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STP Questions 3

November 17th, 2017 Go to comments

Question 1

Explanation

PortFast causes a switch or trunk port to enter the spanning tree forwarding state immediately, bypassing the listening and learning states.

Question 2

Question 3

Question 4

Explanation

Answer A is not correct as we can choose which switch to become root bridge by configuring bridge priority. The switch with lowest bridge priority (value) would become the root bridge.

For answer B, this paragraph from Cisco confirms it is the correct answer:

“When you implement a root bridge in a switching network, you usually refer to the root bridge as the root switch. Each VLAN must have its own root bridge because each VLAN is a separate broadcast domain. The roots for the different VLANs can all reside in a single switch or in various switches.”

Reference: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/5234-5.html

The meaning of answer C is not clear but maybe it means “every VLAN must use the same root bridge” which is not correct as Sw1 can be the root bridge for VLANs 1, 3, 5 but Sw2 can be the root bridge for VLAN 2, 4, 6…

From the quote above we can say answer D is not correct.

Question 5

Explanation

PortFast BPDU guard prevents loops by moving a nontrunking port into an errdisable state when a BPDU is received on that port. When you enable BPDU guard on the switch, spanning tree shuts down PortFast-configured interfaces that receive BPDUs instead of putting them into the spanning tree blocking state. In a valid configuration, PortFast-configured interfaces do not receive BPDUs. If a PortFast-configured interface receives a BPDU, an invalid configuration exists. BPDU guard provides a secure response to invalid configurations because the administrator must manually put the interface back in service.

Reference: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst4000/8-2glx/configuration/guide/stp_enha.html

Question 6

Question 7

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