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STP Questions

April 25th, 2015 in SWITCH 300-115 Go to comments

Question 1

Explanation

If we want to view the spanning-tree status of a specific VLAN, use the “spanning-tree vlan ” command. An example of the output of this command is shown below:

show_spanning-tree_vlan_30.jpg

Question 2

Explanation

SW3 needs to block one of its ports to SW2 to avoid a bridging loop between the two switches. But how does SW3 select its blocked port? Well, the answer is based on the BPDUs it receives from SW2. A BPDU is superior than another if it has:
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

These four parameters are examined in order. In this specific case, all the BPDUs sent by SW2 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). The lower value of port priority, the higher priority that port has. Therefore we must change the port-priority on F1/1 to a lower value than that of Fa1/0. Zero is the lowest value we can assign to a port so we can assign this value to SW2 F1/1 and configure a higher value on Fa1/0. This is the command to complete this task:

SW2(config)#interface f1/1
SW2(config-if)#spanning-tree vlan port-priority 0

Note: If we don’t change the port priority, SW3 will compare port index values, which are unique to each port on the switch, and because Fa1/0 is inferior to Fa1/1, SW3 will select Fa1/0 as its root port and block the other port.

Question 3

Explanation

After powered on, the switches start sending BPDUs to elect a root bridge. A BPDU is superior than another if it has:

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

From the output above, we learn that SW1 is the root bridge for VLAN 1 (from “this bridge is the root” line). SW1 has the “Bridge ID Priority” of 1 because SW1 has been configured with switch priority value of 0, which is also the lowest priority value (highest priority). This value is then added with the VLAN ID (VLAN 1 in this case) so the final value is 1.

Question 4

Explanation

After receiving BPDUs from upstream bridges, the switch add the STP cost of that port and choose the lowest value as its root port -> the STP cost of Fa0/21 is smallest so it is chosen as root port.

Question 5

Explanation

Portfast is often configured on switch ports that connect to hosts. Interfaces with Portfast enabled will go to forwarding state immediately without passing the listening and learning state. Therefore it can save about 30 to 45 seconds to transition through these states. To enable this feature, configure this command under interface mode:

Switch(config-if)#spanning-tree portfast

Question 6

Explanation

The “spanning-tree portfast bpdufilter default” command enables BPDU filtering on Portfast-enabled interfaces. This command prevents interfaces that are in a Portfast-operational state from sending BPDUs. If a BPDU is received on a Port Fast-enabled interface, the interface loses its Portfast-operational status, and BPDU filtering is disabled.

In conclusion, above command only affects ports that were configured with Portfast. It prevents these ports from sending BPDUs (notice that Portfast interfaces still send BPDUs) but the funny thing is that if it receives a BPDU, it will disable BPDU filtering and Portfast features.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_55_se/configuration/guide/3560_scg/swstpopt.html#wp1046220

Question 7

Explanation

Root guard does not allow the port to become a STP root port, so the port is always STP-designated. If a better BPDU arrives on this port, root guard does not take the BPDU into account and elect a new STP root. Instead, root guard puts the port into the root-inconsistent STP state which is equal to a listening state. No traffic is forwarded across this port.

Below is an example of where to configure Root Guard on the ports. Notice that Root Guard is always configure on designated ports.

Root_Guard_Location.jpg

To configure Root Guard use this command:

Switch(config-if)# spanning-tree guard root

Reference: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10588-74.html

Question 8

Explanation

Although RSTP was configured on all ports but only edge-ports allow to run RSTP. RSTP cannot work on a trunk port. If we try to configure RSTP on a trunk port (support Fa0/24) we will receive this message:

%Warning: portfast should only be enabled on ports connected to a single host. Connecting hubs, concentrators, switches, bridges, etc… to this interface when portfast is enabled, can cause temporary bridging loops. Use with CAUTION

%Portfast has been configured on FastEthernet0/24 but will only have effect when the interface is in a non-trunking mode.

Question 9

Explanation

UplinkFast is a Cisco specific feature that improves the convergence time of the Spanning-Tree Protocol (STP) in the event of the failure of an uplink. The UplinkFast feature is designed to run in a switched environment when the switch has at least one alternate/backup root port (port in blocking state), that is why Cisco recommends that UplinkFast be enabled only for switches with blocked ports, typically at the access-layer.

