[rescue] Long network paths

[rescue at sunhelp.org]

~ -----Original Message-----
~ From: Joshua D Boyd [mailto:jdboyd at cs.millersville.edu]
~ Sent: Tuesday, August 06, 2002 11:12 AM
~ To: rescue at sunhelp.org
~ Subject: Re: [rescue] Long network paths
~ 
~ 
~ On Tue, Aug 06, 2002 at 08:07:36AM -0700, Fogg, James wrote:
~ 
~ > And, really big LANs usually need to have the broadcast 
~ domain broken up by
~ > routers and VLAN's.
~ > 
~ > For the home datacenter though, it mostly doesn't matter 
~ (except for the
~ > 4-hub/repeater rule, its a killer). For the networker-geek 
~ home network
~ > (mine) one must run at least 3 VLAN's.
~ 
~ Just out of curiosity, what is the point of VLANs at home?  I never
~ really understood all that well what they are for in the first place,
~ which doesn't help.

DANGER- really long pedantic reply with some good info.

There is a scaleability issue with Ethernet, and thats the collision domain.
A collision domain consists of all devices on an Ethernet bus
(thinnet/10base2) or connected by repeaters (hubs and media converters are
repeaters too). All these devices share the same "communications space" from
a layer 1 (physical) perspective. When one talks, everybody else hears it
nearly instantaneously and there is nothing to prevent/traffic-cop it. When
you have too many devices in a collision domain you spend too much time
retransmitting lost packets. This is the basis of the "you don't get better
than 70% utilization" maxim for Ethernet. As you approach 70% collisions
increase and you spend more time resending than actually moving data. As you
pass ~70% the problem increases exponentially. Also, there is no "magic
number" of hosts/devices per collision domain. It depends on how chatty the
devices are.

Collision domain boundries are established by devices that don't repeat, but
store-and-forward instead. When S-A-F devices "duplicate" traffic on another
interface they must comply with the Ethernet standard and "listen and wait"
before transmitting. This stops the collision problem. Layer 2 switches are
an example of an S-A-F device. Because of this, we say that a switch "breaks
up" a collision domain. Each port on a switch is its own collision domain.

There is a scalability issue with TCP/IP (and other protocols, namely
netbios and IPX), and that's the broadcast domain. Broadcasts are sent with
a MAC of all zeros, so all Ethernet cards will forward the packet into the
O/S to process. This will cause the O/S and hardware to stop and process the
info. If you have buttloads of broadcast traffic your network attached hosts
will literally slow down noticably. Both repeaters and switches will forward
broadcasts, so you have to stop broadcasts at layer 3 (routing). Please note
that some protocols (SNA and netbios are two) don't have a layer 3 and
cannot be routed (Source Route Bridging is used).

VLAN's are a play on the abilities of the Spanning Tree protocol. STP is
used by switches to prevent path loops. A loop would happen if you connected
two ports of a switch together, or connected two switches together by two
paths. In this picture when a broadcast came along it would be endlessly
duplicated on all the switchports (a broadcast storm). STP checks paths for
loops and places a redundant port in a blocked (no forwarding) state. STP
continually checks all ports and will unblock (state = forwarding) the
redundant port if the 1st port link goes away. In reality STP is a little
slicker than this, but this is getting long. In VLAN's STP information is
divided up among specific (configurable) groups of ports and STP data isn't
shared among the groups. Each group (VLAN) becomes independant. The effect
is similar to partitioning a hub (remember partitionalble hubs?). Since
there is NO layer 2 traffic passed between VLAN's, you need a layer 3 device
(router) to span VLAN's. This serves to "break up" your broadcast domain
since routers don't pass broadcasts.

There is a lot more to this stuff, such as VLAN trunking, etc. but I'm outa
room and time. I really dig networking and could go on forever with this
crap. Sorry to have bored anyone. I hate to sound pedantic "(/woods)".