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  1. Completely Clueless TechGromit's Avatar
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    #1

    Default How to determine is a port is in blocking/passive mode

    Hello,

    I have a network issue and I figured this be the place to start.

    ROUTER1-------ROUTER2
    | x x x x x x x x x |
    | x x x x x x x x x |
    | x x x x x x x x x |
    Switch1----------Switch2

    Switch1 and 2 have the same Vlan248, the link between Switch1 and Switch 2 is down. The links between the Switches and Routers are fine, but only Switch2 is passing traffic to the Router2 on VLAN248, although Router1 and Switch1 see each other with CDP neighbor, they are not allowing any traffic to pass, I believe this is because Router1 is blocking VLAN248 to prevent a Spanning Loop. Is there a command I can see if the interface from Router1 to the switch interface is in blocking/passive mode to prove this?
    Last edited by TechGromit; 03-22-2017 at 01:30 PM.
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  3. Senior Member shortstop20's Avatar
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    #2
    Is Router1 actually a router or is it a Layer 3 switch?

    What model is it?

    True Routers do not run spanning-tree as routers break up broadcast domains, thus there is no possibility for loops.
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    #3
    Assuming you are using l3 switches as "routers" you can see what spanning tree is blocking or forwarding with the show spanning-tree summary command. If you know the vlan you filter with the command show spanning-tree vlan X
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    #4
    IF the link between SW1 and SW2 is down then there should be no STP loop for VLAN248.

    You can use "show spanning-tree vlan 248" on Switch1. See what's the state of the interface going to Router 1.

    Is the link between R1 and S1 a layer 2 link or is it routed / layer 3?

    There could be a LOT of reasons why traffic might not pass between these 2 devices (ie. access-lists, VACLs, private VLAN configs, etc.)
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  6. Completely Clueless TechGromit's Avatar
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    #5
    Quote Originally Posted by shortstop20 View Post
    Is Router1 actually a router or is it a Layer 3 switch?

    What model is it?

    True Routers do not run spanning-tree as routers break up broadcast domains, thus there is no possibility for loops.
    Both routers are Cisco WS-C6504, Switch 1 is a Cisco 3750 and Switch 2 is a Cisco 3850.
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  7. Completely Clueless TechGromit's Avatar
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    #6
    Quote Originally Posted by dmarcisco View Post
    Assuming you are using l3 switches as "routers" you can see what spanning tree is blocking or forwarding with the show spanning-tree summary command. If you know the vlan you filter with the command show spanning-tree vlan X
    Hmm both 6504 routers say they are forwarding on all ports, nothing blocking. Can the Switch be where the blocking is occurring?
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  8. Senior Member shortstop20's Avatar
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    #7
    Quote Originally Posted by TechGromit View Post
    Hmm both 6504 routers say they are forwarding on all ports, nothing blocking. Can the Switch be where the blocking is occurring?
    That is possible, yes. Please post the output from Router1 and Switch1 for the following commands:

    show spanning-tree vlan 248
    show int ____ trunk <-----Use the interface on R1 that connects to S1 and vice versa.
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