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Difficulty: Very hardRapid PVST+ Spanning Tree Protocol Operations and Roles

Four Cisco switches (SW1, SW2, SW3, and SW4) are interconnected in a enterprise mesh topology running Rapid PVST+ for VLAN 10. The bridge priorities (base priority values) and MAC addresses for VLAN 10 are configured as follows:

SwitchBase Bridge PriorityMAC Address
SW140960001.AAAA.AAAA
SW2286720002.BBBB.BBBB
SW3327680003.CCCC.CCCC
SW4327680004.DDDD.DDDD

The inter-switch physical links and operational speeds are configured as follows:
- SW1 Gi0/1 <--> SW2 Gi0/1 (1 Gbps)
- SW1 Gi0/2 <--> SW3 Gi0/1 (1 Gbps)
- SW2 Gi0/2 <--> SW4 Gi0/1 (1 Gbps)
- SW2 Gi0/3 <--> SW3 Gi0/2 (100 Mbps)
- SW3 Gi0/3 <--> SW4 Gi0/2 (1 Gbps)

All switch ports use default Rapid PVST+ 802.1w short path costs. Which spanning-tree port role and operational state will interface Gi0/2 on SW4 assume for VLAN 10?

  1. Alternate port in the Discarding stateAnswer
  2. B
    Root port in the Forwarding state
  3. C
    Designated port in the Forwarding state
  4. D
    Backup port in the Discarding state

Answer

Alternate port in the Discarding state
SW1 is the Root Bridge because it has the lowest Bridge ID (4106:0001.AAAA.AAAA). Both SW2 and SW3 elect their direct 1 Gbps connections to SW1 as Root Ports, giving both switches a Root Path Cost of 4. SW4 has two 1 Gbps paths to the root: via SW2 (cost 4 + 4 = 8) and via SW3 (cost 4 + 4 = 8). Because the path costs are tied at 8, SW4 breaks the tie using the lowest upstream Designated Bridge ID. SW2's Bridge ID (28682:0002.BBBB.BBBB) is lower than SW3's Bridge ID (32778:0003.CCCC.CCCC), so SW4 selects Gi0/1 as its Root Port. On the link between SW3 Gi0/3 and SW4 Gi0/2, SW3 advertises a lower Root Path Cost (4) than SW4 (8). Thus, SW3 Gi0/3 becomes the Designated Port for the segment, and SW4 Gi0/2 transitions to an Alternate port role in the Discarding state.

Step-by-Step Solution

1
Determine the Root Bridge for VLAN 10.
SW1 total priority = 4096+10=41064096 + 10 = 4106. Since 41064106 is the lowest bridge priority in the network, SW1 becomes the Root Bridge.
Rapid PVST+ adds the VLAN ID (1010) to the base priority to form the total Bridge Priority.
2
Calculate Root Path Costs for SW2 and SW3.
SW2 Root Port = Gi0/1 (Cost 44). SW3 Root Port = Gi0/1 (Cost 44).
1 Gbps link cost under default short path cost mode is 44.
3
Evaluate SW4 Root Port election between Gi0/1 and Gi0/2.
Path via Gi0/1 (SW2): Cumulative cost =4+4=8= 4 + 4 = 8. Designated Bridge ID =28682:0002.BBBB.BBBB= 28682:0002.BBBB.BBBB.
Path via Gi0/2 (SW3): Cumulative cost =4+4=8= 4 + 4 = 8. Designated Bridge ID =32778:0003.CCCC.CCCC= 32778:0003.CCCC.CCCC.
SW4 chooses Gi0/1 as its Root Port (Forwarding state) because SW2 has a lower Bridge ID than SW3.
When cumulative path costs are tied, the upstream Designated Bridge ID breaks the tie.
4
Determine Designated Port and Alternate Port roles on the link between SW3 Gi0/3 and SW4 Gi0/2.
SW3 advertises Root Path Cost 44 on Gi0/3. SW4 advertises Root Path Cost 88 on Gi0/2. SW3 wins Designated Port role (Forwarding state). SW4 Gi0/2 becomes an Alternate Port in the Discarding state.
The bridge advertising the lower cumulative Root Path Cost on a segment becomes the Designated Bridge for that link.

Key Concept

Rapid PVST+ Port Role Election and State Transitions
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Topics

Rapid PVST+ Spanning Tree Protocol Operations and Roles

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