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Line Protocol Down

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  • 1.  Line Protocol Down

    Posted Jan 19, 2023 10:02 PM
    Nowadays people are using STP less and less.
    Frame relay is also obsolete.
    Sometimes they just use layer 3 directly. 
    So are there any circumstances in which line protocol is down is encountered in real life?


  • 2.  RE: Line Protocol Down

    EMPLOYEE
    Posted Jan 20, 2023 01:36 AM
    An example: 1G optical link running between two switches. On one side auto negotiation is disabled, on another side it is enabled. Signal strength is good. Both ports are administratively up, but line protocol is down, because of this mismatch.

    Another example:  VPN protocols often signal their issues to the upper protocols by putting the virtual interface 'Link protocol: DOWN'.

    While your example with Frame Relay is valid, I am afraid line protocol has  nothing to do with STP (there all ports are UP, some are just not forwarding, blocked). Also you never use Layer 3 directly, you always have Layer 2 protocol and physical media.

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    Ivan Bondar
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    Original Message


  • 3.  RE: Line Protocol Down

    Posted Jan 20, 2023 03:07 AM
    Well I have to disagree with this statement:
    "Also you never use Layer 3 directly, you always have Layer 2 protocol and physical media."

    Between 2 physical links I configure point to point ip addresses. So where is the layer 2 protocol involved?
    In this case it always states that the line protocol is up, although I dont know why because there is only layer 3 and no layer 2 involved. 



    Original Message


  • 4.  RE: Line Protocol Down

    MVP GURU
    Posted Jan 20, 2023 03:12 AM
    What's about the OSI model?

    Given that the Data Link layer = Layer 2 then how can exist Layer 3 without having a Layer 2 and a Layer 2 without having a Layer 1 (Physical)?

    Or are we missing a thing in your line of reasoning?



    Original Message


  • 5.  RE: Line Protocol Down

    Posted Jan 20, 2023 04:15 AM
    Ok so enlighten me if I configure OSPF point to point link, what is the layer 2 protocol involved?
    Vlan tags are already discarded.
    So how do I check the layer 2 and if it is working?
    Original Message


  • 6.  RE: Line Protocol Down

    EMPLOYEE
    Posted Jan 20, 2023 05:18 AM
    Sorry, but you mix non-comparable things. OSPF network type 'point-to-point' has nothing to do with encapsulation on that link.

    If you use OSPF, then L3 protocol must be IP, OSPF doesn't support any other. Ok, there is an IP packet that needs to be forwarded over that link from Device A to Device B. Following OSI model, you MUST use L2 protocol to encapsulate the IP packet. Most of modern networks use Ethernet. I bet you also use Ethernet devices and not InfiniBand or FC, right? ;) So, Device A will use Ethernet encapsulaton to wrap your IP packet and just then it will push it through the link towards Device B.

    Here is an example how network packet encapsulation in Ethernet/IP networks works (taken from profinetuniversity.com):



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    Ivan Bondar
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    Original Message


  • 7.  RE: Line Protocol Down
    Best Answer

    EMPLOYEE
    Posted Jan 20, 2023 05:20 AM
    Vlan tags are already discarded.
    This doesn't mean Ethernet header is not there, right?

    So how do I check the layer 2 and if it is working?
    Check the ARP and MAC address tables.

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    Ivan Bondar
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    Original Message


  • 8.  RE: Line Protocol Down

    Posted Jan 22, 2023 02:41 AM
    Thank you for the clarifications.
    I have forgotten the fundamentals.
    Original Message


  • 9.  RE: Line Protocol Down

    Posted Jan 22, 2023 02:42 AM
    We can also check the LLDP as well if it is enabled.
    Original Message