For example in the topology below:

STP_simple.jpg

Suppose S1 is the root bridge in the topology above. S3 is connected to S1 via two paths: one direct path and another goes through S2. Suppose the port directly connected to S1 is root port -> port connected to S2 will be in Blocking state. If the primary link goes down, the blocked port will need about 50 seconds to move from Blocking -> Listening -> Learning -> Forwarding to be used.

To shorten the downtime, a feature called Uplink Fast can be used. When the primary (root) link fails, another blocked link can be brought up immediately for use. When UplinkFast is enabled, it is enabled for the entire switch and all VLANs. It cannot be enabled for individual VLANs.

Reference: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/10575-51.html

Question 10

Explanation

Every non-root bridge needs to elect a root port. The election of root port is as follows:

1) Based on lowest cost path to the root bridge
2) Then based on lowest upstream Bridge ID (Bridge ID = Bridge Priority + MAC)
3) Then based on lowest upstream Port ID (Port ID = Port Priority + Port Index)

Therefore we can use STP cost and port-priority to select the root port.

Question 11

Explanation

Portfast is often configured on switch ports that connect to hosts. Interfaces with Portfast enabled will go to forwarding state immediately without passing the listening and learning state. Therefore it can save about 30 to 45 seconds to transition through these states. To enable this feature, configure this command under interface mode:

Switch(config-if)#spanning-tree portfast

Question 12

Explanation

BPDUFilter is designed to suppress the sending and receiving of BPDUs on an interface. There are two ways of configuring BPDUFilter: under global configuration mode or under interface mode but they have subtle difference.

If BPDUFilter is configured globally via this command:

Switch(config)#spanning-tree portfast bpdufilter default

BPDUFilter will be enabled on all PortFast-enabled interfaces and will suppress the interface from sending or receiving BPDUs. This is good if that port is connected to a host because we can enable PortFast on this port to save some start-up time while not allowing BPDU being sent out to that host. Hosts do not participate in STP and hence drop the received BPDUs. As a result, BPDU filtering prevents unnecessary BPDUs from being transmitted to host devices.

If BPDUFilter is configured under interface mode like this:

Switch(config-if)#spanning-tree bpdufilter enable

It will suppress the sending and receiving of BPDUs. This is the same as disabling spanning tree on the interface. This choice is risky and should only be used when you are sure that port only connects to host devices.

Reference: http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_55_se/configuration/guide/3560_scg/swstpopt.html

Question 13

Explanation

The “spanning-tree portfast bpdufilter default” command is configured under global configuration mode. To stop receiving unwanted BPDUs (for easier troubleshooting), he can issue the “spanning-tree portfast bpdufilter default” under global configuration mode. This will enable BPDUFilter on all PortFast-enabled interfaces and will suppress the interface from sending or receiving BPDUs. This is good if that port is connected to a host because we can enable PortFast on this port to save some start-up time while not allowing BPDU being sent out to that host. Hosts do not participate in STP and hence drop the received BPDUs. As a result, BPDU filtering prevents unnecessary BPDUs from being transmitted to host devices.

Question 14

Question 15

Explanation

If there are more than one connection between two switches, STP will automatically block one of them to prevent a loop. In particular, STP will block the link with higher priority value. Therefore if we want to force traffic to the secondary link we can lower the priority of the secondary link. For example:

Switch(config-if)#spanning-tree port-priority 48

Remember for switch (Layer 2 device), lower value is preferred over higher value. For router (Layer 3 device), higher value is preferred over lower value.

Question 16

Explanation

Spanning Tree Protocol elects a root bridge based on the Bridge IDs. The root bridge is the bridge with the lowest bridge ID. And Bridge ID = Bridge Priority + MAC Address. Therefore to prevent a switch from becoming the root bridge we can adjust STP priority to the maximum value.

Comments
  1. Need Helping
    May 1st, 2015

    Q6 Dilemma, read it carefully please:
    Understanding BPDU Filtering

    The BPDU filtering feature can be globally enabled on the switch or can be enabled per interface, but the feature operates with some differences.

