https://community.cisco.com/t5/routing/spanning-tree-with-port-priority/td-p/1815059
"You just configure "port priority" on upstream switches on designated ports or modify "port cost" on downstream switches on root ports"
https://packetlife.net/blog/2008/sep/5/spanning-tree-port-costs/
The default cost of the 1 Gig link would be 4. Changing the cost to 128 makes the link less preferred.
The key here is to remember that the cost doesn't have to be set on the far end like priority, because it's the cost , hence setting g0/2 cost to 128 on SW2 is OK in order to select g0/3 as a root and blok g0/2 on SW2.
They have differences though:
If you want to affect how to the local switch elects the root port change the cost on the links. Cost is cumulative throughout the STP domain. The higher cost is the less preferred.
If you want to affect how downstream switch elects its root port change the priority. This is only local significant between the two directly connected switches. Highest priority is less preferred.
Going away from the root of the tree use priority whereas, when going towards the root of the tree use cost.
ref: https://community.cisco.com/t5/switching/stp-port-priority-vs-cost/td-p/1274612#:~:text=Highest%20priority%20is%20less%20preferred,of%20the%20tree%20use%20cost.&text=have%20differences%20though%3A-,If%20you%20want%20to%20affect%20how%20to%20the%20local%20switch,cumulative%20throughout%20the%20STP%20domain.
so: ANS: A
Answers C and D can be eliminated as they would make SW2 root since it has priority 4096.
Answer B can also be eliminated on the surface because first it isn't specifying STP for VLAN 1, and port priority affects the port decision for the neighboring switch (SW4), not the local switch. But SW4 is root so both of its ports will be designated.
I goofed misinterpreted cost as priority, also bothered that it wasn't setting cost on STP instance for VLAN1.
La respuesta correcta es A.
SW4 spanni vlan 1 priority 0 <---- esto fuerza al SW4 ser el root para vlan 1
SW2 int G 0/2
SW2 spann cost 128 <--- este comando realmente no esta haciendo nada, ya que el costo por default de casa puerto es 128. pero igual el puerto G0/2 por orden de preferencia
This is the correct explanation of why option A is correct:
*spanning-tree vlan 1 priority 0 is hardcoding SW4 to be the root as it has the lowest priority
*By changing the cost of the link (which is first looked at before the sender Port-ID) on SW2 causes SW2 to elect the other port as a RP as the other link has lowest cost then 128.
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