Re: VLAN Question

[Bennett Todd <bet () rahul net>]

2003-08-20T20:30:02 David Gillett:
> > To help people get the most out of their switch investments,
> > VLANs allowed partitioning broadcast domains, to buy the
> > performance advantages of switch isolation while allowing
> > multiple smaller networks to be implemented on the same
> > expensive switch.
>
>   I can't buy this.

Sorry, that I can't help.

> I don't think there was ever a time when a chassis switch with four
> 12-port cards cost less than four separate 12-port switches.

VLANs were crafted for the opposite case; after you bought your very
expensive switch, you could have one LAN that used most of its
ports, and the remainder could be allocated to other LANs, rather
than being left empty.

>   And if all VLANs did was allow your one big expensive switch to
> emulate a stack of cheap little switches, almost nobody would ever
> use them.

VLANs were invented when there was no such thing as a "cheap little
switch". Switches were very very expensive, and weren't sold in
4-port or 6-port sizes for small nets.

>   Where partitioning of switches into VLANs starts to pay off is 
> where you have (a) trunking of multiple VLANs from switch to switch,
> and (b) router blades for switch chasses, to route between VLANs.

You're talking today.

I was discussing where VLANs came from, why they were first
implemented --- because that historical background drove the early
implementations and support plan. Leakage between VLANs used to be
normal, expected, and ignored by all --- as long as the leakage were
small enough to not constitute a performance issue. Switch vendors
wouldn't take a bug report about such leakage. VLANs weren't
security barriers.

Things have changed.

-Bennett

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