?– Port 1 on switch Y is the nondesignated port on the segment and is in the blocking state.
?– All designated and root ports are in the forwarding state.
Example: Spanning-Tree Path Cost
The spanning-tree path cost is an accumulated total path cost based on the bandwidth of all
the links in the path. In the ?¬?gure, some of the path costs speci?¬?ed in the 802.1D
speci?¬?cation are shown. The 802.1D speci?¬?cation has been revised; in the older
speci?¬?cation, the cost was calculated based on a bandwidth of 1000 Mbps. The calculation
of the new speci?¬?cation uses a nonlinear scale to accommodate higher-speed interfaces.
Table 2-11 describes the spanning-tree path cost calculations based on bandwidth of a link.
When there is a topology change because of a bridge or link failure, spanning tree adjusts
the network topology to ensure connectivity by placing blocked ports in the forwarding
state.
NOTE Most Cisco Catalyst switches incorporate the revised cost calculations. A key
point to remember about STP cost is that lower costs are better.
Table 2-11 Spanning-Tree Path Costs
Link Speed Cost (Revised IEEE Specification) Cost (Previous IEEE Specification)
10 Gbps 2 1
1 Gbps 4 1
100 Mbps 19 10
10 Mbps 100 100
56 Chapter 2: Medium-Sized Switched Network Construction
Example: Spanning-Tree Recalculation
In Figure 2-27, if switch Z (the root bridge) fails and does not send a BPDU to switch Y
within the max_age time (default is 20 seconds, which equals 10 missed BPDUs), switch
Y detects the missing BPDU from the root bridge.
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