Explain RSTP priority and path-cost

RouterOS Beginner Basics
1

Hello,

I’m currently testing RSTP between two 6.44.3 instances.
I’m not familiar at all with STP and the likes.

I looked at first example (the one involving switches SW1, SW2, SW3 and SW4 and hosts A and B) in RSTP Wiki page (see [1]).

  1. I can read that SW1 settings rely on priority while SW4 rely on path-cost.
    Can you explain why ?

  2. Are those priority and path-cost exclusively used by RSTP (and relatives) or shall I expect other side effects.

[1] https://wiki.mikrotik.com/wiki/Manual:Spanning_Tree_Protocol

2

I looked at first example (the one involving switches SW1, SW2, SW3 and SW4 and hosts A and B) in RSTP Wiki page (see [1]).

  1. I can read that SW1 settings rely on priority while SW4 rely on path-cost.
    Can you explain why ?

Per the Wiki:
In RouterOS the root bridge will be elected based on the smallest priority and the smallest MAC address in this particular order:

  • Bridge priority (lowest)
  • Bridge MAC address (lowest)

In RouterOS root ports are elected based on lowest port path cost, lowest port priority and lowest bridge port ID in this particular order:

  • Port path cost (lowest)
  • Port priority (lowest)
  • Bridge port ID (lowest)

Once the root bridge has been determined, the best path is used to determine which ports should be used to reach that root bridge.


  1. Are those priority and path-cost exclusively used by RSTP (and relatives) or shall I expect other side effects.

These are purely internal to STP/RSTP/MSTP.