Thank you for the information. I am stuggling with the use/purpose of VLANs
and you've answered some questions for me.
In article <[EMAIL PROTECTED]>,
[EMAIL PROTECTED] (John Neiberger) wrote:
>A VLAN is, by definition, a separate subnet. If you decided to separate a
>single LAN into two VLANs, you'll have to change your addressing scheme.
>Once you've done that, you have to route to get from one subnet to the
>other. I don't even like the term "VLAN". The very term seems to cause a
>lot of conceptual problems.
>
>For example, let's say you have one LAN and you decide to create a new VLAN
>for a total of two VLANs. This is absolutely no different than having two
>normal LANs on different ports on a router: you have two separate IP subnets
>and you must route to get from one to the other. The only difference is
>that you can use trunking to pass data for both subnets down the same wire,
>and you can then let a switch split that traffic up and send it to thcorrect
>ports.
>
>Imagine the router with two separate ethernet interfaces, each in its own
>subnet, and these are connected to two separate switches. There is no
>topological difference between that scenario and a router doing ISL or
>802.1q trunking to a switch that is configured for two VLANs. The
>requirements for connectivity are the same: you must have a router to get
>from one subnet to the other. Even though they are physically on the same
>switch, topologically speaking they are on different networks.
>
>I hope this makes sense. I had three people stop by my cube to talk and I
>had three phone calls while trying to write this. :-)
>
>Regards,
>John
>
>> OK.
>> I must be brain dead, today.
>> (and, yes, Chuck, I *have* had my morning dose of Diet Coke :)
>> and, yes, I know, "What's so special about 'today' "?
>> )
>> As far I can understand it so far, about the only benefit that I see
>> from VLANs is reducing the size of broadcast domains.
>>
>> Suppose that I have a switch in the closet with one big flat address
>> space (well, it couldn't be that big with only one switch, now, could
>> it ?>). Then someone says,
>> "You know, we're getting a lot of blah-blah broadcast traffic.
>> Let's VLAN.
>> "
>> OK, fine. We VLAN and put whatever services in each VLAN that are
>> required to handle the broadcasts (e.g., DHCP service). So, now the
>> switch doesn't send broadcasts outside a particular VLAN.
>>
>> But, what's so magic about a VLAN that the switch also decides not to
>> send unicasts outside a VLAN. Before the VLANs, the switch maintained
>> a MAC table and knew which port to go out to get to any unicast address
>> in the entire space. So, why can't it continue to do that after we
>> arbitrarily implement some constraint on broadcast addresses?
>> It seems to me that the same, exact MAC table, with an additional VLAN
>> field would not require that restriction. If it's a broadcast, send the
>> packet only out ports with a VLAN-id that matches the source port's
>> VLAN-id. If it's a unicast, handle it just like we used to.
>>
>>
>> Similarly, even if we have 5 switches, I just don't see the requirement
>> that we (as switch-code designers) must block unicasts and resort to a
>> routing requirement.
>>
>> Even with 500 switches ... well, let's not get ridiculous :)
>>
>>
>> I feel that there is a simple point that I've overlooked, so I will
>> continue to RTFM while I await your responses.>)
>>
>>
>> -------------------------------------------------
>> Tks??? ??? | <mailto:[EMAIL PROTECTED]>
>> BV??? ???? | <mailto:[EMAIL PROTECTED]>
>> Sr. Technical?Consultant,? SBM, A Gates/Arrow Co.
>> Vox 770-623-3430???????????11455 Lakefield Dr.
>> Fax 770-623-3429?????????? Duluth, GA 30097-1511
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>>
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