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.