HTH,
Thank you for these comments - this clears up a lot of confusion for me.
To sum, just to make sure I really have this:
Layer 3 switching is possible without VLANs (however the opposite is not
true. Well at least not without some form of Layer 3 intervention.)
VLANs simply the administration behind Layer 3 switching design.
Physical location (port location) independence is ok in front of the
layer 3 switch that is the the hosts gateway. Up to the hosts distribution
switch.
VLANs extending beyond the distribution layer switch across the core is
generally not a good idea - possible, but not recommended. This is the "flat
earth" design that Priscilla mentioned - VLANs that extend across the entire
internetwork.
Thanks!
Stephen Hoover
Dallas, Texas
----- Original Message -----
From: "The Long and Winding Road"
To:
Sent: Monday, February 17, 2003 6:00 PM
Subject: Re: Does MLS (Layer 3 switching) require VLANs? [7:63147]
> I've been following this thread, and have offered a comment or two along
the
> way. Perhaps I should offer some thoughts here at the source.
>
> note that I have not read any of the exam study materials in question, so
I
> don't know what is or is not being stated in the courseware. I can offer
> that just because it says so in the study materials doesn't mean that's
the
> way it is.
>
> comments below....
>
>
> ""Stephen Hoover"" wrote in message
> [EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> > I am studying for the CCNP Switching exam and it covers VLANs and layer
3
> > switching moderately. It states that Cisco recommends a 1 to 1 mapping
of
> > VLANs to subnets. It also states that VLANs can be used to break up
> > broadcast domains.
>
> this is a reasonable, simple approach, and thus one that appeals to my
> reasonably simple mind.
>
>
> >
> > When you create different subnets, you are already breaking up broadcast
> > domains, so does layer 3 switching require the use of VLANs to actually
do
> > the switching?
>
>
> this is where the confusion, no doubt introduced by the marketing people,
> set in.
>
> suppose you have a router with three ethernet interfaces, and each of
these
> interfaces is plugged into a different hub ( no switch )
>
> hosts on each of these hubs are in the same broadcast domain ( same
> collision domain too, but I digress ) hosts in each of these domains
cannot
> reach hosts ( or servers ) in other domians, on different hubs, without
> routing.
>
> this would be true, even if you had all hosts on the same great big hub
with
> 500 ports. You could have hosts on the same hub, but having different L3 (
> IP ) addresses. communication between hosts on different subnets, even if
> they are on the same hub, require the intrercession of a router.
>
> vlans, made possible by various 802.1 specifications, are really just a
way
> of expressing logical broadcast domains.
>
> layer 3 switching is really routing. an L3 switch has the routing function
> built into it, rather than using a separate piece of equipment.
>
>
> >
> > Say for instance I have 2 hosts on the same layer 3 switch, but the two
> > hosts are on 2 different IP subnets (No VLANs are defined). Host A wants
> to
> > talk to host B. Can the switch not look up the routing info and then
know
> to
> > switch to that port? I am not seeing where the requirement for the VLAN
> > comes into play.
>
> despite what others have said, you can do this. it is wasteful, in that a
> host plugged into an L3 port would require 4 ip addresses because you have
a
> subnet with two hosts ( the PC and the port, and the net number and the
> broadcast address ). whereas if you have a vlan, that vlan is a virutal
port
> that represents the physical ports as a single subnet to the L3 (
routing )
> function.
>
>
> >
> > If VLANs are required for layer 3 switching, is that pretty much
standard
> > across the industry, or that a Cisco only thing?
>
>
> forget this L3 switch versus router distinction. it is confusing, and
> misrepresentational.
>
> think instead in terms of how traffic moves through a network.
>
> think instead of a vlan as a virtual logical construct that represents one
> or more ports as a single broadcast domain to a router. it doesn't matter
> that the router is integrated into the switch hardware with an ASIC and
> code, or is an external device.
>
> HTH
>
>
> >
> > Thanks!
> > Stephen Hoover
> > Dallas, Texas
Message Posted at:
http://www.groupstudy.com/form/read.php?f=7&i=63208&t=63147
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