RSTP Questions

Question 1

Explanation

There are five port roles in RSTP:

* Root port – A forwarding port that is the closest to the root bridge in terms of path cost
* Designated port – A forwarding port for every LAN segment
* Alternate port – A best alternate path to the root bridge. This path is different than using the root port. The alternative port moves to the forwarding state if there is a failure on the designated port for the segment.
* Backup port – A backup/redundant path to a segment where another bridge port already connects. The backup port applies only when a single switch has two links to the same segment (collision domain). To have two links to the same collision domain, the switch must be attached to a hub.
* Disabled port – Not strictly part of STP, a network administrator can manually disable a port

There is no “blocking” port role like STP. The “alternative” and “backup” roles are only in RSTP.

Question 2

Explanation

RSTP is backward compatible with STP 802.1D. If a RSTP enabled port receives a (legacy) 802.1d BPDU, it will automatically configure itself to behave like a legacy port. It sends and receives 802.1d BPDUs only.

Question 3

Explanation

When a Switch (Bridge) discovers topology change, it generates a TCN (Topology Change Notification) BPDU (Bridge Protocol Data Unit) and sends the TCN BPDU on its root port. The upstream Switch (Bridge) responds back the sender with TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit) and TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit)
The upstream Switch (Bridge) (bridge which received the TCN BPDU) generates another TCN BPDU and sends out via its Root Port. The process continues until the Root Switch (Bridge) receives the TCN BPDU.
When the Root Switch (Bridge) is aware that there is a topology change in the network, it starts to send out its Configuration BPDUs with the topology change (TC) bit set. Configuration BPDUs are received by every Switch (Bridge) in the network and all bridges become aware of the network topology change.

The switch never generates a TCN when a port configured for PortFast goes up or down -> it means no TC will be created for PortFast (or Edge Port) -> D is correct.

(Reference: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094797.shtml)

Question 4

Explanation

RSTP is backward compatible with PVST so when two interfaces on two switches are connected, the port that is running RSTP will revert back to PVST (or PVST+) with standard 802.1d timers.

Note: Answer D is not correct as we cannot say which switch is old or new. Moreover, RSTP should be used on new switch while PVST+ should be used on old one.

Question 5

Question 6

Explanation

Switches transmit bridge protocol data units (BPDUs) throughout the STP instance. Each switch sends configuration BPDUs to communicate and compute the spanning tree topology -> Answer C is not correct.