Link keepalive

Hey everyone!


I have an extremly annoying question that I cannot seem to find the answer to. I know that when an interface is shut down or the cable is removed, the interfaces will immediately go down. But what technology ensures that the other side knows that the link is down?

I mean, the router/switch that you issue the shutdown command on will know that the interface is down because you issued the shutdown command on it, but how does the other side of the link know what you just did?

And is it the same technology that makes sure that you know if the cable has been removed or damaged so that the routers/switches know that the link is down?



Mr. Frustrated 


  • Are talking a Layer 2 port or Layer 3 port

    In Layer 2 enviroment you have BPDU - two type including Topology Changed notification BPDU

    In some case when You have multiple switch - fpr example prvate point to point circuit - your interface might be up and up status but actulally you can not able to reach destination 

    CHeck the status on the interface  , and protocol status on the interface 

    I hope I understand your question properly if its not please refrase exactly your goal is 

  • If they are connected back to back this is the loss of physical signal (electric or optic) that will signal the router that the link is down or shutdown. For POS link you have also a command called pos ais-shut that will send the line alarm indication signal that alarms to the remote end of the administrative shutdown (you don't have this feature on Ethernet).

    For indirect connections (through a carrier) you may use OAM (layer 2) or BFD (layer 3) failure detection mechanisms.



  • OK, so for a physical Point-to-Point link, it is the actual pyhsical port on the router/switch that will detect if it is no longer receiving electrodes if copper and light if fiber.

    This also makes sense with the shutdown command - the other side does not know that the shutdown command was issued, only that it is no longer receiving any electrons or light.

    Would anoyne of you know of any document/book describing this? I mean, it would be fun to know if a port on copper sent a DC current of say, 0.5V if connected, and 0V if disconnected. Or maybe it uses some electronic keepalive, like a certain patern in the voltage levels or so (would be a wast just to have a steady "0.5V"). 



  • It's probably going to vary by vendor, and be a range of tolerances.  Things like amps and attenuators raise or lower signal levels to within vendor spec on lots of copper and optical gear.

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