Home > STP Questions 2

STP Questions 2

November 21st, 2019 Go to comments

Question 1

Explanation

With the use of dot1q (802.1Q) encapsulation, control frames (include STP, CDP, VTP…) are tagged with VLAN 1 if the switch native VLAN is changed -> that means STP BPDU is tagged with VLAN 1 when the native VLAN is set to VLAN 99 -> Answer A is correct.

Reference: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-6500-series-switches

Control traffic continues to be accepted as untagged on the native VLAN on a trunked port, even when the “vlan dot1q tag native” command is enabled (the native VLAN is tagged with this command) -> Answer D is correct.

Reference: http://www.cisco.com/c/m/en_us/techdoc/dc/reference/cli/nxos/commands/l2/vlan-dot1q-tag-native.html

Note: Control traffic always sent on VLAN 1 when trunking.

Question 2

Question 3

Question 4

Explanation

The default spanning-tree mode in Cisco switch is PVST+. This spanning-tree mode is based on the IEEE 802.1D standard and Cisco proprietary extensions. PVST+ is same as standard IEEE 802.1D but it runs on each VLAN. In the output we see the line “Spanning tree enabled protocol ieee” under “VLAN 20” so it can say the switch is running in PVST+ mode.

Question 5

Explanation

If PortFast is configured on a port and it receives a BPDU the port will disable the PortFast feature (and the BPDUs are processed normally).

The “spanning-tree portfast trunk” command makes a trunk port become Forwarding immediately after coming up. Therefore this command is suitable only on trunk ports leading to Layer3 devices, e.g. routers or servers

Question 6

Comments
  1. polleke
    January 10th, 2020

    Q1: I think there is a difference in STP BPDUs and PVST+ BPDUs……

    – STP BPDU are always sent untagged over the native VLAN (per default VLAN1) of a trunk.

    – PVST+ BPDU are always sent over the native VLAN1. If VLAN1 is not the native VLAN, it is tagged with VLAN 1, even if VLAN1 has been cleared from the trunks. When you do not allow VLAN1 on the trunk “switchport trunk allowed vlan x”, all data traffic for VLAN1 on this port is blocked, but the control traffic continues to move on the port.

  2. Q1 wording is wrong
    February 9th, 2020

    If native vlan on an INTERFACE is 99 then:
    VLAN 99 STP BPDU are sent Untagged on the INTERFACE
    VLAN 1 STP BPDU are sent Tagged on the INTERFACE

  3. suntzu
    February 22nd, 2020

    Q1

    If the Native VLAN on an IEEE 802.1Q trunk is not VLAN 1:(Here native vlan in the question is 99, /= vlan 1)

    VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.
    VLAN 1 STP BPDUs are also sent to the IEEE STP MAC address on the Native VLAN of the IEEE 802.1Q trunk, untagged.
    Non-VLAN 1 STP BPDUs are sent to the PVST+ MAC address, tagged with a corresponding IEEE 802.1Q VLAN tag.
    Note: Native VLAN STP BPDUs are sent untagged.”

  4. suntzu
    February 22nd, 2020

    Below from the cisco community posted by a member:

    In short, if the native VLAN is VLAN1 then:

    VLAN1 standard STP BPDU is sent untagged
    VLAN1 PVST+ BPDU is sent untagged
    Other VLAN’s PVST+ BPDUs are sent tagged with their appropriate VLAN
    If the native VLAN is different from VLAN1 then:

    VLAN1 standard STP BPDU is sent untagged
    VLAN1 PVST+ BPDU is sent tagged with VLAN1
    Other VLAN’s PVST+ BPDUs are sent tagged accordingly (the one for the native VLAN will be untagged)

  1. No trackbacks yet.