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Spanning tree question - which port will block?

I need a better understanding about how a blocking ports are chosen in a Cisco switching network.  Let's take the simplest example - three switches connected to each other by links of equal costs.  One of the end-points of the link "opposite" to the STP root bridge will be blocked, but which one?

What I'm trying to achieve is accurate control on which port in a ring topology will be blocked - so far I was thinking that it's enough to configure a large enough STP cost on it, but apparently it's not that simple...

Thank you.

3 REPLIES
New Member

Spanning tree question - which port will block?

Port of the switch with highest bridge id will be blocked.

Re: Spanning tree question - which port will block?

Hi,

to build a loop-free spanning-tree, this decision sequence is processed:

  1. Lowest Bridge-ID (composed of a configurable + fixed part)
  2. Lowest Cost to Root-Bridge
  3. Lowest Sender-ID
  4. Lowest Port-ID (composed of a configurable + fixed part)

All of this values are stored in the BPDUs.

Ports that are neither root ports nor designated ports for their segement will be blocked.

There are many discussions about this topic in CSC, e.g. this one:

https://supportforums.cisco.com/message/3737555

Feel free to ask for more details!

HTH

Rolf

Silver

Re: Spanning tree question - which port will block?

Hi,

Vasanth is correct.

I just want to add a little explanation. Let me write the 5 steps or tie-breakers, by which the switches decides what to do.

The first 4 are contained in BPDUs, the 5th criterium is local to the switch.

1. Root Bridge ID - Lowest switch priority+MAC address is better

2. Root Path Cost - Lowest accumulated cost is better. (Note: When designated port election is in place, the cost is not added upon reception of BPDU, like in root election operation, but BPDUs are compared immediately as they arrive.)

3. Sender Bridge ID - Lowest switch priority+MAC address is better

4. Sender Port ID - Lowest combination of priority.port_number on sending port is better

---

5. Receiver Port ID - Lowest combination of priority.port_number on receiving port is better


Appendix (STP cost values):

4 Mb/s 250
10 Mb/s 100
16 Mb/s 62
100 Mb/s 19
1 Gb/s 4
10 Gb/s 2

You can solve ALL the scenarios with these five rules.

If you have any further questions, please feel free to ask!!!

Best regards,

Jan

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