Work out 802.1D/802.1w port and root path costs using the long or short cost table, including aggregated links.
Cost is inversely proportional to bandwidth, and the root path cost is the sum of the port costs a BPDU accumulates on its way from the root. The 16-bit short table saturates at 10 Gb — everything faster also costs 2 — which is why 802.1D-2004 introduced the 32-bit long table. Two links of very different speed can end up with the same cost under the short method, so the spanning tree happily blocks a 100 Gb link in favour of a 10 Gb one until you switch the whole domain to long costs.
RSTP Path Cost
Long-method cost = 20,000,000 ÷ link speed in Mbps, giving 2,000,000 for 10 Mb, 200,000 for 100 Mb, 20,000 for 1 Gb, 2,000 for 10 Gb and 200 for 100 Gb.
802.1D-1998 short cost table
Short method: 100 for 10 Mb, 19 for 100 Mb, 4 for 1 Gb, 2 for 10 Gb and above.
Long-method cost = 20,000,000 ÷ link speed in Mbps, giving 2,000,000 for 10 Mb, 200,000 for 100 Mb, 20,000 for 1 Gb, 2,000 for 10 Gb and 200 for 100 Gb. Cost is inversely proportional to bandwidth, and the root path cost is the sum of the port costs a BPDU accumulates on its way from the root. The 16-bit short table saturates at 10 Gb — everything faster also costs 2 — which is why 802.1D-2004 introduced the 32-bit long table.
Two links of very different speed can end up with the same cost under the short method, so the spanning tree happily blocks a 100 Gb link in favour of a 10 Gb one until you switch the whole domain to long costs.
This calculator takes 4 inputs: Link speed, Cost method, Members in the port channel, Hops to the root bridge. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
No. Cost is carried in the BPDU with no indication of which table produced it, so a mixed domain compares incompatible numbers and picks the wrong root port. Change every bridge in the region together.