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Ghazan Haider 04-07-2004 02:24 AM

Breaking a switched network
 
We have 4x 10/100 switches and are switching to cisco switches. There
are 80 machines with heavy database and file transfer use, latency is
not the issue, throughput is. Now being switches (3com and cisco),
they direct traffic properly as opposed to a hub, but the netbios
broadcasts still light up all LEDs. Ethereal showed these and other
broadcasts are about 2.5% of the overall traffic.

Now to add maybe 20 more computers to make it 100+ machines, should I
break everything into VLANS, add a gigabit hub between the switches or
do something else? We wont go beyond 120 computers in the next
conceivable 5-7 years so scalability isnt an issue either.

I read somewhere breaking everything into VLANS will do it, I dont see
the logic for it (CCNP exams if I remember, VLANS are good for large
networks). Is there a special function to redirect all broadcasts to a
single MAC address?

Secondly, which of cisco's 10/100 switches offer high-speed trunks,
beyond gigabit? Are 24-port/48-port gigabit switches cheap enough to
simply move to them for the future?

Thanks for any input.

Walter Roberson 04-07-2004 03:24 AM

Re: Breaking a switched network
 
In article <2f57764a.0404061824.3384ab6b@posting.google.com >,
Ghazan Haider <ghazan@ghazan.haider.name> wrote:
:Secondly, which of cisco's 10/100 switches offer high-speed trunks,
:beyond gigabit?

One of the C3750 line has a couple of 10 gigabit links as I recall.
--
Disobey all self-referential sentences!

Hansang Bae 04-07-2004 04:34 AM

Re: Breaking a switched network
 
In article <2f57764a.0404061824.3384ab6b@posting.google.com >,
ghazan@ghazan.haider.name says...
> We have 4x 10/100 switches and are switching to cisco switches. There
> are 80 machines with heavy database and file transfer use, latency is
> not the issue, throughput is. Now being switches (3com and cisco),
> they direct traffic properly as opposed to a hub, but the netbios
> broadcasts still light up all LEDs. Ethereal showed these and other
> broadcasts are about 2.5% of the overall traffic.


That's nothing to worry about. Broadcasts were a *huge* problem when
the PCs were 486-33 variety. In fact, I still remember the December
when Doom first came out and brought down our (then bridged) campus
network multiple times a day.

Today, with the CPU of the PCs, broadcasts are not much of a problem.


> Now to add maybe 20 more computers to make it 100+ machines, should I
> break everything into VLANS, add a gigabit hub between the switches or
> do something else? We wont go beyond 120 computers in the next
> conceivable 5-7 years so scalability isnt an issue either.


Why throw in routing to the mix? If you don't need it, don't bother
with it. Having 120 PCs in a subnet is not a huge deal.


> I read somewhere breaking everything into VLANS will do it, I dont see
> the logic for it (CCNP exams if I remember, VLANS are good for large
> networks).


It only makes sense if you need to segment the network at the IP level
(better security, control, etc.)


> Is there a special function to redirect all broadcasts to a
> single MAC address?


Not sure what you mean here.


> Secondly, which of cisco's 10/100 switches offer high-speed trunks,
> beyond gigabit? Are 24-port/48-port gigabit switches cheap enough to
> simply move to them for the future?


Only if your PCs are new enough that they can take advantage of it.
Otherwise, it would be a waste of money. But having the servers on GigE
may help.


--

hsb

"Somehow I imagined this experience would be more rewarding" Calvin
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Chris Thomas 04-07-2004 02:51 PM

Re: Breaking a switched network
 
In article <2f57764a.0404061824.3384ab6b@posting.google.com >,
ghazan@ghazan.haider.name says...
Secondly, which of cisco's 10/100 switches offer high-speed trunks,
> beyond gigabit? Are 24-port/48-port gigabit switches cheap enough to
> simply move to them for the future?


