PLANET WGS3-2820 - Manual (Page 134)

User’s Manual of WGS3-2820/WGS3-5220
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the Root Bridge, your Switch will start sending its own BPDU to all other Switches for permission to become the Root Bridge. If it
turns out that your Switch has the lowest Bridge Identifier, it will become the Root Bridge.
Forward Delay Timer – The Forward Delay can be from 4 to 30 seconds. This is the time any port on the
Switch spends in the listening state while moving from the blocking state to the forwarding state.
#
Notice:
Observe the following formulas when setting the above parameters:
Max. Age _ 2 x (Forward Delay - 1 second)
Max. Age _ 2 x (Hello Time + 1 second)
Port Priority – A Port Priority can be from 0 to 240. The lower the number, the greater the probability the port will be chosen as
the Root Port.
Port Cost – A Port Cost can be set from 0 to 200000000. The lower the number, the greater the probability the port will be
chosen to forward packets.
3. Illustration of STP
A simple illustration of three switches connected in a loop is depicted in Figure 5-7. In this example, you can anticipate some
major network problems if the STP assistance is not applied. If switch A broadcasts a packet to switch B, switch B will broadcast
it to switch C, and switch C will broadcast it to back to switch A ... and so on. The broadcast packet will be passed indefinitely in
a loop, potentially causing a network failure. In this example, STP breaks the loop by blocking the connection between switch B
and C. The decision to block a particular connection is based on the STP calculation of the most current Bridge and Port
settings. Now, if switch A broadcasts a packet to switch C, then switch 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, you are advised to keep the default
factory settings and STP will automatically assign root bridges/ports and block loop connections. 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, however, relatively straight forward.
Bridge ID = 15
Port 3
Port 1 Port 2
A
Bridge ID = 20
Port 1
Port 2 Port 3
C
Bridge ID = 30
Port 1
Port 2 Port 3
B
LAN 1
LAN 2 LAN 3
Portcast = 19
Portcast = 19 Portcast = 19
Portcast = 4
Portcast = 4
Portcast = 19
Portcast = 4
Portcast = 4
Before Applying the STA Rules