Carrier Ethernet

Hello,

they often say, for example http://en.wikipedia.org/wiki/Carrier_Ethernet
that such concepts as "bridge" or "spanning tree" don't scale well to large
networks. What exactly is meant here by "scale" ?

Thanks.

Mark

 

Anyone remember back in the 80's when the telecoms thought that they
could build nationwide networks as a single (or maybe a few large) ATM
cloud?

 

Hi, Mark

Bridges are indeed used in network providers.

Spanning tree is generally not used. If the provider is adding a
bridge, STP would disrupt its network too long time, to converge.

In Ethernet WAN, we consider connectionless paths (CL) and oriented
connection paths (CO). With CO paths, address learning process is
useless.

Best regards,
Michelot

 

Bridging has some tables and overheads that grow faster than linear
with the number of connections and devices across the entire set of
indirectly connected switches.

Every end point connected to a bridged system has at least 1 MAC
address, and any traffic to that device implies that MAC has entries
in some or all the devices that make up the network.

This means linear scaled up table space is needed, but the maintenance
overheads that occur grow more quickly as well.

The other practical problem is that a network needs to be stable to
operate well, and in practice that means that you need ways to limit
changes to parts of the system so their effect stay local.

in bridging a topology change can propagate across the system.

Think of the hierarchy in routing as being selective information
hiding.

Instead of 1 entry per MAC address for 100,000 devices in table for a
big corporate network, you may need just 1 or a few routing entries
saying "xxx corp is that way".

This means any topology change in xxx corp stays within their system -
the rest of the system doesnt need to see it

 

Hi Stephen,

In the user plane, a bridge is not a MAC end point.

Best regards,
Michelot

 

Wording quibble. Given the loaded nature of the word "broadcast" it
might be better to put that as "frames get transmitted out all
connected ports" or "frames get flooded out all connected ports" lest
some new folks think the frames get transformed into actaul broadcast
frames.

rick jones

 

The meaning of "scale" is in the sense of how one quantity must
change when you change another.

One can, for example, discuss the change in the size of the legs
of an animal with its weight. That is:

"How does an animal's leg scale with its weight?"

In many interesting and useful cases, the answer is an exponent.

How does a tree trunk diameter scale with tree height?

How does the speed of a transistor scale with its size?

How does the cost of a transistor scale with it speed or size?

Doesn't scale means that something else happens, such that
the scaling laws aren't useful anymore.

Transistors (and animals) don't scale well down to atomic size.
They also don't scale well up to planetary size.

-- glen