Lab: a redundant triangle that does not melt
Three switches in a ring. With spanning tree, resilience; without it, a storm you can watch in the counters.
Three switches, each cabled to the other two. That is a loop, and at layer 2 a loop has no TTL to stop it.
Do this
- Confirm the topology converges: one port somewhere is Blocking.
- Find out which switch is root, and notice that nobody chose it.
- Make the switch you want to be root the root, by priority.
- Watch the blocked port move.
- Shut the root port on one switch and time how long until the blocked port
SW1(config)# spanning-tree vlan 1 root primary
SW1# show spanning-tree vlan 1
The point
Spanning tree is doing something very specific: it leaves exactly enough ports forwarding to reach everything and blocks the rest. The blocked port is not off — it is listening, and it is your redundancy.
The default root election is by lowest MAC address, which means that without configuration your traffic path is decided by which switch is oldest. Set it deliberately.
Try breaking it
- Turn spanning tree off on all three and ping. Watch the interface counters
- Enable portfast on a switch-to-switch port and see what happens on the next
- Add
spanning-tree bpduguard enableto an access port and then plug a
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