Why EtherChannel is needed?

• ASW# = Access layer Switch, a switch that end hosts connect to.

• DSW# - Distribution layer Switch, a switch that access layer switches connect to.

The connection to DSW1 is congested. I should add another link to increase the bandwidth, so it can support all of the end hosts.

The connection is still congested. I'll add another link.

When the bandwidth of the interfaces connected to end hosts is greater than the bandwidth of the connection to the distribution switch(es), this is called oversubscription. Some oversubscription is acceptable, but too much will cause congestion.

The connection to DSW1 is still congested. I guess I should add another link...

If you connect two switches together with multiple links, all except one will be disabled by spanning tree.

If all of ASW1's interfaces were forwarding, Layer 2 loops would form between ASW1 and DSW1, leading to broadcast storms.

Other links will be unused unless the active link fails. In that case, one of the inactive links will start forwarding.

EtherChannel

EtherChannel groups multiple interfaces together to act as a single interface.

STP will treat this group as a single interface.

Traffic using the EtherChannel will be load balanced among the physical interfaces in the group. An algorithm is used to determine which traffic will use which physical interface. More details on this later!

Some other names for an EtherChannel are:

• Port Channel,

• LAG (Link Aggregation Group).