unanswered Question
Jan 28th, 2009

To support MEC you have to use either LACP or mode "on". Any advantages/disadvantages between the two?

(This would be for all Cisco switches and not servers. Possibly VSS to 3750 stacks.)

I have always used "on" to avoid any negotiation issues.




I have this problem too.
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Roberto Salazar Wed, 01/28/2009 - 11:28

LaCP - advantages are that both switches will make the determination whether the other end is capable of channeling. to me that is an advantage since with "on", it just forces the channel to on, they could very well be NOT channeling but with "on" they will be froced to channel. without any other protection this could cause some issues such as STP loop, instability with the packet switching, etc.

The advantage with "on" is it's simple to implement. Channel protocol independent, if the other end is not LaCP capable then "on" is a good option.

gnijs Wed, 03/04/2009 - 09:47


I have done some tests using VSS and a 3750 MEC. And my conclusion was that LACP or PAGP were actually better than mode "on". Why ?

I discovered that when a link recovered, one of the two sides could be faster to bring up the link. The switch that would be "first" immediately starts forwarding packets across the new link, while the other side might not yet be ready with joining the link to the etherchannel. (i am talking about 0.5-1.5 seconds packet loss here).

Using LACP or Pagp negotiation, the link is only being activated AFTER BOTH sides have negotiated and are READY to forward packets. So using LACP/PAGP, i lost 0 packets !

Second, i once experienced an outage caused by "mode on". for example, if you configure one side of a link for "mode on", and you forget to configure the other side (or connect the fiber to the wrong interface :-), the portchannel WILL come up on the "mode on" side. This bringing up of the portchannel will bring up the vlans and will start forwarding vlan traffic. However, because the other side is wrongly connected or configured, the traffic will be black-holed. Add some CEF load-balancing to the question, and you will arrive in situation where half of your pings could be dropped (something like: .!.!.!.!.!.!



glen.grant Wed, 03/04/2009 - 12:39

When you force on a etherchannel will show on if there is a physical link whether the etherchannel is working or not , this is true also when you force on a trunk . Both of what you said is true and it warns in some cisco docs about blackholeing of traffic in misconfigured "on" modes . We generally use negotiated etherchannels and trunks , everyone has their own opinion on this which is better.

Mhon Baul Fri, 03/27/2009 - 23:55


Will it be posible to implement a MEC with 1 TenGigabit and 1 Gigabit combination in a VSS implementation? Thanks in adveanced!



Mhon Baul Sat, 03/28/2009 - 08:29

hi glen,

Thanks for your response. Can you please give me some idea on what will be the impact if ever i will push this one for example?




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