STP Questions 3
Question 1
Explanation
The command “spanning-tree port priority 0” is better than the command “spanning-tree port priority 16” as interface G1/0 is preferred over other interfaces with lowest priority port ID. So answer E is better than B.
Question 2
Question 3
Explanation
If you configure “spanning-tree portfast” then it will only takes effect on access ports (portfast is still disabled on trunk ports). To enable portfast on a trunk port you need the trunk keyword (“spanning-tree portfast trunk”).
Note: The command “spanning-tree portfast default” can only be configured under global configuration mode but this question requires to “apply on the interfaces”.
Question 4
Explanation
The full macro command is “spanning-tree vlan <vlan-id> root primary”. This command will adjust its priority to a lower value (than the value of the current root bridge), so it will become the new root bridge for the specified VLAN, so this command basically generates a macro. But this keyword does not adjust STP timers so maybe this requirement does not exist in the exam.
If the question asks about “automatically adjust STP timers” only then the answer should be “diameter”:
Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.
Reference:
Question 5
Question 6
Explanation
From the second command output (show spanning-tree mst) we learn that MST1 includes VLANs 10 & 20. Therefore if we want DSW1 to become root bridge for these VLANs we need to set the MST 1 region to root -> The command “spanning-tree mst 1 root primary” can do the trick. In fact, this command runs a macro and sets the priority lower than the current root.
Also we can see the current root bridge for these VLANs has the priority of 32769 (default value + sysid) so we can set the priority of DSW1 to a specific lower value. But notice that the priority must be a multiple of 4096. Therefore D is a correct answer.
Question 7
Explanation
By calculating and assigning the port cost of the switch ports, you can ensure that the shortest (lowest cost) distance to the root switch is used to transmit data. You can calculate and assign lower path cost values (port costs) to higher bandwidth ports by using either the short method (which is the default) or the long method. The short method uses a 16-bit format that yields values from 1-65535. The long method uses a 32-bit format that yields values from 1-200,000,000.
The port cost depends on the port speed; the faster interface speeds indicate smaller costs. MST always uses long path costs.
Reference: http://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/spanning-tree-mst-cost.html
Question 8
Question 9
Question 10
Explanation
We suppose there is no Etherchannel configured between ASW1 and DSW1.
For STP, the cost of 10Gbps is 2 while the cost of 1Gbps is 4 so the paths from ASW1 -> DSW1 -> DSW2 and ASW1 -> DSW2 have the same accumulate STP Path cost (= 4). If multiple paths are available to reach the Root Bridge (Root Switch) with the same accumulated Spanning Tree Path Cost in a Non-Root Switch, select the port connected to the neighbor switch which has the lowest Switch ID value as the Root Port. Therefore the 1Gbps port on ASW1 will be chosen as Root port (as DSW2) has the lowest Switch ID value -> traffic will go from ASW1 -> DSW2 -> core.
Question 11
Explanation
From the output we see DSW2 is running in RSTP mode (in fact Rapid PVST+ mode as Cisco does not support RSTP alone). When a new switch running PVST+ mode is added to the topology, they keep running the old STP instances as RSTP (in fact Rapid PVST+) is compatible with PVST+.
Question 12
Question 13
Explanation
Use the diameter keyword to specify the Layer 2 network diameter (that is, the maximum number of switch hops between any two end stations in the Layer 2 network). When you specify the network diameter, the switch automatically sets an optimal hello time, forward-delay time, and maximum-age time for a network of that diameter, which can significantly reduce the convergence time. You can use the hello keyword to override the automatically calculated hello time.
Please update the link to the questions
can someone kindly explain question 4 and 8?
@Dee I have a seen different dumps saying diameter is correct.
I am not sure on that one @Cetprepare anyone please help.
In Q9 the answer is A ( 128 ) but if this value increase in 32 ( 0-32-64-128….) then why 0 value answer C is not correct ?
On Question 6 why A. spanning-tree mstp 1 priority 0 is not correct answer
Q5. why not D instead of B ? 0 is the lowest priority which can be configured in cisco switches.
@ jonathan “mstp” instead of “mst” ?
Q4 explanation – Macro commands are a set of ordinary CLI commands combined together. Think of it as an alias or shortcut for a script – applying the macro command runs those normal commands in one go.
Example Macro Command that changes priority to make switch the root as well as changes the timers:
SWITCH> (enable) set spantree root 8 diameter 4 hello 2
VLAN 8 bridge priority set to 8192.
VLAN 8 bridge max aging time set to 14.
VLAN 8 bridge hello time set to 2.
VLAN 8 bridge forward delay set to 10.
