VSS best practices for L2 Etherchannels

Answered Question
Dec 20th, 2009

Dear friends,

Is PAGP recommended MEC option for VSS for L2 etherchannels? Or is "mode on" also ok to use?

However, i am forced to use PAGP for atleast 2 L2 Etherchannels as i am using PAGP for dual-active detection.

I am just thinking if i can use the other L2 etherchannels with mode on?

I am forced to use PAGP as i only have 10G ports and i dont want to waste 10G ports for sending just dual-active hellos.

Please advise.

Thanks a lot


I have this problem too.
0 votes
Correct Answer by Reza Sharifi about 6 years 10 months ago

Hi Gauta,

Once you bring up the portchannel and configure it, you can add/remove from the allowed vlan on the portchannel on one side or the other without worrying about the portchannel going down.



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Reza Sharifi Sun, 12/20/2009 - 11:31

Hello Gautam,

When you are configuring MEC and running ePAGP for dual-active detection, it is recommended to do desirable desirable on both sides of the etherchannel.

It is recommended to configure on on only for the VSL links between the 2 switches.



gautamzone Sun, 12/20/2009 - 12:04

Thanks a lot Reza. I was thinking that i can use

PAGP for 2 out of 9 L2 MEC's (for which i am doing dual active detection) and for the rest 7, i thought of using "mode on".

The reason being that PAGP is not as flexible as mode on.

If i do vlan pruning on one side of the etherchannel, then i need to do vlan pruning even on the other side, else the etherchannel may go down.

I am not even sure if i have to console in from one side as i may lose connectivity to the switch.

Thanks a lot anyways



Reza Sharifi Sun, 12/20/2009 - 12:20

Hi Gautam,

Instead of vlan pruning I would specify the vlans I need on the etherchannel.

whatever you decide to do and for consistency, I would keep all of them same if possible and not a mixture of both features.  It became a management burden to keep track of what link is using what feature.

Also, make sure you test it if you decide to go with on and on.  I have tested ePAGP and it works well.



gautamzone Sun, 12/20/2009 - 13:09

Dear Reza,

Do you mean "switchport trunk allowed vlan". If yes, thats what even i meant by vlan pruning.

Sorry if i miscommunicated on this.

But if this is what it is, dont you think that there will be issues when you allow vlans on either side on the portchannel.

Lets say for example if you have vlans a,b,c allowed on both ends on the etherchannel trunk. Tomorrow, if you want to add vlan d,

then when you add it on one side, and take a few seconds and add on the other side, my guess is that you may lose the port channel link because the allowed vlans on both sides is different. Please correct me if am wrong?

Thanks a lot


Correct Answer
Reza Sharifi Sun, 12/20/2009 - 15:34

Hi Gauta,

Once you bring up the portchannel and configure it, you can add/remove from the allowed vlan on the portchannel on one side or the other without worrying about the portchannel going down.



vvasisth Mon, 12/21/2009 - 01:55


Here's is the link and a cut/paste from the VSS best practices:


MultiChassis EtherChannel

A MultiChassis EtherChannel is an EtherChannel with ports that terminate
on both chassis of the VSS. A VSS MEC can connect to any network element
that supports EtherChannel, such as a host, server, router, or switch.
At the VSS, an MEC is an EtherChannel with additional capability. The
VSS balances the load across ports in each chassis independently. For
example, if traffic enters the active chassis, the VSS selects an MEC
link from the active chassis. This MEC capability ensures that data
traffic does not unnecessarily traverse the VSL.

*         L2 MEC enables loop free topology, doubles the uplink
bandwidth as no links are blocked and provides faster convergence than

*         L3 MEC provides reduced neighbor counts, better load-sharing
(L2 and L3 for unicast and multicast), reduced VSL link utilization for
multicast flows and faster convergence than ECMP.

Refer to Multichassis EtherChannels
nfiguration/guide/vss.html#wp1062284>  for more information on MEC.


*         Always run L2 or L3 MEC.

*         Do not use on and off options with PAgP or LACP or Trunk
protocol negotiation.

o    PAgP - Run Desirable-Desirable with MEC links.

o    LACP - Run Active-Active with MEC links.

o    Trunk - Run Desirable-Desirable with MEC links.

Link to MEC configuration:


Since the configuration for MEC using LACP to a switch or server is the
same, there should be no special configuration issues needed.

Also, there is no need to modify queue sizes on these switchports
(unlike some routers). The overruns and drops could be a sign the nic is
not passing traffic correctly right at the time of failover.



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