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Switch Port Priority

jskfan
jskfan asked
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Last Modified: 2014-11-24
in the topology below , SW3 is the Root Bridge.SW4 on the right has 2 interfaces connected to it.
based on what I have read Root Port and blocking port , will be determined based on:
Link Cost = Speed of the interfaces
Bridge ID = in our case SW3 is the Root Bridge
Ports Priority = ??
Port Number= in our case Fa0/21 and Fa0/23

So based on what is the Port Priority is determined ?

Thanks
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I see...
By default port priority is 128 for all interfaces unless you manually change it. Correct ?
So in this case by default Root Port and blocking port  of the switch that is directly connected to the Root Bridge , is determined based on:

1-Link Cost ( Speed of the interfaces connecting the switch to the Root Bridge)
 2- interfaces Port Number of the Root Bridge Switch that is sending BPDU to the directly connected switch
3-Port Priority

I put Port Priority as the last in the list, because by default all interfaces have 128 as the Default

Correct?
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Not correct. After root bridge is selected

1. Link cost to the root bridge (don't confuse with speed, you can set link cost manually, so 2 interfaces with the same speed can have 2 different costs - usually is the same value for interfaces with the same speed but not  necessary) command is
  spanning-tree [ vlan vlan-id ] cost { value | auto }
2. port priority (this is usually better way for path manipulation than manipulating link cost in case of interfaces with the same speed)
3. Interface port number - it is taken into account only when previous two are equal - as tie breaker

Author

Commented:
I meant without manually changing anything, will that be:

Link Cost
Port Number
Port Priority
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Again, it is always in this order:

Link Cost
Port Priority
Port ID (Number) <- this is always last option

Author

Commented:
in your previous comment , you said :
2. port priority (this is usually better way for path manipulation than manipulating link cost in case of interfaces with the same speed)

but if everything left to default, the port priority is always 128.XX , where XX is the port number
The lower port number the better from The root Bridge interface connected to the other switch
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Thank you !!

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