EtherChannel Technologies

Network engineers love redundancy in a network; it allows for lines to fail without affecting the connection to the internet for end users. What’s even better is that using EtherChannel technologies, we can make redundant links and use them to increase the speed to local servers on site.

“EtherChannel” is Cisco’s own term for this port channelling, which represents the ability to bundle multiple Ethernet cables into one virtual line on Cisco devices. We commonly use this to increase the bandwidth of Layer 2 technologies, meaning all traffic within your LAN can be increased.

(config)# interface range FastEthernet0/1 – 4
(config-if)# switchport mode trunk
(config-if)# channel-group 1 mode negotiate
(config)# interface port-channel 1
(config-if)# switchport mode trunk

The above commands can be issued on both switches to join the Ethernet wires plugged into port 1, 2, 3 and 4. Typically each “FastEthernet” line can only handle 100Mbps, but bundling them together in an EtherChannel would theoretically allow 4 times that speed. Unfortunately, in reality the speeds will only really reach 350mbps, which still represents a significant increase in speed.

Alongside the advantage of an increase in speed, you’ll also find that with this set up, one link can go down and nothing too much will change (although the speed will be reduced by 25%).

When multiple links are bundled in this way, you can configure the port channel (the group of ports as one) in the same way that you would configure a physical port. All the members of the EtherChannel will follow the configuration, e.g. switchport mode, allowed vlans, port-security etc.

With EtherChannel, there isa limit of 8 bundled FastEthernet ports due to limitations in the protocol. However this is plenty to up the bandwidth by a large amount!

EtherChannel Negotiation

There are 2 major ways to get EtherChannels to agree to bundling ports, the first is to manually enable it, and the second is using a negotiation protocol. There are two types of negotiation protocol: Cisco has its own method, called Port Aggregation protocol (PAgP), but there is also a known standard called Link Aggregation Control Protocol (LACP). These both work in similar ways in that they will negotiate with the neighbouring switch to bundle its ports. Each protocol has different names for the states of negotiation. The table below describes how the LACP/PAgP negotiation process takes place.