Spanning Tree question

Discussion in 'Cisco' started by dilan.weerasinghe, Sep 27, 2006.

  1. Hi

    I have a home lab with a couple of Cisco 2924 XL switches that I'm
    practising Spanning tree on. I've made 2 connections from each switch
    to the other to provide a redundant connection and see ST in action. On



    the 2924_A switch, there are also connections to my laptop and a Cisco
    PIX firewall, which are labelled.


    Here is the output of sh spanning-tree from both switches (omitting
    unused ports for clarity)


    Switch: 2924_A:


    Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol



    Bridge Identifier has priority 32768, address 00d0.ba2f.ae00
    Configured hello time 2, max age 20, forward delay 15
    Current root has priority 32768, address 0002.fdf6.f380
    Root port is 23, cost of root path is 19
    Topology change flag not set, detected flag not set, changes 10
    Times: hold 1, topology change 35, notification 2
    hello 2, max age 20, forward delay 15
    Timers: hello 0, topology change 0, notification 0


    Interface Fa0/1 (port 13) in Spanning tree 1 is FORWARDING
    Port path cost 100, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 00d0.ba2f.ae00
    Designated port is 13, path cost 19
    Timers: message age 0, forward delay 0, hold 0
    BPDU: sent 5558, received 0


    **link to Cisco PIX Firewall**


    Interface Fa0/9 (port 22) in Spanning tree 1 is FORWARDING
    Port path cost 19, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 00d0.ba2f.ae00
    Designated port is 22, path cost 19
    Timers: message age 0, forward delay 0, hold 0
    BPDU: sent 5506, received 0


    **link to laptop**


    Interface Fa0/10 (port 23) in Spanning tree 1 is FORWARDING
    Port path cost 19, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 0002.fdf6.f380
    Designated port is 13, path cost 0
    Timers: message age 2, forward delay 0, hold 0
    BPDU: sent 64, received 770


    **link to 2924_B**


    Interface Fa0/17 (port 31) in Spanning tree 1 is BLOCKING
    Port path cost 19, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 0002.fdf6.f380
    Designated port is 22, path cost 0
    Timers: message age 1, forward delay 0, hold 0
    BPDU: sent 41, received 765


    **link to 2924_B**


    ---------------------


    Switch: 2924_B


    Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol



    Bridge Identifier has priority 32768, address 0002.fdf6.f380
    Configured hello time 2, max age 20, forward delay 15
    We are the root of the spanning tree
    Topology change flag not set, detected flag not set, changes 7
    Times: hold 1, topology change 35, notification 2
    hello 2, max age 20, forward delay 15
    Timers: hello 0, topology change 0, notification 0


    Interface Fa0/1 (port 13) in Spanning tree 1 is FORWARDING
    Port path cost 19, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 0002.fdf6.f380
    Designated port is 13, path cost 0
    Timers: message age 0, forward delay 0, hold 0
    BPDU: sent 722, received 11


    **link to 2924_A**


    Interface Fa0/9 (port 22) in Spanning tree 1 is FORWARDING
    Port path cost 19, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 0002.fdf6.f380
    Designated port is 22, path cost 0
    Timers: message age 0, forward delay 0, hold 0
    BPDU: sent 769, received 1


    **link to 2924_A**


    -------------------


    I can see that 2924_B has become the Root Bridge because it has the
    lower MAC address.


    On the 2924_A switch, there are two connections to the root bridge,
    fa0/10 and fa0/17. Based on the information above, it seems that fa0/10



    has been placed in Forwarding because it has the lower Port ID.


    I do have a couple of questions, however;


    i) How is the Port path cost for an interface calculated? I know that
    100Mbs is 19 and 10Mbs is 100, but what is actually measured? For
    instance, for fa0/1 on the 2924_A switch, the port path cost is 100
    [because the PIX inside interface is set to 10BaseT]. But is this
    supposed to represent the cost from an extra imaginary cable on that
    port to the root bridge?


    ii) The interface number, e.g. fa0/10, and the port number (e.g. port
    23), for some ports do not corelate. On fa0/10 on 2924_A, the interface



    is fa0/10 but the port number is 23. Where is this port number from?


    iii) All ports also have a 'Designated Port x, path cost y' in their
    information. What does this represent?


    Many thanks in advance.
     
    dilan.weerasinghe, Sep 27, 2006
    #1
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  2. Hi Dilan,

    You may wish to investigate Configuring the Path Cost:

    http://www.cisco.com/en/US/products..._guide_chapter09186a00805a6f71.html#wp1105354

    Hope this helps.

    Brad Reese
    BradReese.Com - Cisco Repair
    http://www.bradreese.com/cisco-big-iron-repair.htm
    1293 Hendersonville Road, Suite 17
    Asheville, North Carolina USA 28803
    USA & Canada: 877-549-2680
    International: 828-277-7272
    Fax: 775-254-3558
    AIM: R2MGrant
    BradReese.Com - Cisco Power Supply Headquarters
    http://www.bradreese.com/cisco-power-supply-inventory.htm
     
    www.BradReese.Com, Sep 28, 2006
    #2
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  3. Thanks Brad, that cleared up the first question.

    However, I am still unsure how the designated port for an interface is
    calculated?

    For instance

    Interface Fa0/10 (port 23) in Spanning tree 1 is FORWARDING
    Port path cost 19, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 0002.fdf6.f380
    Designated port is 13, path cost 0
    Timers: message age 2, forward delay 0, hold 0
    BPDU: sent 64, received 770

    **link to 2924_B**

    Interface Fa0/17 (port 31) in Spanning tree 1 is BLOCKING
    Port path cost 19, Port priority 128
    Designated root has priority 32768, address 0002.fdf6.f380
    Designated bridge has priority 32768, address 0002.fdf6.f380
    Designated port is 22, path cost 0
    Timers: message age 1, forward delay 0, hold 0
    BPDU: sent 41, received 765

    **link to 2924_B**

    Both ports are in VLAN1, so I'm not sure how they have different
    designated ports?

    Hope someone can help...

    Cheers.
     
    dilan.weerasinghe, Sep 28, 2006
    #3
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