VLANs, routing, multicast and HP switches, oh my...

[Kurt Buff]

On Mon, Jun 14, 2010 at 05:41, Ivan Voras <ivoras at freebsd.org> wrote:
> On 06/12/10 23:22, Kurt Buff wrote:
>
>> Again - they'll be putting up to 200 busy machines on each subnet. It
>> seems reasonable to limit the broadcast domains with VLANs.
>
> I know that everyone begins to talk about "limiting the broadcast
> domains" when talking about VLANs sooner or later but I have never
> managed to learn exactly why this would be the biggest benefit of using
> VLANs.
>
> Except if you are explicitly researching broadcast communication, the
> only times a modern Ethernet will see broadcast packets is:
>
> 1) ARP packets when the machines are brought up or contacted the first time
> 2) router announcements, RIP & similar
> 3) Windows NetBIOS / Windows Networking workgroup name resolving
> (analogous to ARP).
>
> Is there really so much broadcast traffic of these categories in a
> network of 200 machines? And except if you are going to divide VLANs so
> that each has a dedicated set of switches and cabling, with each VLAN
> consisting of a dozen machines or so, many of these broadcast packets
> will travel through the same cables and the same switch so you won't
> magically get better performance out of it. You won't get away from
> routing announcements and routing IP between VLANs will also result in
> ARP requests on the destination side.

I knew I should be explaining this better. I can only plead lack of
time - I'm being rushed for lots of things at work and home at the
moment, so haven't spent as much care on the explanation as I should
have. My apologies for that.


We'll be simulating installations of our software and hardware for
customer installations that have WANs between sites, with several
complementary applications, including a multicast app that is critical
to the whole effort. While it's a bit much to expect us to be able to
simulate a WAN at this point, I want to be able to simulate at least
two subnets with routed multicast between them.

One of the subnets will have as many as 200 simulated hosts on it, the
others perhaps not so much. The majority of these machines will be
Windows-based, so I expect broadcast traffic to be higher - but I also
take your point about the packets traveling over the same wire. It
would be best if I could get a multiport router - perhaps a layer3
switch (I'd love to get them an HP 3400cl) - but that costs much money
that I don't have to spend at the moment.

I'll set up the VLANs on that port and see how it goes.

Kurt