02-17-2012 09:02 AM
Hi All,
Can anyone tell me:
I have 2 (a and b) redundancies. (a) goes through 1x 100baseT switch and (b) through 2x 1000baseT and 1x10baseT switches.
Will STP choose route a or b?
Effectively, I am asking, will STP choose the lowest average or does it do it a different way?
Thanks,
Joe
02-18-2012 05:13 AM
If you want to know how STP or RSTP works then google and wikipedia are your friends
Check out this page
http://en.wikipedia.org/wiki/Spanning_Tree_Protocol
Determine the least cost paths to the root bridge.
The computed spanning tree has the property that messages from any connected device to the root bridge traverse a least cost path, i.e., a path from the device to the root that has minimum cost among all paths from the device to the root. The cost of traversing a path is the sum of the costs of the segments on the path.
The table below shows the default cost of an interface for a given data rate.
Data rate | STP Cost (802.1D-1998) | RSTP Cost (802.1W-2001) |
---|---|---|
4 Mbit/s | 250 | 5,000,000 |
10 Mbit/s | 100 | 2,000,000 |
16 Mbit/s | 62 | 1,250,000 |
100 Mbit/s | 19 | 200,000 |
1 Gbit/s | 4 | 20,000 |
2 Gbit/s | 3 | 10,000 |
10 Gbit/s | 2 | 2,000 |
So in your case cost of a is: 19 and cost of b is: 2 x 4 + 100
Cheers,
Michel
02-18-2012 08:50 AM
...what Michel said (assuming the root bridge is reachable using BPDUs up both paths).
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