Campus lan and Wireless lan solution Design Guide

Oversubscription ratios 
For three-tiered designs, increasing the uplink speeds between the access and distribution layer switches may 
also require increasing uplink speeds between the distribution and core layer switches in order to maintain the 
desired oversubscription ratio.
Determining the oversubscription ratio of the uplink between the distribution and core layer switches is fairly 
straight forward. You need to take into consideration the number of ports connecting the distribution layer 
switches to the access layer switches or switch stacks, as well as the speeds at which the ports are operating. 

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Oversubscription between the distribution and core layer switches – 40 Gbps uplinks 
Figure 11. 
For example, let’s say your distribution layer switch is a StackWise Virtual pair that supports a building with 4 
floors. Each floor has two IDFs (wiring closets). Each IDF has an access layer switch stack consisting of four 
48-port switches along with a 2 x 25 Gbps uplink module in two of the switches within the stack. The total 
number of 25 Gbps ports required at the distribution layer switches is 4 uplinks x 2 IDFs per floor x 4 floors = 32 
ports.
This configuration would provide up to 32 x 25 Gbps = 800 Gbps bandwidth between the distribution layer and 
access layer switches. Simply keeping existing 2 x 40 Gbps uplinks would only provide up to 80 Gbps between 
the distribution layer and core layer switches. This would provide an oversubscription ratio of 800:80 or 10:1 
between the distribution and core layers. Depending upon your business requirements, this may be insufficient. 
Increasing Uplink Speeds 
You could choose to add additional 40 Gbps links between the distribution and core layer switches, possibly 
operating in a Layer 3 EtherChannel configuration. However, this would require additional 40 Gbps switch ports 
at every distribution layer and core layer switch. More importantly, it would require additional fiber optic pairs 
between the distribution layer switches and the core layer switches.
In a large campus deployment, the core layer switches may be located in a centralized data center in a different 
building. If insufficient optical pairs exist, then additional optical cabling would need to be pulled between the 
centralized data center and each of the buildings. This could be a very expensive proposition, as existing 
conduit space between the buildings may not be capable of supporting additional cabling, and you run the risk 
of damaging the existing cabling in the conduit – resulting in an extended outage. Installing new conduit may 
involve getting the necessary right-of-way to trench and install underground conduit – on top of the cost to 
install the new fiber optic cable. 
An alternative may be to upgrade the uplink speeds between the distribution layer and core layer switches to 
100 Gbps. 

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