I was doing some STP labs tonight and found something that caught me off guard a bit. I had been meddling with some EtherChannels between a pair of 3750s earlier today, and I forgot to reset the configs before starting on the STP stuff. One my secondary root switch, I ran a show spanning-tree vlan 1 to see what status the ports were in, and I noticed the root path cost.
VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24577 Address 001b.d4fa.bb00 Cost 12
This switch is directly connected to the root bridge via a pair of EtherChanneled FastEthernets, so I just assumed I’d get a cost of 19. I surely didn’t expect a cost of 12. I added a third interface to the channel-group and wound up with this.
VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24577 Address 001b.d4fa.bb00 Cost 9
Obviously there’s some internal math going on with the EtherChannel and STP. Guess what happens when I add a fourth link?
VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24577 Address 001b.d4fa.bb00 Cost 8
It’s interesting to see how the path cost changes in a way to seems disproportionate to the bandwidth.
Send any new math formulae comments this way.