Typical campus networks are designed with oversubscription, as illustrated in Figure 2-18. The rule-of-thumb recommendation for data oversubscription is 20:1 for access ports on the access-to-distribution uplink. The recommendation is 4:1 for the distribution-to-core links. When you use these oversubscription ratios, you may make congestion on the uplinks an infrequent occurrence. QoS is needed for these occasions. If congestion is occurring frequently, the design does not have sufficient uplink bandwidth.
The above passage was taken from the Cisco ARCH. Can some explain what the ratio means? Thanks.
It means if you have 200 1 gigabit switchports at the access layer, you should have 10 gigabits of uplink speed to the distribution layer. (so 1 gigabit of uplink for every 20 gigabits of access ports), and when linking distribution to core, if you have 80 gigabits of distribution uplinks to the access layer, you should have 20 gigabits of uplink bandwidth to the core layer.
The idea being, that even if you have 100 gigabits of switch ports at the access layer, it's not very likely that all of them will be used at the same time. So at a 20:1 ratio, 100 gigabit ports would mean a 5 gigabit uplink from the access layer to the distribution layer, and a 1.25 gigabit uplink from the distribution layer to the core layer.
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Another way at looking at these ratios is they account for how much of the time we expect certain ports to be busy. For example, 20:1 for access ports expects, on average, each port will only generate traffic 5% of the time. If that's true, then on average, an uplink of the same capacity should be able to handle 20 such ports (20 x 5% = 100%).
It must be remembered that these ratios are, as you note, rules-of-thumb, and can be very much incorrect in their estimation of port usage. For example, the bandwidth usage of a workstation which maintains most of its file on its own disk storage is much different from one that stores all its files on a server. Similar differences exist between workstations that download a file, work on it in local memory, then save it (e.g. word processor) vs. a CADD workstations that constantly edits a CADD file on the server.
These ratios can also change depending on edge port bandwidths, i.e. 10 vs. 100 vs. gig. Everything else being equal, faster edge ports, for the same traffic, have less utilization. I.e. 20:1 at 10 Mbps might be 2000:1 at gig. (The latter is something hardware vendors don't mention as these rules-of-thumb often date from 10 Mbps days.) NB: BTW, everything isn't equal as new user network applications tend to consume more bandwidth although generally not 100 times more.
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