Refer to the exhibit. Assuming all links are functional, which path does PC1 take to reach DSW1?
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ENCOR Exam - Question 556
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Correct Answer: C
In the topology shown, DSW2 serves as the root bridge because it has the lowest priority of 24576. According to the Spanning Tree Protocol (STP) rules, all ports on the root bridge (DSW2) must be in the forwarding state. The best path calculation to reach the root bridge from any non-root switch will depend on the lowest cost path. The cost is based on the speed of the connection: 10 Gbps links have a lower cost than 1 Gbps links. Therefore, the path that PC1 would take to reach DSW1 is through ALSW1 to DSW2 and then to DSW1. This path is chosen because ALSW1 to DSW2 is a 1 Gbps link (cost=4) and DSW2 to DSW1 is a 10 Gbps link (cost=2) which totals 6, compared to the direct ALSW1 to DSW1 path which would have a higher cost or may even be blocked by STP. Hence, the correct answer is that PC1 goes from ALSW1 to DSW2 to DSW1.
Discussion
22 commentstckoonOption: C
Oct 10, 2022Path cost from ALSW1 to root bridge(DSW2) via DSW1 is 2 + 2 = cost of 4. ( 10gbps+10Gbps) Path cost from ALSW1 to root bridge(DSW2) is 4 = cost of 4. ( 1Gbps) Therefore the patch cost equal and DSW2 is root bridge Therefore it will via Gi0/2. Answer is C. IF let say the question change the ALSW1 to DSW2 Gi0/2 interface to 100Mbps (cost=19) , the answer will be different. ALSW1 to DSW2 will be blocked. Answer will be D.
chegiiOption: C
Feb 6, 2023STP Rule 1—All ports of the root switch must be in forwarding mode. DSW2 is root because it has lowest priority 24576. Therefore, ALSW1 will block port Gi0/1 The path will be PC1 to ALSW1 to DSW2 to DSW1
Alondrix
Nov 3, 2023True, but forwarding ports on the root switch would be misleading if considered alone. Path cost is considered by the downstream switch based on the received BPDU, not at the root.
StefanOT2Option: C
Jan 14, 2023C. is the answer. But NOT directly because DSW2 is root bridge. It does not matter, if ALSW1 is directly connected to the root bridge or not. The best path to the root is choosen and this has nothing to do with direct connection. When a switch has several ways to the root bridge, the port is chosen based on - The cost of the port (in this case both times 4, so it is equal) - port priority (also equal, as no info given) - switch ID (MAC) Because DSW2 has a lower MAC, the path via Gi0/2 is chosen.
Leoveil
May 18, 2023well explained , well done
rafaelinho88Option: C
Feb 6, 2023In the topology above, we see DSW2 has lowest priority 24576 so it is the root bridge for VLAN 10 so surely all traffic for this VLAN must go through it. All of DSW2 ports must be in forwarding state. And: + The direct link between DSW1 and ALSW1 is blocked by STP. + The direct link between DSW1 and ALSW2 is also blocked by STP. Therefore PC1 must go via this path: PC1 -> ALSW1 -> DSW2 -> DSW1
Deu_InderOption: D
Sep 7, 2022DSW2 is the root bridge. ALSW1 has two paths to DSW2: a direct path and via DSW1. ALSW1 needs to decide which path has less cost. Both the paths have the same cost. Direct path has a cost of 4 since the link is 1GB. The path thru DSW1 has the cost 2 + 2 = 4 as the cost of a 10GB link is 2. So, path costs are equal. Now, my understanding would be: since the port Gi0/1 is smaller than Gi0/2, Gi0/1 will be chosen as root port. And thus the path to root bridge for ALSW1 will be thru DSW1.
onkel_andi
Oct 9, 2022The Packet goes always to the root bridge first
rogi2023
Jul 4, 2023Yes, but following the principle: 1-lowest Roothpath cost; IF equal than 2-lowest Sender BID, If that is equal; 3-lower received portID in BPDU. HTH
Summo
Oct 12, 2022we have two equal cost paths, tie breaking rules in this scenario. Here they are, 1. Lowest Sending Bridge ID 2. Lowest Port Priority (of sender) 3. Lowest Interface number (of sender)
rogi2023
Jul 4, 2023Yes, but following the principle: 1-lowest Roothpath cost; IF equal than 2-lowest Sender BID, If that is equal; 3-lower received portID in BPDU. HTH
Joseph123Option: D
Sep 11, 2022Saw this question on the real exam
VergilPOption: C
Oct 1, 2022I'm going for C In the topology above, we see DSW2 has lowest priority 24576 so it is the root bridge for VLAN 10 so surely all traffic for this VLAN must go through it. All of DSW2 ports must be in forwarding state. And: + The direct link between DSW1 and ALSW1 is blocked by STP. + The direct link between DSW1 and ALSW2 is also blocked by STP. Therefore PC1 must go via this path: PC1 -> ALSW1 -> DSW2 -> DSW1.
kewokil120Option: C
Dec 23, 2022not sure why people are putting D due to Root Bridge Priority.
StefanOT2
Jan 14, 2023Because the Path to the Root Bridge and Root Bridge Priority are 2 different things.
Kan3184Option: D
Aug 2, 2024Guys, the trick is how to calculate path costs. From ALSW1 to root bridge(DSW2) via DSW1 is 2 + 1 = cost of 3. ( 10gbps+20Gbps). This is because the link between DSW1 and DSW2 is Etherchannel 20Gbps. According to the table 2-2 from OSG, 20 Gbps gives us the cost 1, not 2!
