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Spanning Tree

Discussion in 'Networks' started by spanme, Mar 29, 2012.

  1. spanme

    spanme New Member

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    Hi,

    I have read that the basic purpose of spanning tree was to allow redundant physical links between switches, whereby a redundant link was blocked until the main link failed and then the redundant link would move to the forwarding state. The condition of a loop in the network at layer 2 exists because when a device makes an arp request the switch forwards it out of all ports except the entry port, so when you have 2 or more switches connected with redundant links the arp is broadcast back to itself on another port and on it continues looping eternally.

    However, how does this explain someone patching a phone into a data point causing a blocked port? The VoIP phone has a single port that the PC connects to, the VoIP phone itself connects to the data point, the data point goes to the patch panel, patch panel to the switch. There can only be one physical link to the switch, so why is spanning tree blocking the port?

    I thought that you can only cause a loop if you either add a layer 2 device (like a switch) in such a way that it has 2 links to a switch, or you accidentally patch a cable so that the switch has redundant links.

    As the only way out of the switch to the core is via a different network, then the spanning tree problem can only be local.

    I don't understand why the port is being blocked. What am I missing?
     
  2. cisco lab rat

    cisco lab rat Megabyte Poster

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    How long does the port block for?
    Which Spanning Protocol are you using?
    Are you using Voice vlan on the port?
    Are you using the "spanning-tree portfast" command?
    Does it happen when you pull just the P.C into the port?
    Are the trunk links going into FWD/BLK status correctly?
     
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  3. spanme

    spanme New Member

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    Basic spanning tree on a HP switch using dynamic vlan, so port is tagged on voip vlan and untagged on data vlan. Not using any portfast command, not been able to test pc only, connecting phones directly to switch has no problem. You can`t gather much from this particular HP switch syslog, cant remember how long it blocks for but its on and off ever few seconds or so eternally.
    All other phones and computers are fine.
    What are you thinking? Maybe a faulty phone causes it, but how?
     
  4. cisco lab rat

    cisco lab rat Megabyte Poster

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    Can you turn on edge port (Port fast) on the port andcan you confirm that you are running either Rapid Spanning tree or conventional Spanning tree (802.1D)

    From my ex with HP switch I have not experienced any issues with phones causing the port on the switch to block, the only time I have seen this is with Rapid Spanning tree and a port with no port fast, the Rapid Spanning protocol uses an exchange of proposal and agreement messages to converge quickly, if the device off of a rapid spanning tree port cannot take part in the dialogue Rapid Spanning tree will put the port into blocking/listen/learning.

    Also do you know how to monitor a port on a HP switch?, if so can you use wireshark to see if the phone is sending BPDU's.

    Something you might try but a little bit dangerous is to put a BPDU filter on the port that the phone is pulled into and see if that solves the problem, a little dangerous but it would prove that there is an exchange of BPDU's between the switch and the Phone causing a spanning-tree problem
     
    Certifications: Yes I pretty much am!!
    WIP: Fizzicks Degree

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