Commentary

This lab focused on 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. Interestingly, Cisco prefers customers to use one of the dynamic protocols because the protocols perform compatibility checks of several parameters that must match for links to be added to a channel.

This lab calls for three different EtherChannel links, all of which had at least one compatibility issue based on the pre-existing configuration. To complete this lab, you need to configure the three EtherChannels per the lab specifications while also changing the configuration to overcome the compatibility issues.

Also, as an aside, you would likely not see so many combinations of configuration settings in the same LAN. Instead, you would create a configuration and use that convention repeatedly for all EtherChannels. This lab asks for a variety of configurations to help you exercise your configuration skills.

One huge key to interpreting the initial configuration is that the channel-group number interface subcommand causes IOS to create a port-channel 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. Per the sequence of events in this lab, the lab does not show the port-channel interface in the initial configuration. Once the switch creates the port-channel interface, it has all default settings. As a result, it may or may not be correctly configured for trunking at this point, so trunking needs to be configured on the port-channel interface.

On the first point above, 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 port-channel 10 should be added and active in the channel. Then, to make the port-channel act as a trunk, you should add the switchport mode dynamic desirable command to match the particulars of the lab instructions. This tells the switch to initiate the dynamic choice to use 802.1Q trunking. (Additionally, on some older switches, including the simulated switches in Cisco Packet Tracer, 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, 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 lab 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 port-channel 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.

Finally, note that Packet Tracer gets confused if you leave the interfaces up while configuring the subcommands in the configuration solution. To improve your chances of success, follow the convention in the answer config uses a shutdown command to disable the ports, followed by the relevant subcommands, and ends with a no shutdown to enable the ports again.

PortChannel 20 – Dist2 to Access2

For this last combination, note that we chose the duplicate number of port-channel number 20 on purpose to duplicate the same port-channel number used between Dist1 and Access1. The port-channel number just has to be unique on a switch. In a real-life design, the designer would likely use a unique number 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, modify the Dist2 configuration with the channel-group 20 mode desirable command on both physical links.

The other requirement for this EtherChannel is to use 802.1Q trunking, so the same configuration required for the other two port channels 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.

Beyond comparing your answers to this lab’s Answers post, you can test in Cisco Packet Tracer (CPT) or Cisco Modeling Labs (CML). In fact, you can and should explore the lab once configured. For this lab, once you have completed the configuration, try these verification steps. 

• Use the show etherchannel summary command to verify that each of the EtherChannels is up and operational.