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 help 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 to use load balancing on traffic between the redundant links; this help improve the efficient use of bandwidth.
Spanning-tree treats the EtherChannel bundle as a single logical switchport 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 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.
To dynamically establish and maintain the EtherChannel bundle, Cisco Catalyst switches use channeling protocol 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 configuration to form a port-channel.
2. Link Aggregation Protocol (LACP).
LACP works in a similar way 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.
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 table below: