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have collisions! Full-duplex conversations should not have collisions, but in this case, e3 tried to talk to e2 and e2 was busy. That’s a collision. Figure 2-3 shows our collision in action. The kid brother is transmitting on e3, but e2’s output port is occupied, so the death metal is dropped. If only it were that simple in real life. Figure 2-3. Switch fabric collision If you think that this sounds ridiculous and doesn’t happen in the real world, you’re almost right. The reason it doesn’t seem to happen in the real world, though, is largely because Ethernet conversations are rarely always on, and because of buffers. In Figure 2-4, I’ve added input buffers to our simple switch. Now, when port e3 tries to transmit, the switch can detect the collision and buffer the packets until the output port on e2 be<strong>com</strong>es available. The buffers are like little answering machines for Ethernet packets. Now, when you hang up with your girlfriend, the death metal can be politely delivered in all its loud glory since the output port (you) is available. God bless technology. 14 | Chapter 2: Buffers
This is cool and all, but these input buffers are not without their limitations. Just as an answering machine tape (anyone remember those?) or your voicemail inbox can get full, so too can these buffers. When the buffers get full, packets get dropped. Whether the first packets in the buffer get dropped in favor of buffering the newest packets, or the newest packets get dropped in favor of the older packets is up to the guy who wrote the code. So if the buffers can get full, thus dropping packets, the solution is to put in bigger buffers, right? Well, yes and no. The first issue is that buffers add latency. Sending packets over the wire is fast. Storing packets into a location in memory, then referencing them and sending them takes time. Memory is also slow, although the memory used in these buffers is much faster than, say <strong>com</strong>puter RAM. It’s more like the L2 cache in your CPU, which is fast, but the fact remains that buffering increases latency. Increased latency is usually better than dropped packets, right? As usual, it depends. Figure 2-4. Switch fabric with input buffers Buffers | 15
Page 2 and 3: Want to read more? Spreading the kn
Page 4 and 5: Table of Contents Preface. . . . .
Page 6 and 7: Spanning Tree and MLAG 151 14. Firs
Page 8 and 9: Arista Support 377 31. Aristacisms.
Page 10 and 11: Figure 2-1. Simple switch fabric of
Page 14 and 15: Dropped packets might be OK for som
Page 16 and 17: Figure 2-6. Switch fabric with outp
Page 18 and 19: Figure 2-7. Head-of-line blocking V
Page 20 and 21: The kickin’ 10 Gbps data flow fro

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