Stp Tiebreakers - Cisco Catalyst 2000 Configuration Handbook

STP Tiebreakers

When any STP decision has identical conditions or a tie, the final decision is based on
this sequence of conditions:
The lowest BID
1.
The lowest root path cost
2.
The lowest sender BID
3.
The lowest port ID
4.
Path Costs
By default, switch ports have the path costs defined in Table 7-1.
Table 7-1 Switch Port Path Costs
Port Speed
4 mbps
10 mbps
16 mbps
45 mbps
100 mbps
155 mbps
622 mbps
1 gbps
10 gbps
100 gbps
1000 gbps (1 tbps)
10 tbps
By default, Catalyst switches in RPVST+ mode use the "short mode" or 16-bit path or
port cost values. When the port speeds in a network are less than 1 gbps, the short mode
scale is sufficient. If you have any ports that are 10 gbps or greater, however, set all
switches in the network to use the "long mode" or 32-bit path cost scale. This ensures
that root path cost calculations are consistent on all switches.
Default Port Cost "Short
Mode"
250
100
62
39
19
14
6
4
2
—
—
—
Chapter 7: Spanning Tree Protocol (STP) 113
Default Port Cost "Long
Mode"
—
2,000,000
—
—
200,000
—
—
20,000
2000
200
20
2