Switch is now the root switch for active VLAN 8.
Q8 is simply asking which commands can be used to configure LoopGuard.
A will do it for all ports.
B will do it for individual port.
Other options are incorrect.
Q4.
Answer : Diameter.
-You can configure the root bridge with the network diameter for a specific VLAN, and the timer values are computed accordingly. Cisco recommends that, if you must make changes, only configure the diameter and optional hello time parameters on the root bridge for the VLAN.
spanning-tree vlan vlan-id [root {primary | secondary}] [diameter diameter-value [hello hello-time]]
!— This command needs to be on one line.
This macro makes the switch root for the specified VLAN, computes new timer values on the basis of the diameter and hello time specified, and propagates this information in configuration BPDUs to all other switches in the topology.
https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/24330-185.html
Q1.. The priority values are 0, 32, 64, 96, 128, 160, 192, and 224. All other values are rejected.(cisco) ..so definitely E .
https://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/n5k/commands/spanning-tree-port-priority.html
Can someone explain Q1 to me please?
I must be misunderstanding the topology.
I thought port priority should be set on the up-stream switch.
Thanks.
Q5 and Q6: Why choose priority 4096 instead of 0? 0 is a valid priority value and is numerical lower therefor superior to 4096. Or did I miss something?
I think Q2 is B)
Default STP mode for switches is currently Rapid PVST+
Does anyone know why in Q5 and Q6 a priority of 4096 is prefered to a priority of 0 ?
What about Q10 and link between DSW1 and ASW1? We can see TWO red lines there. Could we assume an etherchannel there? In this case STP will take into account the portchannel path cost which MUST be lower than cost of single 10G link. So, preferred path will include DSW1 and answer D is correct, isn’t it?
Q10 is just terribly explained. Can anyone please better explain the answer for Question 10?
Q10 is rather tricky tbh. are those 2 10G links in a port-channel? if yes, than it should have a cost of 1, to which you add 2 from 10G between distribution, making it better than the 4 between dsw2 and asw1. But if is not, then both have same cost to reach root. This means that we should know the port-priority, then port-number…..I think this diagram is missing info. if anyone has some more info, that will be great.
Correction: before port id, the tie breaker is bridge id, which again we dont know. for sure info is missing.
In Q6 “spanning-tree mstp” options are incorrect so the options with “spanning-tree mst” are correct, therefore opt D is correct. Option E is not crrect also because priority 1 is not correct.
In regards to Q5 option D should be the correct one. Priority value of 0 is the lowest available.
in q5, why is priority root correct? there is no option to configure this way. should answer be A B
(config)#spanning-tree vlan 2 root ?
primary Configure this switch as primary root for this spanning tree
secondary Configure switch as secondary root
(config)#spanning-tree vlan 2 priority ?
bridge priority in increments of 4096
is question 1 correctly written? that Q1 does not make sense.
Q1 needs a diagram.
All questions that explain a diagram should have one instead of trying to explain it
Q3
correct answers are not A, D but B, D:
Switch(config)#spanning-tree portfast ?
bpdufilter Enable portfast bdpu filter on this switch
bpduguard Enable portfast bpdu guard on this switch
default Enable portfast by default on all access ports
@Sergio
A and D confirmed correct.
It ask configuration on interface level
Your posted command is on global level
Q.94. #show spanning-tree vlan 200 output
Which two statements about output are true?
It appears that the root bridge has it’s STP timers changed to 10,20,30. I know the defaults are 2,20,15 which is seen in output of the current switch.
I also know that the Forward Delay (30) is the combined time spent in the Listen (15) and Learn (15) state, so how could the correct answers be?
B. The time spent in the listen state will be 30 seconds
F. BPDU’s sent every 10 seconds
The correct answer is:
A: BPDU’s sent every 2 seconds (2 is default so true)
c: The time spent in the learning state will be 15 seconds (15 is default so true)
Q5 I think answer should be A,D
Why don’t make priority 0 rather than 4096?
there is no “priority root” command it must be “root primary” so this one must be wrong.
Making DSW2 priority highest 61440 is another solution, why this is not correct? I will take the exam 2 days later, and confused, please help anyone knows it. thanks.
Q6
Rack1SW1(config)#spanning-tree vlan 1 priority 1
% 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
https://supportforums.cisco.com/t5/lan-switching-and-routing/stp-priority-values/td-p/891247
Q1.
Fake question… better answer is surely E but if the default port priority is configured on DSW2 (as written on the question, default value is 128) also A could be correct…. ugly and needless question…
Q2. Correct answer is A (PVST+).
