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Thread: Spanning Tree

  1. Junior Member
    Join Date
    Feb 2015

    Default Spanning Tree

    Hi guys,

    could someone please exactly describe me how STP root and designated ports will be chosen?

    If I'm right:

    Root ports will be elected in the way of comparing the path costs of each link from a non-root switch to the root one where the path with the lowest path cost wins and will be the root port.
    Designated ports will be selected firstly in the way of comparing the path costs to reach the root bridge (except the root port) on a non-root switch. If a tie exists, that port will be the designated for a LAN segment, which is on the switch with the lower BID value (priority+MAC). If it also wouldn't work, the port priorities are considered if configured (lower wins and will be designated) and finally, the internal port numbers (defined by Cisco) determines the designated port.

    Is this right?

    Thanks in advance!
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  2. SS
  3. Junior Member
    Join Date
    Feb 2015
    I found a solution to my question/problem myself:
    I don't know exactly, for which port role-selection the 4-step process applies (1. lowest root cost, 2. BID, 3. Port-priority, 4. Port ID) but I found if I use it for both of the two elections (both for root and designated ports) I can reach a 100% result in exam questions.
    Thus, it doesn't really matter for which port roles these rules are applied until I can correctly choose the port roles with the 4-step one.

    Nevertheless, I would appreciate if someone could explain me the exact process - just to be clear.

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  4. ProEthicalHacker.com fuz1on's Avatar
    Join Date
    Dec 2014
    silicon valley, where we speak that bay area slang

    CCNA CCENT ITIL SSCA SSVVP WSQD WSQI Cloud+ Mobility+ Security+ Linux+ Network+ Server+ Project+ A+ HIT LPIC-1 CLA11 ACE ace/PACA SSBBP SSLP CMS SCP
    Edit: Check this out - http://www.cisco.com/c/en/us/support.../12013-17.html

    Copy/paste from 'Understanding and Configuring STP on Catalyst Switches'

    STP Path Cost Automatically Changes When a Port Speed/Duplex Is Changed

    STP calculates the path cost based on the media speed (bandwidth) of the links between switches and the port cost of each port forwarding frame. Spanning tree selects the root port based on the path cost. The port with the lowest path cost to the root bridge becomes the root port. The root port is always in the forwarding state.
    If the speed/duplex of the port is changed, spanning tree recalculates the path cost automatically. A change in the path cost can change the spanning tree topology.

    Last edited by fuz1on; 02-26-2016 at 03:18 PM.
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  5. Senior Member
    Join Date
    Nov 2015
    heres info from curriculum:
    A BPDU is a messaging frame exchanged by switches for STP. Each BPDU contains a BID that identifies the switch that sent the BPDU. The BID contains a priority value, the MAC address of the sending switch, and an optional extended system ID.
    The lowest BID value is determined by the combination of these three fields.

    After the root bridge has been determined, the STA calculates the shortest path to the root bridge. Each switch uses the STA to determine which ports to block. While the STA determines the best paths to the root bridge for all switch ports in the broadcast domain, traffic is prevented from being forwarded through the network. The STA considers both path and port costs when determining which ports to block.

    The path costs are calculated using port cost values associated with port speeds for each switch port along a given path. The sum of the port cost values determines the overall path cost to the root bridge. If there is more than one path to choose from, STA chooses the path with the lowest path cost.

    When the STA has determined which paths are most desirable relative to each switch, it assigns port roles to the participating switch ports. The port roles describe their relation in the network to the root bridge and whether they are allowed to forward traffic.

    at first each switch if they are out of box think they are root bridge-thus usually its lowest mac address that determines who gets picked,after that its down to algorithm to calctulate path costs to reach root,and determine that longest path is alternate port thus still listening but in blocked state,but depending on which switch and routes you want to use you can manipulate cost,think its spanning-tree cost 18<19 or electing other switch as root or changing its priority.
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