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VLAN vs subnetting

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Anonymous
October 4, 2004 8:50:47 AM

Archived from groups: comp.dcom.lans.ethernet (More info?)

Hi All,

I am not sure if this is the right forum to ask this question.
Please excuse me if so.

I would like to know when one should design networks using VLAN vs subnets
if one has multilayer switches. What r the considerations?

Thanks,
ASM

More about : vlan subnetting

Anonymous
October 4, 2004 4:03:07 PM

Archived from groups: comp.dcom.lans.ethernet (More info?)

asm wrote:
> Hi All,
>
> I am not sure if this is the right forum to ask this question.
> Please excuse me if so.
>
> I would like to know when one should design networks using VLAN vs subnets
> if one has multilayer switches. What r the considerations?
>
> Thanks,
> ASM

Whaddya mean? Typically you do both - each vlan is made into it's own
subnet if your switch supports layer 3 routing...
Anonymous
October 4, 2004 5:56:26 PM

Archived from groups: comp.dcom.lans.ethernet (More info?)

"asm" <arut@post.com> wrote:

> Hi All,
>
> I am not sure if this is the right forum to ask this question.
> Please excuse me if so.
>
> I would like to know when one should design networks using VLAN vs
> subnets
> if one has multilayer switches. What r the considerations?

Good question. But first, "subnetting" and VLANs are not mutually
exclusive. You can create IP subnets out of VLANs or not.

In my view, VLANs are most useful if you need to create IP subnets that
span large areas. For example, if you want to create a number of
separate campus-wide subnets, as opposed to subnets confined to
individual buildings or individual floors in buildings of a campus, then
a good Link Layer technique to achieve this would be use of VLANs.

An alternative would be to create a completely separate set of physical
LANs which span the campus. That's probably still the most secure
approach, if security is paramount, but it requires more labor to
install all the cable and the greater number of switches and routers in
the combined core networks. Not to mention more work to administer all
these network components.

Another somewhat similar alternative might be use of MPLS (multiprotocol
label switching, RFC 3031), which is conceptually similar to VLANs, but
operates at the Network Layer. MPLS creates shortcuts between IP subnets
that you might want to be tied closer together, perhaps to achieve
faster response times. With MPLS, you would likely end up creating more
IP subnets than the VLAN approach, but you could then create
label-switched paths through the core of the campus network. These LSPs
route packets between certain IP subnets more efficiently than normal IP
routing would do.

Bert
Related resources
Anonymous
October 4, 2004 7:17:28 PM

Archived from groups: comp.dcom.lans.ethernet (More info?)

asm <arut@post.com> wrote:
> Hi All,

> I am not sure if this is the right forum to ask this question.
> Please excuse me if so.

> I would like to know when one should design networks using VLAN vs subnets
> if one has multilayer switches. What r the considerations?

Forget about the term "multilayer switches", it's sales critter language.

If you on the other hand have level-3 devices ( = routers) it's perfectly
doable.

Not all devices needs to be routers, it's perfectly doable to
use trunks to a router where routing will be done between VLAN's.

> Thanks,
> ASM

--
Peter Håkanson
IPSec Sverige ( At Gothenburg Riverside )
Sorry about my e-mail address, but i'm trying to keep spam out,
remove "icke-reklam" if you feel for mailing me. Thanx.
October 5, 2004 12:26:40 AM

Archived from groups: comp.dcom.lans.ethernet (More info?)

"asm" <arut@post.com> wrote in message
news:c8b782e3.0410040350.28f1d45a@posting.google.com...
> Hi All,
>
> I am not sure if this is the right forum to ask this question.
> Please excuse me if so.
>
> I would like to know when one should design networks using VLAN vs subnets
> if one has multilayer switches. What r the considerations?

subnets usually correspond to VLANs for IP if you are building a routed
network - if you use other protocols then there might be other layer 3
structure for each protocol, such as an IPX network per VLAN.

VLAN descriptions often assume that VLANs are "tagged" links between
switches, but VLANs are a common way of logically splitting up traffic
within a switch as well - the simplest ways are to assign a port to a VLAN,
so all traffic on a port is logically tagged inside the switch, or to use a
port as tagged port where the VLAN labels appear outside the switch.

VLANs can be a lot more complicated than that - some switches can put
traffic into a specific VLAN based on MAC address, IP source address,
protocol type and so on - so you can have a layer 3 structure for 2
protocols that are different.

For example, you might have a good reason to select big networks for IPX,
lots of small subnets for IP, and layer 2 only flat structure for non routed
protocols such as NetBEUI and SNA. Or your layer 3 switch may only route
IPv4, so you take IPv6 or appletalk "subnets" down a trunk to an external
software based router.

Most of the applications seem to cause more problems than they solve, and
since most protocols apart from IP are dying out, this is less of an issue
than it used to be.
>
> Thanks,
> ASM
--
Regards

Stephen Hope - return address needs fewer xxs
!