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Stackable NWay Ethernet Switch User’s Guide
Switch Management Concepts 33
Root Bridge. If you set a Hello Time for your 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.
♦
Bridge Max. Age
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 it turns out that your Switch has the lowest
Bridge Identifier, it will become the Root Bridge.
♦
Bridge Forward Delay
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.
Observe the following formulas when you set the above parameters:
1.
Max. Age
≤
2 x (Forward Delay - 1 second)
2.
Max. Age
≥
2 x (Hello Time + 1 second)
♦
Port Priority
A Port Priority can be from 0 to 255. The lower the number, the greater the probability
the port will be chosen as the Root Port.
Illustration of STA
A simple illustration of three Bridges (or the Switch) connected in a loop is depicted in
Figure
5-1
. In this
example, you can anticipate some major network problems if the STA assistance is not applied. For instance,
if Bridge 1 broadcasts a packet to Bridge 2, Bridge 2 will broadcast it to Bridge 3, and Bridge 3 will broadcast
it to Bridge 1 and so on. The broadcast packet will be passed indefinitely in a loop, causing a serious network
failure.
To alleviate network loop problems, STA can be applied as shown in
Figure
5-2
. In this example, STA breaks
the loop by blocking the connection between Bridge 1 and 2. The decision to block a particular connection is
based on the STA calculation of the most current Bridge and Port settings. Now, if Bridge 1 broadcasts a
packet to Bridge 3, then Bridge 3 will broadcast it to Bridge 2 and the broadcast will end there.
STA setup can be somewhat complex. Therefore, you are advised to keep the default factory settings and STA
will automatically assign root bridges/ports and block loop connections. However, if you need to customize the
STA parameters, refer to
Table 5-1
.
Figure 5-1. Before Applying the STA Rules