Answers: L2 EtherChannel 2

Wendell
By Wendell July 8, 2016 13:10

This latest lab asks you to configure layer 2 EtherChannels, but starting with a partial configuration already. Go back and check the lab exercise, create your answer, and check your answer here.

Answers

Figure 1: Distribution/Access Switch Topology

Example 1: Dist1 Config

 

Example 2: Dist2 Config

 

Example 3: Access1 Config

 

Example 4: Access2 Config

 

Commentary

The focus of this lab is layer 2 EtherChannel. There are three main ways to configure EtherChannel: Statically configuring it using the on keyword, dynamically configuring it to use LACP, and dynamically configuring it to use PAgP. Both LACP and PAgP have similar goals but are not interoperable with each other.

For this lab you were given a configuration that was initially configured with three different EtherChannel links, none of which was fully formed or working correctly. Your task was to make the necessary alterations to the configuration to meet the requirements listed. It should be noted however that in production environments it is extremely odd to see multiple EtherChannel methods used, for example it is common for an organization (or its architects) to select a specific EtherChannel protocol which is then used across all of its EtherChannel links.

Note that one huge key to interpreting the initial configuration is that the channel-group number interface subcommand causes IOS to create a portchannel interface by that same number. For instance, on switch Dist1, once the first channel-group 10 command was configured, interface portchannel 10 existed, with all default settings.

 

PortChannel 10 – Dist1 to Dist2

First, focus on Etherchannel 10 between switches Dist1 and Dist2. As initially configured, it has two problems:

  1. The two physical links have different access VLAN settings, which prevents both physical links from being active in the EtherChannel at the same time. One physical link uses a default setting of access VLAN 1, while the other is configured with access VLAN 2 (switchport access vlan 2)
  2. The portchannel interface is not listed in the configuration, so it uses all default interface subcommands. As a result, it may or may not be correctly configured for trunking at this point, so trunking needs to be configured on the portchannel interface.

On the first point, EtherChannels have a list of configuration settings that must agree before new links can be added to an EtherChannel. One of the settings that must agree is the access VLAN setting – even if the channel will be trunking.  To solve this problem, make the physical links use the same access VLAN setting. In this case, we set both links to use the default setting of access VLAN 1 with the command no switchport access vlan 2 on one physical port (or you could have used switchport access vlan 1).

Once completed, both links in portchannel 10 should be added and active in the channel. Then, to make the portchannel act as a trunk, to match the particulars of the lab instructions, you should add the switchport mode dynamic desirable command. This tells the switch to initiate the dynamic choice to use 802.1Q trunking. (Additionally, on some switches, the switches need to be told to use 802.1Q instead of ISL using the switchport trunk encapsulation dot1q command.)

 

PortChannel 20 – Dist1 to Access1

Next we will focus on the link between Dist1 and Access1. Per the initial configuration, Dist1 uses LACP because of its channel-group 20 mode active command, and Access1 uses PAgP per the channel-group 20 mode auto command. (The “auto” option happens to be one of the PAgP options, and the “active” option is one of the LACP options.)  Because LACP and PAgP are not compatible with each other, this EtherChannel channel will never form. The requirements state that this link should use LACP with Dist1 being the initiating device. To meet the lab requirements, change the configuration on switch Access1 to use LACP passively; to configure this use the channel-group 20 mode passive command on both links on Access1.

The other requirement for this EtherChannel is to use 802.1Q trunking, so the same configuration required for portchannel 10 between Dist1 and Dist2 is required here as well. To match the lab requirements, on both Dist1 and Access1, add the switchport mode dynamic desirable command, and optionally the switchport trunk encapsulation dot1q command.

 

PortChannel 20 – Dist2 to Access2

For this last combination, note that we chose the duplicate number of portchannel number 20 on purpose, to duplicate the same portchannel number used between Dist1 and Access1. The portchannel number just has to be unique on a switch. In a real-life design, the designer would likely use a unique number just to make operational tasks more obvious.

For this last EtherChannel, per the initial configuration, both Dist2 and Access2 are configured to use PAgP per the channel-group 20 mode auto command. (The “auto” option one of the PAgP options.) However, the auto parameter causes the switch to wait for the other switch to initiate the channel negotiations, so with both sides using auto, no channel will form. The requirements state that this link should use PAgP with Dist2 being the initiating device, to correct this configuration Dist2 must be modified to use PAgP actively; to configure this use the channel-group 20 mode desirable command on both physical links on Dist2.

The other requirement for this EtherChannel is to use 802.1Q trunking, so the same configuration required for the other two portchannels is required here as well. To match the lab requirements, on both Dist2 and Access2, add the switchport mode dynamic desirable command, and optionally the switchport trunk encapsulation dot1q command.

L2 EtherChannel 2
PPP over Ethernet 2
Wendell
By Wendell July 8, 2016 13:10
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5 Comments

  1. Mubin Ahmed May 31, 19:49

    Hello Mr. Odom, I wanted to clarify if Port channel 20 should include “no switchport access vlan 2” for port Gi1/0 and Gi0/2 since port Gi0/3 (dist) and Gi0/1 (access) is on default vlan?

    Reply to this comment
    • certskills June 6, 10:22

      Mubin,
      I don’t think so, but it’s because of the sequence in this lab. If you look at the lab post, and recall that the default setting for “switchport access vlan” command is VLAN 1, then I think you’ll see that the end result should show a correct config.

      It may be that I tried to do too much with this lab – what do you think?
      Wendell

      Reply to this comment
      • Ruben December 20, 18:32

        This is actually a good exercise because you made us practice all combos in a single exercise instead of separating all the combos into different blogs. I don’t mind this structure 🙂

        Reply to this comment
  2. Sam Ekladious March 13, 01:58

    The two physical links between Dist1 and Access1 and between Dist2 and Access2 have different VLAN settings as well. So, shouldn’t we add “no switchport access vlan 2” under G1/0 of Dist1 and Dist2 and under G0/2 of Access1 and Access2?

    Reply to this comment
    • certskills April 3, 10:50

      Hi Sam,
      Your suggestion wouldn’t hurt, but it wouldn’t change the operation on the port channel, either. The reasoning behind my claim follows this sequence:

      The two links from Dist1 – Access1 will operate as part of a port channel
      The port channel is configured correctly so that it will also operate as a trunk
      The “switchport access vlan” setting matters only when operating as an access link
      Your suggested command changes the access VLAN setting from VLAN 2 to VLAN 1 (which is again ignored while operating as a trunk)

      Hope this helps…
      Wendell

      Reply to this comment
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