D-Link DGS-3208F Product Manual - Page 23

On the Port Level, User-Changeable Parameters, Illustration of STA

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Gigabit Ethernet Switch User's Guide On the Port Level ♦ Root Port Each switch has a Root Port. This is the port that has the lowest Path Cost to the Root Bridge. In case there are several such ports, then the one with the lowest Port Identifier is the Root Port. ♦ Designated Port This is the port on each Designated Bridge that is attached to the LAN segment for which the switch is the Designated Bridge. ♦ Port Priority The smaller this number, the higher the Port Priority is. With higher Port Priority, the higher the probability that the port will be selected as the Root Port. ♦ Path Cost This is a changeable parameter and may be modified according to the STA specification. User-Changeable Parameters The factory default setting should cover the majority of installations. However, it is advisable to keep the default settings as set at the factory, unless it is absolutely necessary. The user-changeable parameters in the Switch are as follows: ♦ Bridge Priority A Bridge Priority can be from 0 to 65535. 0 is equal to the highest Bridge Priority. ♦ Bridge Hello Time The Hello Time can be from 1 to 10 seconds. This is the interval between two transmissions of BPDU packets sent by the Root Bridge to tell all other Switches that it is indeed the 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 that 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, 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 16

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Gigabit Ethernet Switch User’s Guide
16
On the Port Level
♦
Root Port
Each switch has a Root Port. This is the port that has the lowest Path Cost to the Root
Bridge. In case there are several such ports, then the one with the lowest Port Identifier is the Root
Port.
♦
Designated Port
This is the port on each Designated Bridge that is attached to the LAN segment for
which the switch is the Designated Bridge.
♦
Port Priority
The smaller this number, the higher the Port Priority is. With higher Port Priority, the
higher the probability that the port will be selected as the Root Port.
♦
Path Cost
This is a changeable parameter and may be modified according to the STA specification.
User-Changeable Parameters
The factory default setting should cover the majority of installations.
However, it is advisable to keep the
default settings as set at the factory, unless it is absolutely necessary. The user-changeable parameters in the
Switch are as follows:
♦
Bridge Priority
A Bridge Priority can be from 0 to 65535. 0 is equal to the highest Bridge Priority.
♦
Bridge Hello Time
The Hello Time can be from 1 to 10 seconds.
This is the interval between two
transmissions of BPDU packets sent by the Root Bridge to tell all other Switches that it is indeed the
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 that 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, 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