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
Message Posted at:
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