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.
Port Cost
– A Port Cost can be set from 0 to 65535. 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
In this example, only the default STP values are used.
