Spanning tree question

ceejay3001ceejay3001 Member Posts: 36 ■■□□□□□□□□
a topology of for switches in a diamond shape with one connected to a router.

the switches are all connected and running stp. there are 3 vlans running over vtp.

what is the root switch

i cant seem to figure this close ---can you help.

if i do a sh spanning-tree -it shows only local info - therefore if this is not the stating its the root bridge -which one is then??-you only can console into one switch.

i know the lowest priority\mac elects. but how can you work out which neighbor switch is the root. even if i do sh cdp neighbor - yes you can see which ports is connected.but this doesnt mean its the root does it.

need to some how match lowest id to a neighbor device i guess

how can i do this.


  • jrambarjrambar Users Awaiting Email Confirmation Posts: 29 ■□□□□□□□□□
    I had a question on my exam similar to this.
  • David_HXDavid_HX Member Posts: 37 ■■□□□□□□□□
    I was thinking the same thing about consoling into the only switch which you *do* have access to,, then, utilizing the:

    SWITCH-A#show cdp neighbor detail

    command of the switch to see the administrative IP address of VLAN 1 of the immediately connected (trunked ?) SWITCH-B which is connected.

    Then (once again, from the console connection) TELNET into that next SWITCH-B which does not allow direct console access.

    You *can* TELNET into Switches,, you just cannot TELNET *from* Switches to another Switch. (let me know if I am wrong here)

    While TELNETTED into the next Switch over (lets call it "B"),, you could then execute the command:

    SWITCH-B# show spanning-tree

    and examine the screen output to see if the SWITCH-B is or is not the Root Bridge (Switch),, either by it actually saying so in the display "This is the Root Bridge" (or words to that effect),, or by further analyzing the Bridge priority number, looking for the lowest number which would be the Root Bridge.

    note: if all Priority numbers are equal. (3276icon_cool.gif,, then look for the lowest 4 digits of the Switch MAC address which is also listed.

    Of course, while TELNETTED into Switch-B,, it would also serve you well to exceute command:

    SWITCH-B#show cdp neighbor detail

    In order to grab the IP adx. of the next switch in line,,, SWITCH-C

    Then back out of the current TELNET session with SWITCH-B and TELNET into SWITCH-C and once again utilize the:

    SWITCH-C#show spanning-tree

    command to further analyze the output while looking either for the line saying "This Bridge is Root" (or words to that effect) or looking at the Spanning-Tree Bridge priority number (once again,, comparing it to previous numbers,, looking for the lowest Bridge Priority ID (or MAC number) which would designate the ROOT BRIDGE.

    I think this will work......

  • lwwarnerlwwarner Member Posts: 147 ■■■□□□□□□□
    Do we know the bandwidth of the links? If so, all the information needed is available from show spanning-tree output on any single switch. For example, assume all switch ports are 100 Mbps and we see this on one of the switches:

    SW1#show spanning-tree

    Spanning tree enabled protocol ieee
    Root ID Priority 32769
    Address 000a.f4fe.80c0
    Cost 38
    Port 11 (FastEthernet0/11)
    Hello Time 2 sec Max Age 20 sec Forward Delay 15 sec

    The STP cost of a 100 Mbps link is 19, so from this output we know the root bridge is 2 hops out port fa0/11.

  • David_HXDavid_HX Member Posts: 37 ■■□□□□□□□□
    Now *that* shows some real thought Bill.

    Thanx ! icon_wink.gif

    Now....... How could you find and verify the Port to MAC Adx. association if the:

    SWITCH-A#show mac-address-table

    command dosn't show every updated MAC address. (max_time'ed out)

    In other words, how could you cause SWITCH-A to refresh it's CAM with all connected ports' devices Layer-II addresses ?

    Pinging the switches B'cast address maybe,, or any unused IP address on the switches sub-net to stimulate a "flooding" looking for a device ?

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