Create Port Channels and Boost Bandwidth on Cisco Switches

Port Channels are a quick way to get more bandwidth by aggregating multiple connections in one virtual pipe. For instance, tie four gigabit ports together into a channel and it becomes a four gigabit connection.

Channels can also offer redundancy fault tolerance for physical connections. If one of the links involved in a channel loses connection, the channel will continue on with the existing ports and three quarters of the bandwidth.

Ports involved in a channel must be on the same blade in a modular switch like a Catalyst 4500 or 6500.

Configuring port channels has become much easier in recent IOS versions. First, designate the desired ports into a channel group.

My_Switch(config)# interface GigabitEthernet2/1
My_Switch(config-if)# description Core Connection
My_Switch(config-if)# channel-group 2 mode desirable

My_Switch(config)# interface GigabitEthernet2/2
My_Switch(config-if)# description Core Connection
My_Switch(config-if)# channel-group 2 mode desirable

The desirable option will create a channel to another Cisco switch in etherchannel format and drop a single channel if necessary. In contrast, using the on option would force a port channel, but would drop the entire channel if a single link.

Etherchannel is Cisco's proprietary channel protocol, sometimes called PAGP or Port Aggregation Protocol. In order to create channel with a server or non-Cisco switch, the channel will have to be configured in LACP (Link Aggregation Control Protocol) format, which is a multivendor standard.

My_Switch(config)# interface GigabitEthernet2/1
My_Switch(config-if)# description Core Connection
My_Switch(config-if)# channel-group 2 mode passive

My_Switch(config)# interface GigabitEthernet2/2
My_Switch(config-if)# description Core Connection
My_Switch(config-if)# channel-group 2 mode passive

Using the active option instead of the passive mode option will force the ports into a LACP channel without negotiation much like the on option for PAGP. The entire channel will go down if a single line is disconnected and will not be fault tolerant.

Next, create a virtual port channel interface.

My_Switch(config)# interface Port-channel4
My_Switch(config-if)# description Core Connection
My_Switch(config-if)# switchport

The virtual port channel configuration merely controls the aggregate port. For instance, if the port-channel interface is disabled, or shut down as Cisco calls it, then the channel will not work even though all four member ports are enabled.

Display active channels with the show neighbor command.

My_switch> show pagp neighbors

My_switch>show lacp neighbors

The output gives much detail about the channels and their state.

Disable channels
Channels are a handy tool, but most Cisco switches ship in auto mode by default and that can sometimes causes problems with workstations that do not understand how to disregard the channel auto-negotiation. The switchport mode access command disables channel negotiation as well as disabling vlan trunking negotiation. It prepares the port for use by workstations.

My_Switch(config)# interface gig6/5
My_Switch(config-if)# description My favorite PC
My_Switch(config-if)# switchport mode access

Channels are often a quick way to add bandwidth and add redundancy with existing hardware.