PVST+ is based on the IEEE 802.1D standard and Cisco proprietary extensions. It is the default spanning-tree mode used on all Ethernet port-based VLANs. The PVST+ runs on each VLAN on the switch up to the maximum supported, ensuring that each has a loop-free path through the network.
N.B.: some hw platform should have Rapid-PVST+ or MST as default STP (ex. Packet Tracer has PVST as default STP…), but in this Q&A surely A is the correct answer.
Q5. Fake question..
the diagram is missing, it is not specified if the bridge priority is default for all the switches and then asks for Vlan10 only while there is no answer for Vlan10 alone.
The answer is to trust and according to a certain logic it should be A and B.
Q10 I think we could assume the 2 10Gb links are not in ether channel, after all this is STP question.
If that’s the case one of the links is going to be in blocking state, DSW2 is the root, lowest priority 24576
cost to DSW2 from ASW1 is 4,
cost to DSW2 via DSW1 is 4.
Now this is STP tie breaker when it comes to selecting port roles
-Lowest Sender Bridge ID – this is us, DSW2 has lower ID from the one of DSW1, 28672<24576
-Lowest Sender Port Priority
-Lowest Sender Port Number
B is correct
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https://vceguide.com/which-are-connected-to-2-access-switches/
Q1 ‘S picture is here
@certprepare
Q5 definitely needs diagram, the question state is noncence
Q.5
Surely E is not correct the command does not exist.
B SURELY IS CORRECT BECAUSE they mentioned that is the lowest.
D is correct because if B is correct because of the word (4096 THE LOWEST), so zero is less than 4096 and is a valid value.
With extended system IDs, we add 10 (value of the Vlan) to the priority value to obtain the bridge ID. So D is correct.
Q.5
Surely E is not correct the command does not exist.
B SURELY IS CORRECT BECAUSE they mentioned that is the lowest.
D is correct because if B is correct because of the word (4096 THE LOWEST), so zero is less than 4096 and is a valid value.
With extended system IDs, we add 10 (value of the Vlan) to the priority value to obtain the bridge ID. So D is correct.
Q.10
B is correct. explanation is very simple, but we suppose that the CORE is not the root bridge, the logic says that no switch is mentioned in the CORE part (it is a smog). So the root bridge is DSW2 because of its lowest priority.
A.not correct because we have to pass over the root bridge
C.Is not correct because we attempted the DSW2, the root bridge, we should not go to the DSW1 to attempt the CORE.
So B or D can be correct.
The cost is the same, 1G cost is 4, 2*10G cost is 2*2.
So the first criteria to decide if a port on ASW1 will become root port is the cost.
Now we have the same cost 4=4.
The second criteria to decide is the senders bridge ID (BPDU senders bridge ID). The lowest is the winner.
So the bridge ID of DSW2 is lower because the priority is lower than the DSW1 priority.
This is why the root port on ASW1 is the port connected to the DSW2 bridge, the other port to the DSW1 will be in blocked state.
ALRIGHT GUYS, CHECK THIS OUT!!
Question 5
Diagram is: with 1 core, 2 DSW connected with EtherChannel, 2 ALSW, 2 PCs per ALSW. Per diagram current root for VLAN 10 is DSW2. Question is how to make DSW1 the primary root for VLAN 10. (Choose two)
A. DSW2: change bridge priority to 61440 (highest)
B. DSW1: change bridge priority to 4096 (lowest)
C. DSW1: change port priority but in global configuration mode
D. DSW1: change bridge priority but value is 0
E. DSW1: change bridge priority but command is priority root
Answer: B E
I AGREE ANSWER B COULD BE CORRECT IF THE BRIDGE PRIORITY ON DSW2 IT’S USING DEFAULT PRIORITY OR IS LOWER HIGHER THAN 4096…
ANSWER A IS CORRECT SINCE IS THE HIGHEST BRIDGE PRIORITY…
ANSWER E SEEMS AWFULLY INCORRECT SINCE THERE IS NO COMMAND “PRIORITY ROOT” THE ACTUAL COMMAND TO MAKE A SWITCH THE ROOT IS “ROOT PRIMARY” FOR ANY VLAN.
PLEASE CAN YOU EXPLAIN WHY THESE ANSWERS WERE SELECTED AS RIGHT???
For Question 11
Refer to the exhibit. Which statement describes the result if a switch that is running PVST+ is added to this network?
What is the correct answer?
Is it C?
In cisco document: https://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches/72836-rapidpvst-mig-config.html#rpvst
“When any RSTP port receives legacy 802.1D BPDU, it falls back to legacy STP and the inherent fast convergence benefits of 802.1w are lost when it interacts with legacy bridges”
Q14 answer should be B and C