You're right, machines on one "vlan" would not be able to reach hosts on the
other side. Another problem would be connectivity to the router. If you
had a switch split up into two "vlans" that used the same ip subnet, you
would have to have two links connected to a router to allow them access to
the rest of the network, so you're going to need two different interfaces on
a router. But since you can't have two different interfaces in the same
subnet, you're up a creek without a paddle. Of course, you could enable
bridging, but then you've completely negated the purpose of the exercise.
With true VLANs, you would have to renumber the hosts, but then you could
have a single trunk connection to a router with one subnet per VLAN
subinterface.
-John
> 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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>
>
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