    At the global level, you can enable BPDU filtering on Port Fast-enabled interfaces by using the spanning-tree portfast bpdufilter default global configuration command. This command prevents interfaces that are in a Port Fast-operational state from sending or receiving BPDUs. The interfaces still send a few BPDUs at link-up before the switch begins to filter outbound BPDUs. You should globally enable BPDU filtering on a switch so that hosts connected to these interfaces do not receive BPDUs. If a BPDU is received on a Port Fast-enabled interface, the interface loses its Port Fast-operational status, and BPDU filtering is disabled.

    At the interface level, you can enable BPDU filtering on any interface by using the spanning-tree bpdufilter enable interface configuration command without also enabling the Port Fast feature. This command prevents the interface from sending or receiving BPDUs.

  2. Syed Kashif Shahab
    May 12th, 2015

    The thing is if the interface which is port-fast enabled … should not send or receive BPDUs. if it does then the port-fast feature of that interface would shut down hence making the port to go through the normal phases before its in forwarding state….i.e. listening >> learning >> forwarding ———–states rather than immediate up-state/forwarding state which is the selling point of Port-fast.

  3. Libert
    May 25th, 2015

    Q8,

    I’m not sure that the correct answer is C. Trunk ports can be enabled for PortFast. It would be common setting for virtual server hosts.

    “Enable Port Fast on an access port connected to a single workstation or server. By specifying the trunk keyword, you can enable Port Fast on a trunk port.

    Note To enable Port Fast on trunk ports, you must use the spanning-tree portfast trunk interface configuration command. The spanning-tree portfast command will not work on trunk ports.”
    http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3560/software/release/12-2_55_se/configuration/guide/3560_scg/swstpopt.html#wp1046220

    Maybe it’s correct because it’s not specified weather additional keyword “trunk” was used in the configuration.
    Should “An administrator recently configured all ports for rapid transition using PortFast” imply that only spanning-tree portfast command was used. It’s quite misleading…

  4. KeyF
    June 5th, 2015

    Q8,
    I think C is correct, or partially corretc. Agree that “spanning-tree portfast trunk” interface command allows to enable portfast on trunk, but the sentence “An administrator recently configured all ports for rapid transition using PortFast” seems more oriented to have used the global config command “spanning-tree portfast default” that is quick to enable the PortFast feature on all ports…but the feature effectively works only for nontrunking ports.
    Finally, A B and C are wrong…so only C remains.

  5. jk
    June 10th, 2015

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  6. ABC
    June 13th, 2015

    Hii i failed in exam.To retake the exam is it compulsary to give it as early as possibke?

  7. SHIPEZ
    July 2nd, 2015

    Q8 .The answer is correct C.
    The problem is how the question(multiple choices were formulated).In stead of talking about Portfast they talk about RSTP Which is wider and not specifically meaning Portfast

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    August 4th, 2015

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  9. Ashish
    August 7th, 2015

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    August 30th, 2015

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    September 6th, 2015

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  12. Bla
    September 9th, 2015

    q3. I think the answer should be B: even if the priority is 0 there may be another switch with the same priority and with lower mac but ” this bridge is the root ” ensure that this is the root.

  13. $1L@
    October 6th, 2015

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  14. CCNP-soon?
    October 11th, 2015

    Bla, I agree..

    I also think it’s
    B. The switch priority for VLAN 1 and the macro specifies “This Bridge is the root”

  15. Alb
    October 16th, 2015

    Q6

    Information on cisco.com – link in the explanation above – says clearly “This command prevents interfaces that are in a Port Fast-operational state from sending or receiving BPDUs”.

    The explanation above skipped the receiving part of that statement and moved on to the consequence of a received BPDU and that’s a bit misleading, I think. If a port can’t send or receive BPDUs, it means the command effectively disables STP on all such ports as they can’t communicate with STP and can’t move through STP states.

  16. TK
    October 20th, 2015

    Q6. The command effectively disables STP on all portfast enabled ports. So no spanning-tree process. Answer should be B.

  17. TrogDOR
    November 3rd, 2015

    @Bla,

    For Question 3, I tested this out on a Catalyst 2950. The answer is D. The key item to look for in the output is:

    Bridge ID Priority 1 (priority 0 sys-id-ext 1)

    See, the priority is “0” but it adds the “sys-id-ext” to the Bridge priority to get a value of 1. You can see this same thing happen if you set the priority to a different value, like say 4096.

