What is EtherChannel?
The process used to bundle ports together in an ethernet switch to achieve higher bandwidth is known as EtherChannel or link aggregation.
Link aggregation can be useful when interconnecting switches together and when interconnecting other devices to a switch, e.g. you can use EtherChannel when you need to connect a switch to a server with multiple network adaptors or when interconnecting two distribution switches.
EtherChannel helps improve operation when ethernet switches are interconnected using multiple physical interfaces.
In a more traditional network function, Spanning-Tree Protocol blocks one redundant link to avoid Layer 2 loops, what Etherchannels does is use load balancing on traffic between the redundant links; this helps improve the efficient use of bandwidth.
Spanning-tree treats the EtherChannel bundle as a single logical switch port and adjusts the spanning-tree cost to reflect the increased bandwidth.
How EtherChannel Works.
In an Ethernet Channel group, If there is a failure on a physical link, the EtherChannel only loses the bandwidth that the link supplied.
If the physical link comes back up it is dynamically added back into the EtherChannel.
Spanning-tree sees the EtherChannel bundle as a single logical switch port and adjusts the spanning-tree cost to reflect the increased bandwidth.
To bundle multiple physical Ethernet ports together, you must use the channel-group command, this enables a single logical interface called a port-channel. A port-channel can be an access port or it can be a trunk port.
You can aggregate up to eight 10/100 ports together to enable a port channel on Cisco Catalyst switches, You can also bundle or aggregate up to eight-gigabit ports if available. A switch can have multiple port channels.
EtherChannel Protocols.
To dynamically establish and maintain the EtherChannel bundle, Cisco Catalyst switches use channeling protocols like:
1. Port Aggregation Protocol (PAgP)
PAgP (Cisco proprietary) enables the Catalyst switches to learn the capabilities of each interface assigned to an EtherChannel group and activate interfaces of similar configurations to form a port channel.
2. Link Aggregation Protocol (LACP).
LACP works similarly to PAgP, only its multi-vendor or standard channeling protocol.
However, link protocols such as DTP, VTP, CDP, and STP still transmit and receive frames over a port channel.
STP/EtherChannel Cost
The spanning tree reproduces the increased bandwidth provided by EtherChannel.
The default cost for a 100 Mbps link is 19 and if a port-channel is created that has only two 100 Mbps links, the spanning-tree cost will be 9.
A port-channel with six or more 100 Mbps physical ports will have an STP cost of 5. The cost is calculated based on the number of ports assigned to the port channel. see the table below:
In Summary, EtherChannel is widely used in enterprise networks for its ability to increase bandwidth, provide load balancing, and ensure redundancy. It’s an essential component in designing resilient and high-performance network infrastructures.
How to Configure EtherChannel.
