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Wired Networks Thread, Core Switch issues in Technical; As for the issue: MAC address table is needed to know which devices are on which ports ARP table maps ...
  1. #16
    DMcCoy's Avatar
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    As for the issue:

    MAC address table is needed to know which devices are on which ports
    ARP table maps IPs to MAC addresses and is needed to know which IPs are on which VLAN/port.
    If you run out, then you will has issues as you have found, it may not learn any new MAC/ip locations until it removes or expires an old one

    You may be able to help slightly with a shorter arp timeout before it's removed, but this is not a good solution.

    If you had been *just* over then you may get away with 120 second timeout or so, but over twice the number of machines as entries is going to be a problem. At least if it is a 1910, it was cheap.
    Last edited by DMcCoy; 7th September 2013 at 11:08 PM.

  2. Thanks to DMcCoy from:

    sluggster66 (9th September 2013)

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    twin--turbo's Avatar
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    Quote Originally Posted by DMcCoy View Post
    As for the issue:

    MAC address table is needed to know which devices are on which ports
    ARP table maps IPs to MAC addresses and is needed to know which IPs are on which VLAN/port.
    If you run out, then you will has issues as you have found, it may not learn any new MAC/ip locations until it removes or expires an old one

    You may be able to help slightly with a shorter arp timeout before it's removed, but this is not a good solution.

    If you had been *just* over then you may get away with 120 second timeout or so, but over twice the number of machines as entries is going to be a problem. At least if it is a 1910, it was cheap.
    It will learn a new address almost instantly, but that requires a broadcast. As every clinet sends a request any destination not in the ARP table will cause a broadcast. As this continues the switch will get bogged down sending, reciving and processsing arp entries and will grind to a halt very quickly. It's CPU will spend time on constant persistance ARP work and not data transfer.

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    Update:

    The switch has been changed and everything working again.

    Thanks for all the help.

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