    SW03(config)#spanning-tree vlan 1 priority 4096

    VLAN0001
    Spanning tree enabled protocol rstp
    Root ID Priority 4097
    Address 000d.2955.a300
    This bridge is the root
    Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

    Bridge ID Priority 4097 (priority 4096 sys-id-ext 1)

  18. Study for Switch
    November 7th, 2015

    For Q6, answer would be A. Please read below excerpt from CCNP Foundation guide

    BPDU Filter behaves differently if applied globally or on a per-port basis.

    When enabled globally, BPDU Filter has these attributes:

    It affects all operational PortFast ports on switches that do not have BPDU Filter configured on
    the individual ports.
    If BPDUs are detected, the port loses its PortFast status, BPDU Filter is disabled, and the STP
    sends and receives BPDUs on the port as it would with any other STP port on the switch.
    Upon startup, the port transmits ten BPDUs. If this port receives any BPDUs during that time,
    PortFast and PortFast BPDU Filter are disabled.

    When enabled on an individual port, BPDU Filter has these attributes:
    It ignores all BPDUs received.
    It sends no BPDUs.

  19. lik
    November 22nd, 2015

    Question 3, so ok… we have three switches. Made a loop, and set them manually with priority 0. Everybody root? No.
    Look at the ref:
    http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html
    The switch with the lowest bridge ID in the network wins this election process.
    Remember that one root switch is identified per-VLAN. After the root switch identification, the switches adhere to these rules:
    STP Rule 1—All ports of the root switch must be in forwarding mode.

    So correct is A. The bridge priority is 1 and all ports are forwarding.

  20. lik
    November 22nd, 2015

    @TrogDOR – BTW “sys-id-ext 1” is vlan number, read the manuals.

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    November 30th, 2015

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    December 28th, 2015

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  23. niko
    January 5th, 2016

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  26. Stikazzi
    February 20th, 2016

    Q3:
    in case there are multiple switches with priority 0 the lowest MAC representing the STP instance defines the root bridge. So priority 0 in not enough. You should alway look at the sentence “this bridgew is root” to make sure the bridge is root.

  27. RP
    March 5th, 2016

    Q6.

    The answer is A. Although BPDUFilter is initially mentioned, the actual question does not refer to it. It simply asks what happens to a port enabled for portfast when it receives a BPDU. A bit sneaky mind.

  28. Speedy 6 Nipples
    March 14th, 2016

    Q6:

    I think the answer maybe D.
    Please see following output from my lab:

    *Mar 4 03:01:56.604: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to up
    *Mar 4 03:01:58.591: %LINK-3-UPDOWN: Interface GigabitEthernet1/0/10, changed state to up
    *Mar 4 03:01:59.593: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet1/0/10, changed state to up
    *Mar 4 03:02:38.642: %SPANTREE-7-RECV_1Q_NON_TRUNK: Received 802.1Q BPDU on non trunk GigabitEthernet1/0/10 VLAN1.
    *Mar 4 03:02:38.642: %SPANTREE-7-BLOCK_PORT_TYPE: Blocking GigabitEthernet1/0/10 on VLAN0001. Inconsistent port type.
    *Mar 4 03:02:38.647: %LINEPROTO-5-UPDOWN: Line protocol on Interface Vlan1, changed state to down
    *Mar 4 03:02:53.647: %SPANTREE-2-UNBLOCK_CONSIST_PORT: Unblocking GigabitEthernet1/0/10 on VLAN0001. Port consistency restored.

    Also,

    Look at the following output from the spanning-tree interface:

    ASW1#sh spanning-tree interface g1/0/10

    Vlan Role Sts Cost Prio.Nbr Type
    ——————- —- — ——— ——– ——————————–
    VLAN0001 Desg BKN*4 128.10 P2p *TYPE_Inc

  29. SWITCH
    March 15th, 2016

    @Speedy- The answer is indeed correct, see below.

