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Exam 350-401 topic 1 question 119 discussion

Actual exam question from Cisco's 350-401

Question #: 119
Topic #: 1

Refer to the exhibit.

Link1 is a copper connection and Link2 is a fiber connection. The fiber port must be the primary port for all forwarding. The output of the show spanning-tree command on SW2 shows that the fiber port is blocked by spanning tree. An engineer enters the spanning-tree port-priority 32 command on G0/1 on SW2, but the port remains blocked.
Which command should be entered on the ports that are connected to Link2 to resolve the issue?

A. Enter spanning-tree port-priority 4 on SW2.
B. Enter spanning-tree port-priority 32 on SW1.
C. Enter spanning-tree port-priority 224 on SW1.
D. Enter spanning-tree port-priority 64 on SW2.

Show Suggested Answer

Suggested Answer: B 🗳️

by Tuborger at Nov. 3, 2020, 9:36 a.m.

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rezavage

Highly Voted 4 years, 6 months ago

B is correct. whenever a tie happens on two ports that receiving the same advertised cost and want to choose the RP , the port that is receiving the bpdu from the upstream switch port with lower priority will win the race and will becom the root port. so, in order to change the root port on SW2 you have to lower the port priority on switch 1.

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jmaroto

Highly Voted 4 years, 2 months ago

B is correct. Check in lab, when the switch 2 receive the bpdu with port priority it change the Altn Blocked port to root port. Mar 8 23:24:09.971: RSTP(2): updt roles, received superior bpdu on Et0/1 *Mar 8 23:24:09.971: RSTP(2): Et0/1 is now root port *Mar 8 23:24:09.971: RSTP(2): Et0/0 blocked by re-root *Mar 8 23:24:09.971: RSTP(2): Et0/0 is now alternate *Mar 8 23:24:09.971: RSTP[2]: Et0/0 state change completed. New state is [blocking] *Mar 8 23:24:09.971: RSTP[2]: Et0/1 state change completed. New state is [forwarding] *Mar 8 23:24:09.971: RSTP(2): starting topology change timer for 35 seconds *Mar 8 23:24:09.971: STP[2]: Generating TC trap for port Ethernet0/1

upvoted 6 times

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RichardIzarra

Most Recent 6 months ago

Selected Answer: B

Cuanto menor sea el valor numérico de la prioridad del puerto, mayor será la prioridad de ese puerto. En otras palabras, debemos cambiar la port-priority en Gi0/1 de SW1 (no en Gi0/1 de SW2) a un valor menor que el de Gi0/0.

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RichardIzarra

6 months ago

Selected Answer: B

Tal vez la simulacion tiene sentido con un sw cat: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-5/configuration_guide/lyr2_lyr3/b_165_lyr2_lyr3_9300_cg/b_165_lyr2_lyr3_9300_9500_cg_chapter_00.html The port priority in Spanning Tree Protocol (STP) determines the role of a port when multiple paths to the root bridge exist. The range for port priority is 0 to 240, with increments of 16, and the default value is 128. The priority works as follows: Lower values mean higher priority. For example, a port with a priority of 16 will be preferred over a port with a priority of 128 if the costs to the root bridge are the same. Invalid values (not in increments of 16) are rejected. Valid priorities: 0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240

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AbdullahMohammad251

8 months ago

Selected Answer: B

To influence the root port election on SW2, we need to adjust the port priority on the upstream switch (SW1,) local port priority values on SW2 don't matter. The lower the port priority value, the higher the priority. By default, the port priority is 128, with valid values ranging between 0, and 240 in increments of 32. When multiple paths to the root bridge have the same cost, the port priority is the tiebreaker. In this scenario, we want SW2 to use interface Gi 0/1 as its root port and interface Gi 0/0 as its alternate port. To achieve this, we must configure the port priority of Gi 0/1 on SW1 to 32, which is lower than the default priority of '128' on Gi 0/0. By doing so, interface Gi 0/1 on SW1 will advertise BPDUs with a higher priority than those advertised from interface Gi 0/0. SW2 will thus receive the higher priority BPDUs on link 2, choosing Gi 0/1 as its root port.

upvoted 2 times

AbdullahMohammad251

8 months ago

https://www.ciscopress.com/articles/article.asp?p=2995351&seqNum=2#:~:text=Modifying%20STP%20Root%20Port%20and%20Blocked%20Switch%20Port%20Locations https://www.astorinonetworks.com/2011/06/30/spanning-tree-port-priority/#:~:text=We%20need%20to%20look%20at%20changing%20the%20port%2Dpriority%20upstream%20on%20Cat1%20so%20that%20when%20Cat1%20sends%20BPDUs%20to%20Cat2%2C%20Cat2%20will%20compare%20the%20received%20port%2Dpriority%20values%20and%20choose%20the%20lowest%20one.

upvoted 1 times

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[Removed]

1 year ago

Selected Answer: B

B is correct the default port priority is 128, lowest priority wins, we should just make both interfaces on the primary link lower than 128 at both sides.

