Layer 2 EtherChannels

• When configuring Layer 2 channels, use the channel-group interface configuration command on the interface port or ports.
  • This command creates the port-channel logical interface.
• You cannot put a Layer 2 interface into a manually created port-channel interface.

Configure PAgP EtherChannel

When configuring EtherChannel use the following general template:

Switch(config)# interface type mod/num
Switch(config-if)# channel-protocol pagp
Switch(config-if)# channel-group number mode {on | [auto | desirable] [non-silent]}

• Lab testing showed that if ommited, verification defaults to PAgP negotiation protocol.
• Older switch models, such as Catalyst 2950, offer only PAgP option therefore the channel-protocol command is not even available.
• Channel group number can be from 1 to 64.
• Each interface in the EtherChannel bundle must be assigned to the same channel group number.
• PAgP defaults to silent mode with the Auto and Desirable mode, by default. It means it will not transmit PAgP packets.

Figure 1: This is the example topology used for this practice lab configuration

SW4#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
SW4(config)#int fa0/1
SW4(config-if)#switchport
SW4(config-if)#channel-group 1 mode desirable
Creating a port-channel interface Port-channel 1

 

!

SW4(config-if)#int fa0/2
SW4(config-if)#switchport
SW4(config-if)#channel-group 1 mode desirable

!

SW4(config-if)#int fa0/3
SW4(config-if)#switchport
SW4(config-if)#channel-group 1 mode desirable

!

SW4(config-if)#int fa0/4
SW4(config-if)#switchport
SW4(config-if)#channel-group 1 mode desirable

The configuration of SW5 follows the same basic steps as with SW4. Only this time we’re using int range to configure a range of interfaces for EtherChannel with one swing.

SW5#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
SW5(config)#int range fa0/1 – 4
SW5(config-if-range)#switchport
SW5(config-if-range)#channel-group 1 mode desirable
Creating a port-channel interface Port-channel 1
SW5(config-if-range)#

Verification

• SUmeans port channel 1 is a Layer 2 EtherChannel and is in use.
• The P flags on the ports indicate that they are active in the channel.
• Although assigned to PO1 channel group, port Fa 0/48 has a D flag because the physical connection on the other end does not exist.

Configure LACP EtherChannel

The following template configures LACP, inlcuding some parameters:

Switch(config)# lacp system-priority priority
Switch(config)# interface type mod/num
Switch(config-if)# channel-protocol lacp
Switch(config-if)# channel-group number mode {on | passive | active}
Switch(config-if)# lacp port-priority priority 

• The system priority defaults to 32, 768.
  • Ranges from 1 to 65, 535.
  • A lower system priority allows a switch to become the main decision maker about the EtherChannel.
  • If left to the default value, the lower MAC address wins.
• The lacp port-priority command allows you to configure an interface to be active port or on standby.
  • You can configure more interfaces than the maximum eight the system allows to be active.
  • By configuring ports with low port-priority, you ensure that they are active on the channel. With a higher priority they are reserved as standby.
  • For example, you may configure 4 ports with a lower priority to make it active in the channel. Then you may configure another 4 with higher priority value than the fist four - which puts them on standby. If one of the lower priority ports goes down, one of the standby ports become active.
  • The default priority is 32, 768. If all ports are left in default value, the lower port numbers (in interface number order) are used to select the active ports.
  • LACP port priority value can range from 1 to 65, 535.

The following is the SW4 configuration example (refer to the same topology diagram on figure 1):

SW4#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
SW4(config)#lacp system-priority 150
SW4(config)#int rang fa0/1 – 4
SW4(config-if-range)#channel-protocol lacp
SW4(config-if-range)#channel-group 1 mode active
Creating a port-channel interface Port-channel 1

If a port priority were to be configured, it would look something like this:

SW4(config-if-range)#lacp port-priority 150

Similarly, configure SW5 as follows:

SW5#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
SW5(config)#int range fa0/1 – 4
SW5(config-if-range)#channel-protocol lacp
SW5(config-if-range)#channel-group 1 mode active
Creating a port-channel interface Port-channel 1

Verify

• All ports Fa0/1 – 4 are active in the channel group
• The negotiation protocol used is LACP. 

Configure Layer 3 EtherChannel

• When configuring Layer 3 EtherChannels, you must manually create the por-channel logical interface first, using the interface port-channel global config command.
• After that , you can put the logical interface intot the channel group by using the channel-group interface configuration command.
• Use the no switchport interface command to put the interface in Layer 3 mode. 
• If you decide to move an IP address from a physical interface to an EtherChannel, delete the address from the physical interface first, then configura it on the port-channel.

SW5 config t
SW5(config)# interface port-channel 2
SW5(config-if)# no switchport
SW5(config-if)# ip address 10.10.10.1 255.255.255.0
SW5(config-if)# no shut

!

SW5(config)#int ra fa0/1 – 4
SW5(config-if)#no switchport
SW5(config-if)#channel-prot lacp
SW5(config-if)#channel-group 2 mode active

References:

1. Configuring EtherChannels – Catalyst 3550 Software Configuration Guide 12.1(13)EA1

This entry is not an authoritative guide. These are merely notes and rehash of the primary text materials and resources that I use. For a thorough guide of the BCMSN course, consider purchasing Building Cisco Multilayer Switched Networks (BCMSN) (Authorized Self-Study Guide) (4th Edition)by Richard Froom, Balaji Sivasubramanian, and Erum Frahim and CCNP BCMSN Official Exam Certification Guide (4th Edition) by Dave Hucaby ; as well as following the links on the reference section of this entry.