Re: Re: [fw-wiz] Vlan's as effective security measures?

From: Brian Ford (
Date: 02/12/04

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    Date: Thu, 12 Feb 2004 13:13:55 -0500


    And cars crash and cars burn and people are dying in cars all the
    time. And cars can be made to carry disease and explosives and kill many
    people with just one car and driver! So let's all abandon our cars and
    start walking to work every morning. If we're late the boss will
    understand because cars are dangerous. ;-)

    You should probably research the switch that you buy and use in order to
    make sure that it doesn't do these things.

    Your mileage may vary!

    Liberty for All,


    At 12:00 PM 2/10/2004 -0500, wrote:
    >Message: 4
    >Date: Mon, 09 Feb 2004 12:52:31 -0800
    >From: John Hall <>
    >To: "Ware, Larry" <>
    >Cc: "''"
    >Subject: Re: [fw-wiz] Vlan's as effective security measures?
    >1. A surprising number of network devices' VLAN implementations
    > will leak packets between VLANs under heavy loads, or in some
    > cases randomly all the time.
    >2, Some switches have a single forwarding database which includes
    > VLAN tags and a host presenting a carefully chosen MAC address
    > can sometimes hijack traffic for a host on another VLAN.
    >3. Some switches flood ARP requests across VLANs.
    >4. Some switches flood all traffic under heavy load.
    >5. Few switches and routers have adequate configuration security.
    >Don't depend on VLANs to guarantee the separation of two networks
    >that *must* be separated. Your security is only as good as the
    >weakest element in your infrastructure and the security of most
    >switches (and to a lesser extent routers) is pretty weak.
    >Ware, Larry wrote:
    > >Forgive a long out of field, and now working on getting back up to speed
    > >firewall admin, but would someone care to educate me concerning the security
    > >issues related to VLAN's? I have lots of them, and need to know why a VLAN
    > >is not an effective adjunct to firewall and router security policies.
    > >-larry
    > >

    Brian Ford
    Consulting Engineer, Security & Integrity Specialist
    Office of Strategic Technology Planning
    Cisco Systems Inc.

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