03-06-2007 10:48 PM - edited 03-03-2019 04:03 PM
Please find attached diagram on this.
Hi, can you clarify this?
Imagine my "root" bridge has been identified. Please find attached diagram. Then I have two switches "Switch2" and "Switch3". I understand that at that point a root port for each switch should be determined determined based on the lowest cost to get to root bridge. If I look at "Switch3", what happens if I have two paths with same cost to get to "Root"?
Which path the STP algorithm would take at that point thus making that the "Root Port" and the path with the highest cost the "blocked" port??
Solved! Go to Solution.
03-07-2007 03:39 AM
Friend,
STP uses the following order sequence to decide on its parameters
Step 1. Lowest Root BID
Step 2. Lowest Path Cost to Root Bridge
Step 3. Lowest Sender BID
Step 4. Lowest Port ID
In your case the step 1 & 2 would yield the same result and hence the deciding factor would be the 3 point which is the lowest sender bridge ID (which could be either be the root or the swith2 in your case)
If you had both the links terminated on the same root switch directly, then the deciding factor would have been the port ID and the lowest port would be in forwarding state by default.
HTH, rate if it does
Narayan
03-07-2007 03:39 AM
Friend,
STP uses the following order sequence to decide on its parameters
Step 1. Lowest Root BID
Step 2. Lowest Path Cost to Root Bridge
Step 3. Lowest Sender BID
Step 4. Lowest Port ID
In your case the step 1 & 2 would yield the same result and hence the deciding factor would be the 3 point which is the lowest sender bridge ID (which could be either be the root or the swith2 in your case)
If you had both the links terminated on the same root switch directly, then the deciding factor would have been the port ID and the lowest port would be in forwarding state by default.
HTH, rate if it does
Narayan
03-07-2007 07:24 AM
Spanning Tree Algorithm is used to calculate a loop-free path.
All switch ports are in blocking mode to begin with. It takes approx 30 seconds until packets can be forwarded.
Step 1 : Elect Root Bridge - Lowest bridge priority, if there is a tie then switch with lowest bridge ID
Step 2 : Elect Root Ports - Locate redundant paths to root bridge; block all but on root. Root Path Cost is cumulative cost of path to root bridge. Ports directly connected to Root Bridge will be root ports, otherwise lowest root path cost used.
Step 3 : Elect Designated Ports - Single port that sends and receives traffic from a switch to and from Root Bridge - Lowest cost path to Root Bridge.
To learn more
03-07-2007 07:40 AM
If there is a tie with the cost-path the second criteria stp will look at is the lowest port number if the switch is using multiple interfaces.
03-07-2007 08:00 AM
Ah, that's right. I missed that point.
Thanks.
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