L2 EtherChannel 1

#EtherChannel takes a big step towards improving STP. With two links between switches, but without using EtherChannel, each instance of STP could use only one of the links blocking the other link. With a layer 2 EtherChannel, STP treats the links as one, balancing the traffic over the links when in a forwarding state. This next lab takes a quick and easy look at Layer 2 EtherChannel Config.

Requirements

In this lab, you practice the various options to enable a Layer 2 EtherChannel on a Cisco switch. The variety of requirements listed here are used just to exercise all the options. In particular:

• Use these EtherChannel Numbers:
  • 20 between SW1 and SW2
  • 21 between SW1 and SW3
  • 22 between SW2 and SW4
• Configure for the following requirements:
  • EC 20: statically enable on both ends
  • EC 21: use PAgP, with SW1 initiating the EC, and SW3 waiting for SW1
  • EC 22: use LACP, with SW2 initiating the EC, and SW4 waiting for SW1
• The switches have all default configuration with the exception of their hostnames.

Figure 1: Three EtherChannels, with Two Links Each

Initial Configuration

While you might be able to configure EtherChannel based on the information supplied so far, the initial configurations of the three switches can also be helpful. Example 1 and 2 show the beginning configuration state of SW1 and SW2.

Example 1: SW1 Config

hostname SW1
!
interface GigabitEthernet0/1
 no shutdown
!
interface GigabitEthernet0/2
 no shutdown 
!
interface GigabitEthernet0/3
 no shutdown
!
interface GigabitEthernet0/4
 no shutdown 
!
interface FastEthernet0/1
 no shutdown
 switchport mode access

Example 2: SW2 Config

hostname SW2
!
interface GigabitEthernet0/1
 no shutdown
!
interface GigabitEthernet0/2
 no shutdown 
!
interface FastEthernet0/1
 no shutdown
 switchport mode access

Example 3: SW3 Config

hostname SW3
!
interface GigabitEthernet0/1
 no shutdown
!
interface GigabitEthernet0/2
 no shutdown

Example 4: SW4 Config

hostname SW4
!
interface GigabitEthernet0/1
 no shutdown
!
interface GigabitEthernet0/2
 no shutdown

Answer on Paper, or Maybe Test in Lab

Next, write your answer on paper. Or if you have some real gear, or other tools, configure the lab with those tools.

Verifying EtherChannel takes just a quick look at the output from the show etherchannel number summary command. This command should list all the ports in the working EtherChannel with a status of P, meaning the port is in the channel. Troubleshooting why a port is not in the EtherChannel requires more effort, such as checking the variety of interface settings that must match on one switch, and on neighboring switches, before a switch will form the channel and add all ports to it. See the ICND2 Official Cert Guide “Troubleshooting EtherChannel” section for more details.

Do This Lab with Cisco’s VIRL

You can do these labs on paper and still get a lot out of the lab. As an extra help, we have added files for the Virtual Internet Routing Lab (VIRL) software as well. The .VIRL file found here is a file that when used with VIRL will load a lab topology similar to this lab’s topology, with the initial configuration shown in the lab as well. This section lists any differences between the lab exercise and the .VIRL file’s topology and configuration.

Download this lab’s VIRL file!

Network Device Info:

This lab happens to use some interface numbers that cannot be replicated in VIRL. The .VIRL file supplied here translates the lab to use the ports listed in the following table.

Host device info:

This table lists host information pre-configured in VIRL, information that might not be required by the lab but may be useful to you.

Handy Host Commands:

To see PC IP address: ifconfig eth1

Ping example: ping -c 4 10.1.1.1

Trace example: traceroute 10.1.1.1

To connect to another node within the topology: telnet 10.1.1.1