Difference Etherchannel Layer2 & Layer3

[N.ima0102]

Hi

What difference are there between Etherchannel layer 2 & layer 3?and where layer 2 is used and where layer 3 is used and Anything about advantage and dis-advantage of these methods.

Thanks in advance

[Giuseppe Larosa]

Hello Nima,

etherchannel allows to bundle multiple links and to make them appear as only one link.

The best way to build the bundle is the use of standard based LACP protocol as it permits a form of signalling on each member link ( each member link status is negotiated with the other side) and this allows to detect misconfigurations and errors in cabling that may lead to different member links connected to two different devices ( not supported).

Cisco etherchannel allows to build bundles in a silent not negotiated way, but experience says that nowdays this is not recommended as unconditional bundling cannot detect some errors like having different member links connected to two different switches. Also there is no knowledge if the other side is acting as a bundle or not.

A layer2 etherchannel is seen as a single link by STP allowing to use all the member links for better performance. Most of the times a layer2 etherchannel is configured as a trunk ( 802.1Q more often) and can carry multiple Vlans/broadcast domains.

Layer2 etherchannels are best suited for interconnection of L2 LAN switches providing at the same time better performance and link fault tolerance.

In order to setup a L2 etherchannel acting as a trunk member links have to be configured at the same way:

same speed

same duplex

same trunking encapsulation protocol

same native vlan ( for 802.1Q)

the same list of Vlans permitted on each member link

A Layer3 etherchannel is like  a single port configured in routed mode with no switchport, there is no concept of Vlans here, an IP address is associated to the port-channel logical interface, no ip address is configured under member interfaces. Layer3 port channel are used for interconnecting routers with routers or routers with multilayer switches for the same reasons of increased performance and link fault tolerance. No L2 signalling protocol including STP plays a role on Layer3 port channel.

In order to setup a L3 etherchannel acting as a trunk member links have to be configured at the same way:

same speed

same duplex

no switchport

L3 etherchannels are used in Service Provider networks to scale over 10GE speeds between core routers like CRS or ASR 9000 using bundles of 4 or more tengigabit interfaces.

Hope to help

Giuseppe

[N.ima0102]

Thanks a lot for your attention.

May you give one example for usage of L3 and L2?

I want to know where I use L2 and where use L3 etherchannel.

Thanks in advance

[Giuseppe Larosa]

Hello Nima,

between LAN switches L2 etherchannel is more common

L3 etherchannel is used when routers are involved

Hope to help

Giuseppe

[issofunky]

SO CONCISE AND CLEAR! :    BEST NUTSHELL EVER.

[Nikolay Somov]

Hi Giuseppe Larosa

In terms of advantages / disadvantages are they the same (L3 Po and L2 Po)?

And yet, if I have an L3 Switch on one side (for example, a nexus 7710), and on the other, an ASR9012 router: then I can configure L3 Po on the L3 Switch and on the ASR L3 Po - everything is clear, everything works. I can also configure L2 Po on the L3 Switch, and on the ASR I can configure L3 Po - everything works. 

Difference second metod? 

[shivakumarmanakoji]

Very well explained, Thank you

[JohnHHunter]

Over 7 years later and your wonderful explanations are still helping others! Thank you. 

jhh

[ciscojose1107]

excellent explanation!

[ColeEdwards04124]

Thank you for your post so many years ago. Best explanation I have seen. Typing this with guarded hopes of a response.

Reading what has been stated so far, I think layer 2 is my answer. I am a hobiest for a home network and lack experience. I am only tinkering with this for experience.

I have a pfsence router and a 3750x switch and then a cheap managed switch down the path. A tl-sg108e which is capable of handling vlans and lacp. 

I was thinking setting up both routes from pfsence to 3750x and then 3750x to TI-sg108e with link aggregation. I think it's clear that switch to switch would be layer 2. Just double checking, would you also recommend layer two for router to 3750x? 

Thanks 

[Giuseppe Larosa]

Hello @ColeEdwards04124 ,

thanks for your kind words .

>> Just double checking, would you also recommend layer two for router to 3750x? 

The answer is it depends on who is doing inter VLAN routing if you are using the C3750X as a L3 switch and the pfsense router provides internet access you can be fine with a L3 etherchannel.

If instead, all the inter VLAN routing is performed by the router pfsense ( called router on a stick) in this case you would need a L2 etherchannel on the C3750x side to the pfsense.

From performance point of view C3750x can perform inter VLAN routing in hardware so it i has far better performance.

The pfsense router can perform NAT providing Internet access to some or all of the internal subnets.

C3750x lacks the NAT feature.

Hope to help

Giuseppe

[paulmeasom]

Comprehensive and easily understood explanation.Thanks

[sojiadvanced]

Thanks for this detailed explanation

[osmanomar]

If you have a 2950 switches everywhere then thats were you would  use  Layer 2 etherchannel.

If you have 3550 switches or above then thats were you would use Layer 3 etherchannel.

[dbeattie]

An Etherchannel is just a bundle of links. The question of L2 or L3 is just as valid for a single link. A L2 link in initiated using the "switchport" command and can be access or trunk. It has a single or multiple VLANs assigned, it runs spanning tree and frames are switched into the "backplane" of the switch. To route traffic in this way needs an SVI or similar construct. A L3 link is initiated using the "no switchport" command and can be a single interface or may have sub-interfaces. It does not normally run spanning tree and packets are routed onto the "backplane" of the switch/router. Making the link and Etherchannel changes none of this.

So the question remains: how do you know when to use L2 or L3. Generally, the question comes back to the usual thoughts around subnet and failure domain sizes. If you need a VLAN to be available on multiple ports on a switch you should use a L2. In fact this is often seen as the default for a switch. If you want to prevent this, or you want (arguably) greater control of the routing, you might select L3, especially since it does not require a Spanning Tree Instance.

There is one further twist, which is that in some instances you may opt to use multiple parallel L3 links and use a routing protocol capable of ECMP rather than using a L3 Etherchannel. This tends to be done for resilience reasons, but is rare (in my experience).

Hope this helps,

Dave