Rapid Spanning Tree

Question 1

RSTP

Answer 1

802.1W

Question 2

Port states

Answer 2

• Discarding
  The switch port is enabled but the port is not forwarding any traffic to ensure that a loop is not created.
• Learning
  The switch port modifies the MAC address table with any network traffic it receives. The switch still does not forward any other traffic.
• Forwarding
  The switch port forwards all network traffic and updates the MAC address table as expected.

Question 3

Port roles

Answer 3

• Root port
  A network port that connects to the root switch or an upstream switch.
• Designated port
  A network port that receives and forwards frames to other switches.
• Alternate port
  A network port that provides alternate connectivity toward the switch through a different switch.
• Backup port
  A network port that provides link redundancy toward the shared segment within the same collision domain.

Question 4

Port types

Answer 4

• Edge port
  A port at the edge of the network where hosts connect to the Layer 2 topology with on interface and cannot form a loop.
• Non-Edge port
  A port that has received a BPDU.
• Point-to-point port
  Any port that connect to another RSTP switch.

Question 5

RSTP handshake

Answer 5

RSTP tries to handshake with the device connected to the other end of the cable. If a handshake does not occur, the other device is assumed to be non-RSTP compatible and the port defaults to regular 802.1D.

Question 6

Building the RSTP topology

Answer 6

RSTP uses a synchronization process to add a switch to the RSTP topology without introducing a forwarding loop.

Question 7

RSTP topology process

Answer 7

1. As the first two switches connect to each other, they verify that they are connected with a point-to-point link by checking the full-duplex status.
2. They establish a handshake with each other to advertise a proposal (in configuration BPDU’s) that their interface should be the DP for that segment.
3. There can be only one DP per segment, so each switch identifies whether it is the superior or inferior switch, using the same logic as 802.1D.
4. The inferior switch recognizes that it is inferior and marks its local port as the RP. At the same time, it moves all non-edge ports to a discarding state.
5. The inferior switch sends an agreement (configuration BPDU) to the root bridge, which signifies to the root bridge that synchronization is occurring on that switch.

6 The inferior switch moves its RP to a forwarding state. The superior switch moves it DP to a forwarding state too.

7. The inferior switch repeats the process for any downstream switches connected to it.