Answers: L2 EtherChannel 1

Wendell
By Wendell January 21, 2016 09:05

Today’s lab: Check your static EtherChannel config. It’s pretty simple, but worth a quick review. Answers below the fold as usual. And check the original lab post for the lab exercise.

Figure 1: Three EtherChannels, with Two Links Each

Answers

Example 5: SW1 Config

Example 6: SW2 Config

Example 7: SW3 Config

Example 8: SW4 Config

 

Commentary

Cisco uses the term EtherChannel to refer to their link aggregation technology. Link aggregation overcomes some of the shortcomings of the Spanning Tree Protocol (STP). For example, if EtherChannel was not being run on the links between SW1 and SW2, then one of the links would automatically be blocked by STP to ensure that a switching loop does not form; this will lose half of the available throughput. EtherChannel combines both links into a single logical link; because of this, STP considers this a single connection between the devices and does not block any of the ports.

Static EtherChannel configuration uses on mode per the channel-group mode interface subcommand; however, using this mode has the potential for causing problems if only a single side is configured. Cisco supports two protocols to negotiate the use of EtherChannel: Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP). PAgP is a Cisco proprietary version of link aggregation that was introduced before LACP. LACP is a standard-based solution and is defined in 802.3ad.

For EtherChannel 20, between SW1 and SW2, both interfaces (G0/1 and G0/2) on both switches need to be configured in on mode with the channel-group 20 mode on command.

For EtherChannel 21, between SW1 and SW3, SW1’s G0/3 and G0/4 use the channel-group 21 mode desirable command, which uses PAgP, and initiates the negotiation. SW3’s G0/1 and G0/2 use the channel-group 21 mode auto command, which uses PAgP, but passively waits for the other switch to initiate negotiations.

For EtherChannel 22, between SW2 and SW4, SW2’s G0/3 and G0/4 use the channel-group 22 mode active command, which uses LACP, and initiates the negotiation. SW4’s G0/1 and G0/2 use the channel-group 22 mode passive command, which uses LACP, but passively waits for the other switch to initiate negotiations.

Note that all the interfaces used default configuration, so there should be no issues with mismatched interface settings preventing any interfaces from joining a Layer 2 EtherChannel. See the ICND2 Official Cert Guide for more details on how to check for those dependencies, and for a broader look at EtherChannels.

L2 EtherChannel 1
EIGRP and Delay
Wendell
By Wendell January 21, 2016 09:05
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8 Comments

  1. Ricardo October 15, 22:37

    There are some typos on the channel-group numbers.

    Correct answer for SW2:

    hostname SW2
    !
    interface GigabitEthernet0/1
    Channel-group 20 mode on
    !
    interface GigabitEthernet0/2
    Channel-group 20 mode on
    !
    interface GigabitEthernet0/3
    Channel-group 22 mode active
    !
    interface GigabitEthernet0/4
    Channel-group 22 mode active
    !
    interface FastEthernet0/1
    switchport mode access

    Reply to this comment
    • certskills October 19, 08:26

      Hi Ricardo,
      Yep, you are right!
      I fixed the mistakes so that the channel-group numbers used in the answers match the numbers suggested in the lab (and as noted in your suggested answer). Thanks for the heads up,
      Wendell

      Reply to this comment
      • Liooh December 11, 15:36

        “For EtherChannel 21, between SW1 and SW3, SW1’s G0/3 and G0/4 use the channel-group 20 mode desirable command, which uses PAgP, and initiates the negotiation. SW3’s G0/1 and G0/2 use the channel-group 20 mode auto command, which uses PAgP, but passively waits for the other switch to initiate negotiations.”

        Should it be channel-group 21 instead of 20 ?

        Reply to this comment
  2. Gabriel Moran September 12, 11:10

    Small detail but the g0/3 and g/04 were missing from sw2 on the previous example given. Was a bit confusing to me as the instructions state explicitly to define the EtherChannels on the various links but then 2 links were missing from the config given 🙂

    Reply to this comment
  3. Sanjin July 23, 23:49

    Hi Wendell,

    cuold You please explain to me how to configure Ether L2 channel as a trunk? Do i need to configure ports like a trunk first? What about native VLAN and VLAN-s we want to trunking? Thank to you for any information.

    Sanjin

    Reply to this comment
    • certskills July 25, 10:51

      Hi Sanjin,
      Sure. For example, on SW1, get into config mode, and type the “interface ether channel 20”. That will put you into config mode for the Etherchannel 21 interface. Then configure all your favorite interface commands, eg, switchport mode trunk. Think of the “etherchannel x” interfaces like you would any physical layer 2 interface.
      Wendell

      Reply to this comment
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