Stubby Post – Path Cost of EtherChannels

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.

jac
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