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TOPIC: STP

STP 8 years 1 month ago #27672

  • nk007
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If i have two switch A and B.

I have connect the switch like as below:

Switch A port1
>Switch B port1
Switch A port2
>Switch B port2 (redundancy)

Anyhow One of this port will be block.

I like to know how one of this port block and another one is allowing even though it has same MAC address and priority.


Kindly explain me.

Thanks in advance.
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Re: STP 8 years 1 month ago #27680

  • S0lo
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Hello nk007,

Before answering, may I ask how did you know that the two switches have same MAC addresses? :? Or did I misunderstand you. As far as I know, all interfaces (hence switches) have distinct MAC addresses, even if they come from different vendors. STP (and in-fact ethernet) is based on the concept of having differing MACs. It will not work as it should otherwise.
Studying CCNP...

Ammar Muqaddas
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Re: STP 8 years 1 month ago #27691

I think the MAC nk007 is referring to is the Root Bridge ID. If so, then in the example given, let us assume switch A has lower bridgeID than switch B, then one port on switch B will be root port and the other will be blocking. The way the root port is decided is

1. Since the Root Bridge ID is same on both the BPDU recieved on both the ports, there is tie break and the next condition is, the lowest cost of the BPDU.
2. In this case the cost will be same since BPDU are recieved from the root directly. Now there is again a tie break, so this tie is broken by
3. Switch A will be sending the port information in the BPDU, in this case port1 for the BPDU sent on port 1 and port 2 for the BPDU sent on port2. So the switch B will select the port on which the lower port value BPDU is recieved.
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Re: STP 8 years 1 month ago #27692

  • S0lo
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I think the MAC nk007 is referring to is the Root Bridge ID

Even if we assume so, Bridge IDs still can't be the same on both switches since a Bridge ID is formed by combining the priority and the MAC (which is never the same).
1. Since the Root Bridge ID is same on both the BPDU recieved on both the ports, there is tie break and the next condition is, the lowest cost of the BPDU.
2. In this case the cost will be same since BPDU are recieved from the root directly. Now there is again a tie break, so this tie is broken by
3. Switch A will be sending the port information in the BPDU, in this case port1 for the BPDU sent on port 1 and port 2 for the BPDU sent on port2. So the switch B will select the port on which the lower port value BPDU is recieved.

I agree with you. And I did not know that 3rd tie breaker so thanks :). It seams also there is a per port configurable priority on some switches that can be used to tie break before the ports own MAC or identifier is used.
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Ammar Muqaddas
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Re: STP 8 years 1 month ago #27693

  • Patiot
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Yes SOLO, when the first two tie breakers fail to break the tie the third tie breaker will try its shot .

In the third tie breaker info, actually the port priority of the connected switch (in this case SWITCH-A`s sending port priority) will influence the port selection criteria of the concerned switch ( in this case switch-B ). This is actually done with the downstream concept with the core being the root bridge in mind .

Thanks
Patiot
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