In RSTP, a switch with four interfaces that connects to other RSTP switches, has been selected as the root bridge. What port type will be associated with the root bridge interfaces?

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In RSTP, a switch with four interfaces that connects to other RSTP switches, has been selected as the root bridge. What port type will be associated with the root bridge interfaces?

  • all will be alternate ports
  • all will be designated ports
  • some will be root ports and some of them will be alternate ports
  • all will be root ports

The correct answer is:

All will be designated ports.

Detailed Explanation (RSTP Port Roles and Root Bridge Behavior)

1. Overview of RSTP (Rapid Spanning Tree Protocol)

Rapid Spanning Tree Protocol (RSTP), standardized as IEEE 802.1w, is an evolution of the classic Spanning Tree Protocol (STP) designed to improve network convergence times and overall efficiency in preventing loops in Ethernet networks. RSTP achieves faster convergence by introducing new port roles and states while maintaining compatibility with traditional STP.

2. Understanding the Role of the Root Bridge

In an RSTP network, the root bridge is the central reference point for spanning tree calculations. It is elected based on the lowest bridge ID (BID), which consists of:

  • Bridge Priority (default: 32768)
  • MAC Address (used as a tie-breaker if priorities are the same)

Once elected, the root bridge has a special role in the topology:

  1. It is the most authoritative switch in the network.
  2. All other switches determine the shortest path to it.
  3. All its connected ports are always designated ports.

3. RSTP Port Types

In RSTP, a switch can classify its interfaces into the following port roles:

  1. Root Port (RP):

    • This is the port that has the best path (lowest cost) to the root bridge.
    • Only non-root bridges have root ports.

  2. Designated Port (DP):

    • A port that forwards traffic and is the best port on a given segment.
    • It is responsible for sending BPDUs (Bridge Protocol Data Units) downstream to non-root bridges.
    • Every segment (collision domain) must have one designated port.

  3. Alternate Port:

    • This port acts as a backup to the root port.
    • It is in a discarding state but can quickly transition to forwarding if needed.

  4. Backup Port:

    • This port acts as a backup to the designated port on the same switch.
    • It is also in a discarding state.

  5. Disabled Port:

    • A port that is administratively shut down.

4. Why All Ports on the Root Bridge Are Designated Ports

Since the root bridge is the ultimate reference point in the topology:

  • The root bridge has no root port because it is the final destination in the spanning tree structure.
  • No port on the root bridge can be an alternate port because alternate ports are only used as backups for root ports, and the root bridge has no root port.
  • Every port on the root bridge must be a designated port because it is responsible for forwarding BPDUs to downstream switches.

5. Scenario: A Root Bridge with Four Interfaces

Let’s assume a network where a switch has four interfaces and has been elected the root bridge. These interfaces connect to other switches in the network, forming the following topology:

       (Root Bridge)
      +-------------+
      |     SW1     |
      +-------------+
     /      |       \
   SW2     SW3      SW4

  • Since SW1 is the root bridge:

    • It has no root ports.
    • Each of its ports will forward BPDUs to the downstream switches.
    • Thus, all four interfaces will be designated ports.

  • Downstream switches (SW2, SW3, and SW4) select a root port:

    • Each non-root switch will choose its best path to SW1.
    • The lowest-cost path to SW1 will be the root port for each downstream switch.

6. What Happens If Another Switch is Elected as Root?

If another switch with a lower BID (lower priority or MAC address) becomes the root bridge, then the original root bridge (SW1) will:

  1. Select its best interface towards the new root bridge as its root port.
  2. Other ports may remain designated ports or become alternate ports depending on the topology.

However, as long as a switch is the root bridge, all its interfaces will always be designated ports.

7. Conclusion

In the scenario where a switch is the root bridge and has four interfaces connecting to other RSTP switches:

All of its ports will be designated ports because:

  • A root bridge does not have a root port.
  • The root bridge must forward BPDUs to all connected switches.
  • Every collision domain must have a designated port, and since the root bridge is the highest authority, it assumes the designated role on all interfaces.

Thus, the correct answer is:

“All will be designated ports.”