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262 Intel® Blade Server Ethernet Switch Module IXM5414E
the discarding state to the forwarding state. For IEEE 802.1w operation this is the
time that a designated port on the bridge spends in the learning state while moving
from the disabled state to the forwarding state when both operPointToPointMAC
and operEdgePort are false.
/ NOTE
Observe the following formulas when setting the previously described
parameters:
• Max. Age ≤ 2 x (Forward Delay - 1 second)
• Max. Age ≥ 2 x (Hello Time + 1 second)
Port Priority You can set a port priority from 0 to 240. The lower the number, the greater the
probability that the port will be chosen as the root port.
Port Path Cost
You can set a port cost from 1 to 200000000, or specify auto. The lower the number,
the greater the probability that the port will be chosen to forward packets. If you
specify auto the switch will assign the port cost based on the link speed.
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 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 an STP using values other than the defaults can be complex. Therefore, keep the default
factory settings and the STP will automatically assign root bridges, ports and block loop
connections. However, influencing STP to choose a particular bridge 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.