What are the possible reasons why STP would choose to use a FastEthernet link over an EtherChannel?
Below is a network diagram where this is happening.
2 Answers
One possible answer is because the top switch in the diagram is the root switch. The other switches have their root ports connected to the root switch. That means the etherchannel will be blocking to avoid a loop.
Here's a general algorithm that STP uses.
- The root switch has all its ports designated (i.e. forwarding).
- Every other switch selects the port with the lowest cost to the root. that's the root port (also forwarding)
- For every other network, the one port with the lowest cost/priority/MAC will be the designated port (forwarding)
- Every other port will be blocking
answered Aug 12, 2014 at 16:13
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you right that the top switch is the root switch. Please correct me if i'm wrong, but isn't it that the etherChannel should be prioritized over the single link? Commented Aug 12, 2014 at 16:18
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Not necessarily. Spanning tree doesn't know what the best path from A to B is, it just calculates a loop fre path based on cost/priority. If you follow the algorithm as I described, you'll see that you end up with exactly the network you have. Commented Aug 12, 2014 at 16:37
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@AllanChua The LAG/Etherchannel link is prioritized, but its combined root cost is higher than that of the direct root link. You need to either change the root bridge, interface costs, or use MSTP or RPVST+ to create additional trees and root bridges. Additionally, there's a design flaw in that network as it does not have the fastest links towards the root bridge.– Zac67 ♦Commented May 22 at 12:50
You need to think in terms of the total cost of the paths to the root. As long as the two uplinks to the root have equal cost, they will both be forwarding because the relevant calculation is whether the uplink cost is less than the uplink cost plus the etherChannel cost.
Should you want the etherChannel active, one way to do it is to increase the cost of one of the uplinks, so that uplink A cost > uplink B cost + etherChannel cost. If you turn the problem around, and think "how can I get this link to block", that may make it easier to get your head around.
show spanning-tree