I suggest you look seriously at the 3750. There is a model which has
24 gig ports, plus 1-4 gig uplinks. (There is a similar model with 10
gig uplinks, just announced.) The 3750s are stackable, which means
you can build a 48, 72, 96 etc. port switch. The big advantage of
the 3750 over other models is that it has a 32 gb backplane, and the
stack interconnect is also 32 gb, so you don't have to choke on a
slow interswitch link. The price point of the 3750 is low enough
that it makes rolling out desktop gb entirely reasonable, which is
what we're doing. You do have to worry about overbooking the uplink,
but you can use up to 4 gb links on each switch in a stack, so you
can get a lot of uplink bw if you really need it. Stacks can be mix
and match, so you could have one 24p gb along with a 48p 10/100, for
example.

All Macs and most PCs being sold today come with gb nic cards. Heck,
my laptop (Thinkpad) has a gb nic. Many will only do about 600 mb/s
(PCI bus limitation), and gb is not going to be ten times as fast as
FE, but it may well be 4-6 times as fast. For our applications
(intensive number crunching), it makes a worthwile difference.

/Chirs, UCLA Academic Tech Svcs

Ghazan Haider 04-07-2004 07:07 PM

Re: Breaking a switched network
 
Thanks everyone for the response...


> Why throw in routing to the mix? If you don't need it, don't bother
> with it. Having 120 PCs in a subnet is not a huge deal.


I thought having 120PCs simultaneously on a subnet would be bad. If
120 FE connections to switches interconnected with gbit connections is
good in a single subnet, I wont have to do much here.

> > Is there a special function to redirect all broadcasts to a
> > single MAC address?

>
> Not sure what you mean here.


Most broadcasts are either DHCP or NetBIOS. They're both answered by
one server with a single MAC address. Is there a way broadcast
datagrams can be forwarded to a single MAC address... say map
192.168.0.255 to one MAC address rather than send it out to everyone?
That will make things very efficient and shouldnt take much processing
on behalf of the router.

> > Secondly, which of cisco's 10/100 switches offer high-speed trunks,
> > beyond gigabit? Are 24-port/48-port gigabit switches cheap enough to
> > simply move to them for the future?

>
> Only if your PCs are new enough that they can take advantage of it.
> Otherwise, it would be a waste of money. But having the servers on GigE
> may help.


Thats a good idea, one server is already running off an uplink gigabit
port. I'll have to read more whitepapers before moving to workstations
with 64-bit pci gbit cards and appropriate switches. cisco gbit
baseline switches are pricey compared to a few others however.

Walter Roberson 04-07-2004 08:52 PM

Re: Breaking a switched network
 
In article <2f57764a.0404071107.25f38b7a@posting.google.com >,
Ghazan Haider <ghazan@ghazan.haider.name> wrote:
:Most broadcasts are either DHCP or NetBIOS. They're both answered by
:one server with a single MAC address. Is there a way broadcast
:datagrams can be forwarded to a single MAC address... say map
:192.168.0.255 to one MAC address rather than send it out to everyone?
:That will make things very efficient and shouldnt take much processing
:on behalf of the router.

I don't know. You might be able to fudge it somehow. But is it
a good idea?

Broadcasts include:
- ARP (address resolution protocol). Expected to be answered by the device
that has the target MAC
- DHCP. Expected to be answered by all/most DHCP servers listening on the
subnet. It is true that tne non-servers do not "really" need to hear
these packets
- NETBIOS. If you are running NETBIOS, then every NETBIOS-running machine
needs to hear the NETBIOS broadcasts. That's how resources get locked
in NETBIOS: a station that wants to use a resource broadcasts a
"I want this resource" message, and if it does NOT get back a reply within
a certain timeframe, it assumes that nothing else cares; if a system
is using the resource, it instead sends back a "Sorry, that resource is
busy" message.
--
Are we *there* yet??

Terry Baranski 04-08-2004 12:10 AM

Re: Breaking a switched network
 
On 7 Apr 2004 12:07:48 -0700, ghazan@ghazan.haider.name (Ghazan
Haider) wrote:

>Thanks everyone for the response...
>
>
>> Why throw in routing to the mix? If you don't need it, don't bother
>> with it. Having 120 PCs in a subnet is not a huge deal.

>
>I thought having 120PCs simultaneously on a subnet would be bad. If
>120 FE connections to switches interconnected with gbit connections is
>good in a single subnet, I wont have to do much here.


