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Difficulty: Very hardSpanning Tree Protocol Operations

An enterprise network topology consists of four switches (Switch-1, Switch-2, Switch-3, and Switch-4) running 802.1D Spanning Tree Protocol (STP) using default revised 802.1D-1998 / 802.1w path cost values (10 Gbps = 2, 1 Gbps = 4, 100 Mbps = 19). The switches are configured with the following parameters:

* Switch-1: Priority 4096, MAC address 0011.2233.4455
* Switch-2: Priority 8192, MAC address 0011.2233.4422
* Switch-3: Priority 32768, MAC address 0011.2233.4411
* Switch-4: Priority 32768, MAC address 0011.2233.4433

The physical interconnections and link speeds between the switches are:
* Switch-1 to Switch-2: 10 Gbps
* Switch-1 to Switch-3: 1 Gbps
* Switch-2 to Switch-4: 10 Gbps
* Switch-3 to Switch-4: 1 Gbps
* Switch-2 to Switch-3: 100 Mbps

Which THREE of the following statements accurately describe the converged STP topology, root path costs, and port roles? (Select THREE)

  1. Switch-4 selects its connection to Switch-2 as its Root Port because it yields a lower Root Path Cost of 4 compared to a cost of 8 via Switch-3.Answer
  2. B
    Switch-3 is elected as the Root Bridge because it possesses the lowest MAC address (0011.2233.4411) among all switches in the topology.
  3. On the link segment connecting Switch-3 and Switch-4, Switch-3 hosts the Designated Port while Switch-4 blocks its port because Switch-3 has a lower MAC address.Answer
  4. D
    Switch-4 selects its port connected to Switch-3 as its Root Port because Switch-3 has a lower Bridge ID than Switch-2.
  5. Switch-3 places its port connected to Switch-2 into an Alternate/Blocking state because Switch-2 advertises a lower Root Path Cost (cost of 2) on that segment.Answer

Answer

The three correct statements are: Switch-4 selects its connection to Switch-2 as its Root Port due to a lower Root Path Cost of 4; Switch-3 hosts the Designated Port on the segment to Switch-4 based on a lower MAC address tie-breaker; and Switch-3 blocks its port connected to Switch-2 because Switch-2 has a lower Root Path Cost on that segment.
The Root Bridge is Switch-1 (lowest priority of 4096). Switch-4 reaches Switch-1 with a path cost of 4 via Switch-2 (10G + 10G) versus a path cost of 8 via Switch-3 (1G + 1G), making the port connected to Switch-2 its Root Port. On the segment between Switch-3 and Switch-4, both switches have an equal root path cost of 4; Switch-3 wins the Designated Port role because its MAC address is lower than Switch-4's MAC address. On the segment between Switch-2 and Switch-3, Switch-2 advertises a lower root path cost (2) than Switch-3 (4), making Switch-2's port Designated and causing Switch-3 to block its port on that link.

Step-by-Step Solution

1
Determine the Root Bridge
Switch-1 is elected Root Bridge
Bridge ID consists of Priority + MAC address. Switch-1 has the lowest numerical priority (4096).
2
Calculate Root Path Costs for each switch
Switch-2 cost = 2 (10G direct link); Switch-3 cost = 4 (1G direct link); Switch-4 cost via Switch-2 = 2 + 2 = 4, via Switch-3 = 4 + 4 = 8
Path cost accumulates cost of egress interfaces toward the Root Bridge.
3
Identify Root Ports for Switch-2, Switch-3, and Switch-4
Switch-2 Root Port -> Switch-1; Switch-3 Root Port -> Switch-1; Switch-4 Root Port -> Switch-2
Each non-root switch chooses the single interface with the lowest total path cost to the Root Bridge.
4
Determine Designated and Blocking Ports on remaining segments
Segment Switch-2 <-> Switch-3: Switch-2 is Designated (cost 2 < 4), Switch-3 blocks. Segment Switch-3 <-> Switch-4: Path costs tied (4 = 4), Switch-3 has lower MAC (0011.2233.4411 < 0011.2233.4433), so Switch-3 is Designated and Switch-4 blocks.
Segment with lowest advertising root path cost wins Designated status; tie-breakers use lower Bridge ID (Priority then MAC).

Key Concept

Spanning Tree Protocol Convergence Mechanics and Path Cost Calculation
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Topics

Spanning Tree Protocol Operations

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