Deu_Inder
Sep 7, 2022And Gi0/2 on ALSW1 will be blocked.
ivokolOption: C
Oct 1, 2022PC1-ALSW1-DSW2-DSW1
onkel_andiOption: C
Oct 9, 2022Packets always goes to the root bridge first.
yousif387
Nov 14, 2022not mandatory, if the port is in FWD state it will check the MAC address based on that it will FWD the frame
91fdc83
Dec 24, 2024that's absolutely incorrect. it comes down to root port selection, which is the main issue in this question and depends on what calculation method and protocol are being used
bora4motionOption: C
Nov 28, 2022I'm going with C.
nushaduOption: C
Dec 6, 2022DSW2 is root stp switch (it has low priority), so all traffic goes over it
nushaduOption: C
Dec 23, 2022ALSW1 to DSW2 [ROOT] to DSW1
rami_mmaOption: C
Mar 28, 2023"spanning-tree pathcost method long" command is very important to answer this question.
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Jun 25, 2023Yeah... I got this answer wrong because I assumed otherwise. But we are supposed to know that short method is the default. SW-1#show spanning-tree pathcost method Spanning tree default pathcost method used is short
Lukaszaw
Sep 5, 2022C is correct
Lukaszaw
Sep 6, 2022sorry , D is correct indeed
nushadu
Dec 13, 2022C. this question about root priority, lower priority is root switch stp: sw2#show spanning-tree VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 32769 Address aabb.cc00.1000 Cost 100 Port 65 (Port-channel2) Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 32769 (priority 32768 sys-id-ext 1) Address aabb.cc00.4000 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 300 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Et0/0 Desg FWD 100 128.1 Shr Et0/2 Desg FWD 100 128.3 Shr Et0/3 Desg FWD 100 128.4 Shr Po2 Root FWD 100 128.65 Shr sw2#
nushadu
Dec 13, 2022priority has been changed: sw2(config)#spanning-tree vlan 1 priority 24000 % Bridge Priority must be in increments of 4096. % Allowed values are: 0 4096 8192 12288 16384 20480 24576 28672 32768 36864 40960 45056 49152 53248 57344 61440 sw2(config)#spanning-tree vlan 1 priority 24576
nushadu
Dec 13, 2022now it is root for vlan1 sw2(config)#do s spann VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24577 Address aabb.cc00.4000 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 24577 (priority 24576 sys-id-ext 1) Address aabb.cc00.4000 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 15 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Et0/0 Desg FWD 100 128.1 Shr Et0/2 Desg FWD 100 128.3 Shr Et0/3 Desg FWD 100 128.4 Shr Po2 Desg FWD 100 128.65 Shr sw2(config)#^Z sw2#sh
nushadu
Dec 13, 2022now it is root for vlan1 sw2(config)#do s spann VLAN0001 Spanning tree enabled protocol ieee Root ID Priority 24577 Address aabb.cc00.4000 This bridge is the root Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Bridge ID Priority 24577 (priority 24576 sys-id-ext 1) Address aabb.cc00.4000 Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec Aging Time 15 sec Interface Role Sts Cost Prio.Nbr Type ------------------- ---- --- --------- -------- -------------------------------- Et0/0 Desg FWD 100 128.1 Shr Et0/2 Desg FWD 100 128.3 Shr Et0/3 Desg FWD 100 128.4 Shr Po2 Desg FWD 100 128.65 Shr sw2(config)#^Z sw2#sh
owenshinobi
Mar 26, 2023I'm no sure but. NOTE The original IEEE specification did not account for links faster than 1 Gbps. Specifically, 1 Gbps links were assigned a port cost of 1, 100 Mbps link a cost of 10, and 10 Mbps links a cost of 100. Any link faster than 1 Gbps (i.e., 10 GE) was automatically assigned the same port cost of 1 Gbps links (i.e., port cost of 1). refer link: https://www.ciscopress.com/articles/article.asp?p=2832407&seqNum=4#:~:text=The%20default%20port%20cost%20is,a%20port%20cost%20of%20100.
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May 24, 2024C is correct
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May 24, 2024after i read some comments i see many people confused about this. don't make it hard on your self, just think simple. we have 3 switches forming a circle, we all know that DSW2 is the root bridge and all RB ports are forwarding. in this case, we know for sure that the only connection left is the one between ALSW1 and DSW1, this link should be blocking on 1 of it's ports.
91fdc83Option: D
Dec 24, 2024Cisco SWs use Rapid PVST by default which uses different cost values. That being the case , G0/1 becomes the root port because its cost to reach the root bridge (10g+10g= 2000+2000) is smaller than directly to the root thru g0/2 (1g = 20000), which enters the alternate state and doesn't forward traffic.
simcosOption: C
Feb 6, 2025The answer is indeed C, here my explanation. First thing to check is the cost to the root from ALSW1, and the 2 path have equal cost (direct link = 4, non-direct link = 2+2 = 4). What do we check next? The bridge ID (which is the HEX representation of the priority + the bridge MAC), the lowest wins. A root bridge always has the lowest bridge ID. Conclusion: the root port for ASLW1 is the port directly connected to DSW2, hence traffic will go ALSW1 --> DSW2 --> DSW1