    S1(config-if-range)#do sh run | inc bpduf
    spanning-tree portfast bpdufilter default
    S1(config-if-range)#do sh run | sec interface FastEthernet0/2
    interface FastEthernet0/2
    spanning-tree portfast
    !
    S1(config)#int fa0/1
    S1(config-if)#shut
    S1(config-if)#
    *Mar 1 01:08:58.796: STP SW: 1 virtual port link down single: Fa0/1.1
    *Mar 1 01:08:58.796: STP SW: Fa0/2 new listening req for 1 vlans
    *Mar 1 01:08:58.796: STP SW: deleted vlan 1, ext id 5E6B5A8
    *Mar 1 01:08:58.796: STP SW: 1 virtual port being removed single: Fa0/1.1
    *Mar 1 01:09:00.801: %LINK-5-CHANGED: Interface FastEthernet0/1, changed state to administratively down
    *Mar 1 01:09:01.807: %LINEPROTO-5-UPDOWN: Line protocol on Interface FastEthernet0/1, changed state to down
    *Mar 1 01:09:13.803: STP SW: Fa0/2 new learning req for 1 vlans
    *Mar 1 01:09:28.810: STP SW: Fa0/2 new forwarding req for 1 vlans

  30. SWITCH
    March 15th, 2016

    @Speedy- To keep this short, I suspect that you have 2 switches: 1 with a port type as trunk, and 1 with a port type of access. The error clearly references trunking ie. the dot1q message. Try it again, and make sure that the ports are set to access or just default, and use the ‘debug spanning-tree switch state’ and ‘debug spanning-tree switch pm’ commands to verify that the output I pasted above is normal. HTH.

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    March 27th, 2016

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  32. Anonymous
    May 13th, 2016

    On my test, there was a question that stated that you’re an engineer, who’s been instructed to configure a switch so it won’t participate in spanning-tree elections. Enabling which (one) of the following will meet that objective?

    1) BPDU Guard
    2) Portfast
    3) BPDU filter
    4) Root Guard

    I ruled out the first 2, but am not sure beyond that. By ensuring that the switch doesn’t become root, Root Guard effectively meets the objective, right? But it would shut down the port, if it caught a whiff of something suspicious, so maybe that wouldn’t be the best option. BPDU filter would effectively shuts down spanning-tree, stopping participation in the election process, but leaves the switch vulnerable to storms. What should be the correct answer?

    Thanks

  33. Anonymous
    May 16th, 2016

    @Anonymous, the best answer would be 3: BPDU filter will disable spanning tree on that switch. It will not send any BPDU’s, therefore the switch is not competing in the spanning tree election process (no BPDU, no bridge ID know, no way it can become root)

  34. student
    June 5th, 2016

    Q3 , the most logical answer and that it is used is B, just look at “This Bridge is the root” and directly find who is the root, but the problem in this answer is the part “the macro specifies”.

    The only macro i know is “#spanning-tree vlan 1 root primary”, that do the calculations to choose a priority depending on the network at a minimum of 4096 priority, the macro doesn’t go any lower, so technically this is not the answer neither.
    So just go with bingo or trust certprepare, this is another tricky-wrong Cisco question.

  35. jo
    June 22nd, 2016

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    July 15th, 2016

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  37. thanhle
    July 20th, 2016

    Q6:

    I think A is incorrect

  38. Juan RC
    August 29th, 2016

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  41. Sam
    December 9th, 2016

    Q6.
    At the global level (this is the key word), you can enable BPDU filtering on Port Fast-enabled interfaces by using the spanning-tree portfast bpdufilter default global configuration command.

    This command prevents interfaces that are in a Port Fast-operational state from sending or receiving BPDUs. The interfaces still send a few BPDUs at link-up before the switch begins to filter outbound BPDUs. You should globally enable BPDU filtering on a switch so that hosts connected to these interfaces do not receive BPDUs.
    “If a BPDU is received on a Port Fast-enabled interface, the interface loses its Port Fast-operational status, and BPDU filtering is disabled.” which means normal STP operation would start…..

  42. Sam
    December 9th, 2016

    Q16.
    I am so sceptical about the answer cuz what if u change all the switches’ priority to the max? if our switch has the lowest mac by any chance it would become the root, although it has the highest Possible priority value!!!!

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    December 12th, 2016

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    January 17th, 2017

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