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I am studying for my CCNA. The book makes a statement that I do not completely understand:

"Layer 2 segmentation at the distribution layer practically eliminates network diameter as an issue."

I'm not sure what it means by "Layer 2 segmentation". Are they talking about VLANS? If so, how does this eliminate network diameter as an issue?

This is on page 7 (last paragraph) of LAN Switching and Wireless (CCNA Exploration), Cisco Press.

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up vote 2 down vote accepted

Distribution layer is layer 3, right? So yes, they are talking about limiting the size of your layer 2 network. Remember that broadcasts get sent to everyone on your layer 2 network, so segmenting it with routers or layer 3 capable switches helps keep things flowing. VLANs are one way to achieve this. Also, STP can cause issues when the layer 2 domain is too big. Just remember that switching is great, but routing is better suited for large networks.

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Core is 1, Distribution is 2 and Access is layer 3. I'm going with your response, because I think it explains what the book means by "Layer 2 segmentation." Any time I see "Layer 2," especially with the capital 'L', I assume they are talking about the Data Link Layer of the OSI model. Thanks for the clarification. – Kedeki Nov 19 '10 at 17:55
I think by Layer2, the book exactly means the MAC layer (OSI Data Link) not the Cisco distribution layer. They must mean : MAC layer segmentation of the distribution layer using a L3 device such as a router or a L3 switch. This results in breaking up a broadcast domain into smaller ones. Imagine 10 switches trunked together on the distribution layer. The network diameter in this case is 10 since a packet may have to cross 10 switches before getting to its final destination. In this case, L2 frames are forwarded between switches. Now, consider each of these switches is connected to a router or – user105036 Dec 24 '11 at 6:55

Layer 2 segmentation refers to VLANs, or Bridges that connect different Layer 2 (data link layer of the OSI reference model) segments.

nTier references within Cisco speak - i.e. Core layer, Access layer, Distribution/Aggregation layer are informal and have no direct association to the OSI reference model at all.

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I will assume TCP/IP over Ethernet. TCP/IP & Ethernet networks are typically segmented based on several factors with the most being user base or end point base. I.E. How many IP end points that need very fast access to one another being the chief concern. Other factors also come into play too... User population, etc... but let's focus on performance or speed.

The reason I focus on speed is because of the statement of "eliminates network diameter as an issue." Network diameter is mostly a speed issue at hand.

The way you achieve the best or fastest possible speed or bandwidth is to minimize the number of devices the traffic must flow through (routers and switches) and lowering the number of devices connected to a device collapsing the end points (switch); that is I'm assuming a star configuration here.

By reducing hops, you eliminate latency and I/O. You also have physical constraints you eliminate such as fiber to copper, etc... By reducing the number of devices, you reduce switch I/O (ARP lookups)

So the diameter is often the size in terms of end points and devices your endpoints need to talk via.

The common rule of thumb you often see in TCP/IP & Ethernet books is that the faster the network, the smaller the network diameter. Network diameter being the two things mentioned above (hop count, end points per subnet).

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