Let me give you an example. Let’s say we have three switches, SW1, SW2 and SW3. These switches are connected in a triangle.
Without any Etherchannels, spanning-tree will work and blocks some interfaces to create a loop-free topology…life is good.
Now we try to configure an Etherchannel between SW1 and SW2, we use a protocol like PaGP for negotiation. For some reason, there’s a misconfiguration on one of the physical interfaces. Since we use PaGP, it’s able to detect this and this physical interface will be removed from the etherchannel or the etherchannel won’t come up…life is good.
Now we want to configure a static Etherchannel between SW2 and SW3. First we configure the static etherchannel on SW2, but for some reason…someone configures the wrong interfaces on SW3. Here’s what might happen:
- SW2 has an Etherchannel and will only send one BPDU to SW3.
- SW3 doesn't have an Etherchannel and will send two BPDUs (one of each physical interface).
- SW3 expects to receive two BPDUs, one on each interface. It's only receiving one though.
- Since SW3 doesn't receive a BPDU, it might put the interface in forwarding mode.
- We now have a loop.
Spanning-tree can do its job but only when your interfaces are configured correctly. When using static Etherchannels, there’s nothing protecting you from misconfiguration.