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djedeen

1 year, 9 months ago

Selected Answer: B

Lower port priority on upstream (bpdu sender) will be selected, default value is 128.

upvoted 2 times

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teikitiz

1 year, 11 months ago

Selected Answer: B

This link really helped refreshing this topic. https://www.omnisecu.com/cisco-certified-network-associate-ccna/how-spanning-tree-protocol-stp-select-root-port.php

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NTGuru

2 years, 1 month ago

Correct Answer B Order of precedence 1. A lower Root Bridge ID 2. A lower path cost to the Root 3. A lower Sending Bridge ID 4. A lower Sending Port ID

upvoted 2 times

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HungarianDish_111

2 years, 2 months ago

Selected Answer: B

root port election: configure "port priority" on upstream switches on designated ports or modify "port cost" on downstream switches on root ports https://community.cisco.com/t5/routing/spanning-tree-with-port-priority/td-p/1815059

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x3rox

2 years, 3 months ago

There is a problem with this question. The Root Switch is SW1 so SW2 needs to find the closest port to the root. Since we have a tie in cost SW2 will select the port receiving the lowest priority from the upstream switch. In the output we see that G0/1 (Fiber) is ALREADY the lowest priority on SW1 (32.2). So it should've been elected as RP NOT G0/0. WHY IS G0/1 IN BLOCKING STATE???

upvoted 3 times

x3rox

2 years, 3 months ago

Forget it. I misunderstood the output. The column PrioNbr is local. ok -So.... SW needs to W2 will select the port receiving the lowest priority from the upstream switch. Asumming that it has the default values SW2 will receive BPDUs indicating Priorities as follows: 128.1 for G0/0 and 128.2 for G0/1. SW will elect the lowest between the two which is G0/0 as a Root Port ( Link 1 - Cooper ). So, in order to change this selection the admin will need to go ahead the change the port priority on SW1 to a lower value for G0/1 so that SW2 will elected as it's Root Port. Answer B is correct as it will change the default 128.2 to 32.2 for G0/1. Now SW2 will see that 32.2 on G0/1 is lower than the receiving priority of 128.1 that G0/0 has and will make G0/1 it's Root Port (Link 2 - Fiber) - DO NOT LOOK at the column Prio.Nbr, these are local values that don't do anything in this scenario.

upvoted 2 times

x3rox

2 years, 3 months ago

Switch(config-if)#spanning-tree port-priority ? <0-224> port priority in increments of 32

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x3rox

2 years, 3 months ago

from GNS3 vios_l2-ADVENTERPRISEK9-M Experimental Version 15.2(20170321:233949)

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nushadu

2 years, 5 months ago

In my current lab, I could not find 32 value, only 64, root sw: sw2(config-if)#spanning-tree port-priority ? <0-192> port priority in increments of 64 sw2(config-if)#spanning-tree port-priority 32 % Port Priority in increments of 64 is required sw2(config-if)#spanning-tree port-priority 64 sw2(config-if)#do s runn interface port-channel 2 Building configuration... Current configuration : 136 bytes ! interface Port-channel2 switchport switchport trunk encapsulation dot1q switchport mode trunk spanning-tree port-priority 64 end sw2(config-if)#

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nushadu

2 years, 5 months ago

far-end before changing priority (see Po2 port): sw1# sw1#show spanning-tree vlan 1 | i Po2|Et0/3 Et0/3 Root FWD 100 128.4 Shr Po2 Altn BLK 100 128.66 Shr sw1# !!! after sw1# sw1#show spanning-tree vlan 1 | i Po2|Et0/3 Et0/3 Altn BLK 100 128.4 Shr Po2 Root FWD 100 128.66 Shr sw1#

upvoted 1 times

nushadu

2 years, 5 months ago

I would say "B" but I did not have this option in my lab 32 (only 64), maybe in the newest version IOS ...

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Eddgar0

3 years, 1 month ago

Selected Answer: B

B is correct because in this scenario sw2 will select the root port of the lowest port priority of the advertising switch (switch that send the bpdu) in this case the SW1.

upvoted 5 times

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youtri

3 years, 6 months ago

notice that Ethernet port priority by default is 128 tested by eve ng Switch(config-if)#spanning-tree port-priority 32 % Port Priority in increments of 64 is required

upvoted 4 times

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