It depends on the network's traffic profile. If traffic flow is such
that the inter-switch links get overloaded, EtherChannel can be a nice
fix. But this in of itself doesn't have anything to do with
subnetting -- separating devices onto different networks just moves
the bottleneck to layer 3 (routers).

>Most broadcasts are either DHCP or NetBIOS. They're both answered by
>one server with a single MAC address. Is there a way broadcast
>datagrams can be forwarded to a single MAC address... say map
>192.168.0.255 to one MAC address rather than send it out to everyone?
>That will make things very efficient and shouldnt take much processing
>on behalf of the router.


I'm with Hansang in that the effect of broadcasts on modern networks
tends to be overstated. IMO, total bandwidth usage is what you should
concern yourself with and 2.5% worth of broadcasts isn't significant.

-Terry

Hansang Bae 04-08-2004 03:57 AM

Re: Breaking a switched network
 
In article <2f57764a.0404071107.25f38b7a@posting.google.com >,
ghazan@ghazan.haider.name says...
> I thought having 120PCs simultaneously on a subnet would be bad. If
> 120 FE connections to switches interconnected with gbit connections is
> good in a single subnet, I wont have to do much here.


Usually not a major deal. Otherwise, everyone would use a /25 mask.


> Most broadcasts are either DHCP or NetBIOS.


DHCP needs to be a broadcast because you don't know what the server's IP
is. I don't believe you're running pure NetBIOS. If you were, you
wouldn't be talking about IP. But it looks like you're talking about
NetBIOS over TCP (NBT). Mostly, it's used for finding resources and can
be controlled by setting the proper node type and or using a WINS
server. Again, in a 120 PC subnet, I don't think it's a big deal.



> They're both answered by
> one server with a single MAC address. Is there a way broadcast
> datagrams can be forwarded to a single MAC address... say map
> 192.168.0.255 to one MAC address rather than send it out to everyone?
> That will make things very efficient and shouldnt take much processing
> on behalf of the router.


You're making much ado about nothing. You don't want to do this because
broadcast frames are there for a reason (for the most part). Things
like dhcp, arp etc., can break if you don't watch out.


> Thats a good idea, one server is already running off an uplink gigabit
> port. I'll have to read more whitepapers before moving to workstations
> with 64-bit pci gbit cards and appropriate switches. cisco gbit
> baseline switches are pricey compared to a few others however.


Cisco is synonymous with pricey. Depending on the model, you do get
what you paid for. But you should definitely consider other vendors
like Nortel, HP and (gasp) Dell.


--

hsb

"Somehow I imagined this experience would be more rewarding" Calvin
*************** USE ROT13 TO SEE MY EMAIL ADDRESS ****************
************************************************** ******************
Due to the volume of email that I receive, I may not not be able to
reply to emails sent to my account. Please post a followup instead.
************************************************** ******************

chris@nospam.com 04-08-2004 06:33 AM

Re: Breaking a switched network
 
On Thu, 08 Apr 2004 03:57:20 GMT, Hansang Bae <uonr@alp.ee.pbz> wrote:

>In article <2f57764a.0404071107.25f38b7a@posting.google.com >,
>ghazan@ghazan.haider.name says...
>> I thought having 120PCs simultaneously on a subnet would be bad. If
>> 120 FE connections to switches interconnected with gbit connections is
>> good in a single subnet, I wont have to do much here.

>
>Usually not a major deal. Otherwise, everyone would use a /25 mask.
>
>
>> Most broadcasts are either DHCP or NetBIOS.

>
>DHCP needs to be a broadcast because you don't know what the server's IP
>is. I don't believe you're running pure NetBIOS. If you were, you
>wouldn't be talking about IP. But it looks like you're talking about
>NetBIOS over TCP (NBT). Mostly, it's used for finding resources and can
>be controlled by setting the proper node type and or using a WINS
>server. Again, in a 120 PC subnet, I don't think it's a big deal.



DHCP traffic is probably not significant. The major component of his
broadcast traffic is probably browser traffic. Setting up a WINS
server will probably help this quite a bit.

-Chris


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