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New access switch becomes STP root bridge

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  • 1.  New access switch becomes STP root bridge

    Posted Mar 06, 2018 02:29 PM
      |   view attached

    Hi guys,

     

    I have implemented a network which consists of a core switch and some access switches. All the access switches are connected to the core switch through two interfaces.

    I have set the core switch as the root bridge with priority 0 for STP, and I can see STP is working fine and the core switch is the root. However, when I have added a new access switch and connected to the core switch, the new access switch has become the root. I don't know why since the priority of the core switch is 0 and the priority of the new access switch is the default (7). The configuration is very simple since I only have created the vlans and enable STP. What can be the reason of this?

    Here you are some outputs from the core switch (CORE-Stack-3810M) and the new access switch (Piso5DC-2930F-24G-PoEP-4SFPP). I have attached these outputs and the switchs' configs.

     

    Piso5DC-2930F-24G-PoEP-4SFPP# show spanning-tree

    Spanning Tree Information

    STP Enabled [No] : Yes
    Mode : RPVST
    Extended System ID : Enabled
    Ignore PVID Inconsistency : Disabled
    RPVST Enabled VLANs : 1,100


    Switch MAC Address : 98f2b3-b7cae0
    Root Guard Ports :
    Loop Guard Ports :
    TCN Guard Ports :
    BPDU Protected Ports :
    BPDU Filtered Ports :
    Auto Edge Ports : 1-28
    Admin Edge Ports :

    VLAN Root Mac Root Root Root Hello
    ID Address Priority Path-Cost Port Time(sec)
    ----- --------------- ---------- ---------- -------------------- ---------
    1 98f2b3-b7cae0 32,768 0 This switch is root 2
    100 98f2b3-b7cae0 32,768 0 This switch is root 2

    Piso5DC-2930F-24G-PoEP-4SFPP#

     

     

    CORE-Stack-3810M# show spanning-tree config

    Spanning Tree Information

    STP Enabled [No] : Yes
    Mode : RPVST
    Extended System ID : Enabled
    Ignore PVID Inconsistency : Disabled
    Default Path Cost [8021t] : 8021.t
    RPVST Enabled VLANs : 1,100


    Switch MAC Address : ecebb8-1f5865

    Root Guard Ports :
    Loop Guard Ports :
    TCN Guard Ports :
    BPDU Protected Ports :
    BPDU Filtered Ports :
    Auto Edge Ports : 1/1-1/16,2/1-2/16

    Admin Edge Ports :

    Max Age Forward Hello Admin Root
    VLAN Priority (sec) Delay(sec) Time(sec) Bridge
    ---- -------- ------- ---------- --------- ----------------
    1 32768 20 15 2 Not Configured   <<<<< shouldn't be 4096 for the root?
    100 32768 20 15 2 Not Configured <<<<< shouldn't be 4096 for the root?

     

    Regards,

    Julián

    Attachment(s)

    txt
    stp_problem.txt   4 KB 1 version


  • 2.  RE: New access switch becomes STP root bridge

    Posted Mar 06, 2018 10:14 PM

    Hi guys,

     

    Any idea?

     

    Regards,

    Julián



  • 3.  RE: New access switch becomes STP root bridge

    Posted Mar 07, 2018 10:47 AM

    Hi guys,

     

    I have tried to increase the priority on the access switch, and also disable and enable STP, but it keeps being the root switch. Any idea?

     

    Regards,

    Julián



  • 4.  RE: New access switch becomes STP root bridge

    Posted Mar 07, 2018 06:23 PM

    Hi guys,

     

    The problem persists. I have also realized when I connect the new access switch, not only it becomes the root bridge, but also the trunk which connects to the core switch has the designated role (as it should be for the root bridge), but its state is blocking.

    The configuration of the new access switch is the same as the other access switches. If I don't connect the new switch, the core switch is the root bridge (it has priority 0), but when I connect the new switch it becomes the root bridge. This is my topology:

     

    stp_problem.jpg

     

    One question, to force a switch to be the root bridge, is it enough to apply the command "spanning-tree priority 0"? It is what I made only in the core switch.

    Until now the only solution I have is to have STP enabled in all switches except in the new switch.

    As example, attached the configurations of an access switch, the new switch and the core switch. Please your help.

     

    Regards,

    Julián

    Attachment(s)

    txt
    new_switch.txt   699 B 1 version
    txt
    access_switch.txt   1 KB 1 version
    txt
    core_switch.txt   1 KB 1 version


  • 5.  RE: New access switch becomes STP root bridge
    Best Answer

    Posted Mar 07, 2018 08:40 PM

    Greetings!

     

    Your switches are operating in RPVST mode, so the spanning-tree priority is set at the VLAN level, not at the switch level — there is a separate root for every VLAN. 

     

    Because it looks like you've only set a global priority on each switch, the per-VLAN priority is set to default for each VLAN on both switches, and the same rules apply for breaking a tie during the root election as do for MSTP/RSTP — the lowest MAC address becomes the root.

     

    There are two approaches you can take to fix this.  First, if you wish to continue using RPVST mode, you can set the priority for each VLAN using a command similar to the following:

     

    switch(config)# spanning-tree vlan 1,100 priority 0
    

    Or, alternately, you can switch to MSTP mode to use a single global priority value to determine the root bridge for the spanning tree, rather than using a separate tree for each VLAN:

     

    switch(config)# spanning-tree mode mstp

     



  • 6.  RE: New access switch becomes STP root bridge

    Posted Mar 07, 2018 09:36 PM

    Hi Matthew,

     

    I know in RPVST mode the priority is at per-VLAN basis, and it is true that the new access switch has lower MAC that the core switch, but the core switch has priority 0, I typed the command:

     

    (config)# spanning-tree priority 0

     

    If I don't specify any VLAN in the command as you say, doesn't that command apply to all VLANs? If I don't specify any VLAN, doesn't the command take effect?

    If the command doesn't take effect if no VLAN is specified, what you say makes much sense.

     

    Regards,

    Julián



  • 7.  RE: New access switch becomes STP root bridge

    Posted Mar 08, 2018 09:01 AM

    Hi Matthew,

     

    Unlike I guessed, applying the command without specifying any VLAN doesn't take effect in RPVST and leaves the switch with the default priority. Once I applied the command including the VLANs on the core switch, I connected the new accesss switch and didn't become the root bridge, the core switch kept being the root. Many thanks for your help!

     

    Regards,

    Julián