EtherChannel

In Ethernet networks, multiple physical links between devices may appear to provide more bandwidth and enhance reliability.

However, this approach creates a problem.
Spanning Tree Protocol (STP) prevents Layer 2 loops by blocking redundant links.

Without EtherChannel

When several parallel links exist between switches, STP disables the extra ones to avoid loops.

Figure 1 – Many Physical Links Without EtherChannel

• 🔴 Only one link per redundant pair remains active.

• 🔴 Extra links stay idle, wasting available bandwidth.

If we look at SW2, we can verify this behavior using the following command:

SW2# show spanning-tree

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- ----
Gi0/1            Root FWD 4         128.1    P2p
Gi0/2            Altn BLK 4         128.2    P2p
Gi0/3            Desg FWD 4         128.3    P2p
Gi0/4            Altn BLK 4         128.4    P2p

If we examine the output on SW2, we can see that:

• The interfaces highlighted in green (Gi0/1 and Gi0/3) are in Forwarding (FWD) state.

• The interfaces highlighted in red (Gi0/2 and Gi0/4) are in Blocking (BLK) state.

This means that although four physical links are connected, only two are actively forwarding traffic.
The other links remain unused due to STP.

As a result, adding more physical links does not improve bandwidth utilization.

With EtherChannel

EtherChannel solves this limitation by bundling multiple physical links into a single logical channel.
From the perspective of STP, routing protocols, and the switch itself, this bundle appears as one single interface.

Figure 2 – EtherChannel Logical Bundling

• ✅ All links are active together, providing aggregated bandwidth.

• ✅ If one link fails, the remaining links continue forwarding traffic.

• ✅ STP does not block individual member links because it sees only one logical interface.

If we now look again at SW2, we observe a different result: