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PortFast Questions

November 20th, 2018 in SWITCH 300-115 Go to comments

Question 1


If a BPDU is received on a port where BPDU guard is configured, that port is put into errdisable state (nearly the same as shutdown state) immediately. BPDU Guard is often configured on a PortFast-enabled port to prevent a switch from connecting to. When that switch begins to send BPDU to a BPDU guard port, it will be blocked immediately.

Question 2

Question 3


Answer A is not correct as PortFast only skips listening and learning state, not “skip all spanning-tree states” (which includes forwarding state).

Question 4


The Portfast feature can be configured on both access or trunk port. This feature instructs the port to skip listening and learning state and move to forwarding state immediately.

“Skip 802.1D timers” here means “skip the listening and learning timers” so in fact these two answers have same meanings.

Question 5

Question 6

Question 7


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.

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

The first and second underlined sentences are very contradictory. The first one says it prevents an interface from receiving BPDUs while the second one says when it receives BPDUs, BPDU filtering is disabled!

The fact is this command will prevent an interface from sending BPDUs only. But if it receives BPDUs, it will lose its Port Fast feature and return to a normal switching port (with STP enabled).

Note: There is another important thing we want to mention here: there are two ways to configure BPDU filtering feature, one in global configuration mode and one under a specific interface:

Configuring BPDU filter globally:
Switch(config)#spanning-tree portfast bpdufilter default
Configure BPDU Filter on the interface:
Switch(config-if)#spanning-tree bpdufilter enable (this overrides the global bpdufilter command above)

But the effect of these two commands are different and you should remember:
+ When BPDU filtering is enabled globally; and if BPDUs are seen, the port loses its PortFast status, BPDU filtering is disabled, the port returns to normal state
+ When BPDU filtering is enabled on a specific port, it prevents this port from sending or receiving BPDUs (so if BPDUs are seen, they will be dropped)

Question 8

  1. trueteller
    December 17th, 2018


    For sure C is correct however I am not sure about D answer:


    “Port Fast immediately brings an interface configured as an access or trunk port to the forwarding state from a blocking state, bypassing the listening and learning states. You can use Port Fast on interfaces connected to a single workstation or server, as shown in Figure 18-1, to allow those devices to immediately connect to the network, rather than waiting for the spanning tree to converge.”
    Learning state is one of the states that it bypass.
    How about B answer instead of D? D is only partially true.

  2. kukocerothe
    February 27th, 2019


    The correct answer is D, “BPDU Filtering”, because this spanning-tree feature prevents a port from sending BPDUs.

  3. Zane Dworkmine
    February 27th, 2019

    Question 4.

    The answer for the question number 4 is A and B, this because Portfast feature it skip the timers that are configured in STP, it bypasses the states listening and learning.

    Option C, is false, you can configure a trunk port with portfast, with the command “spanning-tree portfast trunk”

    Option D is false, Portfas does not have a feature that puts a port into errdisable state.

    Option E is incorrect, because what Portfast do is bypassing the listening and learning states, in order to put a port into forwarding state.

  4. Dr. Janite Cownser
    February 27th, 2019

    Question No. 5.

    C and D are the correct answer.

    A is incorrect, Portfast does not skip all spanning-tree states, which are blocking, listening, learning and forwarding. This answer is inplying that the forwarding state is skipped too.

    B is incorrect, Portfast does not put a port into an error disabled state, it just put a port into a forwarding state.

    C is incorrect, Portfast skips the learning and listening states, not just the learning state.

    Kindly regards from your friend Dr. Janite Cownser.

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