Illustration Of Stp - IBM BladeCenter Installation And User Manual

Max. Age
Forward Delay
Port Priority
Port Cost

Illustration of STP

A simple illustration of three bridges (or three switches) connected in a loop is
depicted in this section. In this example, you can anticipate some major network
problems if the STP assistance is not applied. If bridge A broadcasts a packet to
bridge B, bridge B will broadcast it to bridge C, and bridge C will broadcast it to
back to bridge A, and so on. The broadcast packet will be passed indefinitely in a
loop, potentially causing a network failure.
STP can be applied as shown in the following illustration. In this example, STP
breaks the loop by blocking the connection between bridges B and C. The decision
to block a particular connection is based on the STP calculation of the most current
bridge and port settings. If bridge A broadcasts a packet to bridge C, bridge C will
drop the packet at port 2, and the broadcast will end there.
Setting up STP using values other than the defaults can be complex. Therefore,
keep the default factory settings, and STP will automatically assign root bridges,
ports, and block loop connections. However, influencing STP to choose a particular
switch as the root bridge using the Priority setting, or influencing STP to choose a
particular port to block using the Port Priority and Port Cost settings is relatively
straightforward.
switch, and it is not the root bridge, the set hello time will be used if and
when your switch becomes the root bridge.
Note: The hello time cannot be longer than the Max. Age. Otherwise, a
configuration error will occur.
The Max. Age can be from 6 to 40 seconds. At the end of the Max. Age, if
a BPDU has still not been received from the root bridge, your switch will
start sending its own BPDU to all other switches for permission to become
the root bridge. If your switch has the lowest bridge identifier, it will become
the root bridge.
The Forward Delay can be from 4 to 30 seconds. This is the time that any
port on the switch spends in the listening state while moving from the
blocking state to the forwarding state.
Note: Observe the following formulas when setting the previously
described parameters:
v Max. Age ≤ 2 x (Forward Delay - 1 second)
v Max. Age ≥ 2 x (Hello Time + 1 second)
You can set a port priority from 0 to 255. The lower the number, the greater
the probability that the port will be chosen as the root port.
You can set a port cost from 1 to 65535. The lower the number, the greater
the probability that the port will be chosen to forward packets.
Appendix D. Understanding and troubleshooting the Spanning Tree Protocol
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