Thanks.
>A VLAN is, by definition, a separate subnet.
Well, not by any definition that I've yet read :)
But, I was essentially asking *why* it has to be a different subnet.
That is not discussed anywhere that I've read.
But, anyway, as I posted, I think that the answer is ARP.
If ARP broadcast is not forwarded then we'll not be able to find the MAC
address of a destination IP outside our own VLAN (at least not without
Proxy ARP -- and we've just introduced a router, again !!!
-------------------------------------------------
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
=================================================
-----Original Message-----
From: John Neiberger [mailto:[EMAIL PROTECTED]]
Sent: Tuesday, January 16, 2001 12:48 PM
To: Bob Vance; [EMAIL PROTECTED]
Subject: Re: why is routing needed with VLANs
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 the
correct 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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