May I amend that first statement? Switches should replace hubs so that you
can have dedicated bandwidth per port rather than shared bandwidth. I don't
think switches should replace routers, however. That philosophy is what led
us to the need for kludges like VLANs. ;-) Note that I use the term switch
to mean a Layer-2 switch. If it does Layer 3 stuff and marketing still
calls it a switch, then that's a different story.
Priscilla
At 12:59 AM 7/25/01, Priscilla Oppenheimer wrote:
>At 10:39 PM 7/24/01, Guy Russell wrote:
> >Cisco recommends that a subnet to VLAN relationship exist, but you are
> >correct, it is not necessary, nor is it in practice...
> >
> >If this was the case, then we would not need expensive switches in
addition
> >to routers to "subnet our network... we would save some money, and simply
> >buy a router, maybe add some access lists to it, and voila...
>
>You don't need switches. You didn't actually buy that marketing stuff did
>you? I didn't. ;-) Switches were supposed to be faster, less expensive, and
>easier to configure than routers. Well, they aren't. See more controversial
>statements below...
>
> >A Vlan is an administrative boundry. The purpose is to reduce traffic on
> >segments, both collision and broadcast, and to provide some security.
"most
> >companies who implement this do so to restrict access via departments..
> >
> >So I would not teach it as a subnet, but just as it sounds... a Virtual
> >LAN..
> >
> >appearing to be an independant lan, but actually is part of an existing
> >Lan...
> >
> >Not to hard to understand...
> >
> >----- Original Message -----
> >From: "Peter Van Oene"
> >To:
> >Sent: Tuesday, July 24, 2001 9:06 PM
> >Subject: Re: VLAN routing [7:13465]
> >
> >
> > > I personally feel that the concept of VLANs, simple as it is, is
>extremely
> > > misunderstood by most people in the industry and for that reason, I
>strive
> > > to present it accurately. Consider these common questions:
> > >
> > > Do I need a router between my VLANs?
>
>If you want the VLANs to communicate with each other. Are these trick
>questions? ;-) I realize there are cases where you don't want them to
>communicate. I guess that is what you are getting at.
>
> > > Do I need an IP address on my VLANs?
>
>Some sort of network-layer addressing is required for end stations to
>communicate using typical applications. There are some cases where
>network-layer addressing is not used, of course, but that sort of
>communication is being phased out.
>
> > > Can I route between VLAN 1 and VLAN 2 with just a switch?
>
>No, not a Layer 2 switch.
>
> > > Can I have multiple subnets on the same VLAN?
>
>Yes, but they won't communicate without a router. A station trying to
>communicate with a station in a different subnet ARPs for its default
>gateway. Sure there are exceptions with strangely behaving IP stacks and
>errors with subnet mask configurations, etc., but let's consider the
>typical case.
>
> > >
> > > I've heard these types of questions many times and all of them would be
> > > reduced if people stopped thinking about VLANs in a layer three
context.
> >If
> > > one could prevail upon students the concepts of collision and broadcast
> > > domains and the role routers play in bounding and interconnecting them,
> >the
>
>The collision and broadcast domain concepts are easy. (Well, they should
>be.) Understanding traffic flow and protocol behavior is the problem.
>
> > > concept of VLANs would be a very logical next step. Telling people
that
> > > VLANs are subnets simply confuses the matter and in reality is anything
> >but
> > > the truth. Certainly a one to one mapping of VLANs to IP subnets would
>be
> > > ideal, but is not always the case nor is a requirement.
>
>No, it's not always a requirement, but it often is.
>
> > >
> > > In IP training, I do see the value of having people think about VLANs
> > > containing single IP subnets, but I still feel you must make sure
people
> > > understand the exclusivity of the two concepts.
>
>Yes, I agree with that. But from a design and troubleshooting point of
>view, it's important to consider how they affect each other also.
>
> >IP is still pretty low on
> > > the chart of most popular protocols in use on end stations last I
checked
> >:)
>
>I doubt that. What else is in use? AppleTalk is being turned off on many
>networks. Besides, it behaves essentially like IP, with AARP, etc. NetWare
>5 is IP. NetWare 4 end stations broadcast a GetNearestServer and hence are
>affected by VLAN implementation. NetBIOS runs on TCP/IP these days. Using
>NetBEUI instead is considered a bad idea. I agree that legacy stuff never
>goes away so it's still out there, but IP has won on campus networks and,
>of course, on the most-widely used WAN (the Internet). Tell me if I'm
>wrong, but that's what I see in my consulting practice.
>
>It's been a long day. If I'm full of it (that might get past the filters!
>;-), I apologize.
>
>Priscilla
>
> > >
> > > Pete
> > >
> > >
> > >
> > > *********** REPLY SEPARATOR ***********
> > >
> > > On 7/24/2001 at 5:47 PM Priscilla Oppenheimer wrote:
> > >
> > > >At 01:03 PM 7/24/01, Peter Van Oene wrote:
> > > >>I beg to differ slightly on the concept of VLANS. A VLAN, as I'm
sure
> >you
> > > >>know, is a broadcast domain and makes no assumption of nor has any
> > > >>dependance upon layer three protocols.
> > > >
> > > >I agree in theory. But in practice a VLAN is an IP subnet (on IP
> >networks)
> > > >and it helps to think of it this way when designing and
troubleshooting.
> >I
> > > >get this from Dan Farkas, CCIE and CCSI, author of switching papers at
> > > >Certificaton Zone. http://www.certificationzone.com. (No, this is not
> > > >another ad for that great service. ;-) But Dan says that he encourages
> >his
> > > >students to use the terms broadcast domain, subnet, and VLAN
> > > >interchangeably. It helps them understand traffic flow, etc. I think
it
> > > >might help the poster....
> > > >
> > > >Priscilla
>________________________
>
>Priscilla Oppenheimer
>http://www.priscilla.com
________________________
Priscilla Oppenheimer
http://www.priscilla.com
Message Posted at:
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