Campus lan and Wireless lan solution Design Guide

 
Example oversubscription ratio – access-layer switch stack with 25 Gbps uplinks 
Figure 6. 

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For example, instead of deploying four 48-port switches with fixed 4 x 10 Gbps uplinks in a switch stack 
configuration, you may want to deploy four 48-port switches along with a 2 x 25 Gbps uplink module in two of 
the switches within the stack.
This configuration provides up to 100 Gbps uplink bandwidth in a MEC configuration, when both switches with 
uplinks are operational. Fewer uplink ports are required at the distribution layer StackWise Virtual pair, and 
fewer optical fiber pairs are needed between the distribution layer StackWise Virtual pair and each of the access 
layer switch stacks. However, when deciding to upgrade the uplink speeds between the access and 
distribution-layer switches, you should keep in mind the following: 
● 
The optical transceiver modules which connect the distribution layer switches to the access layer switch 
platforms have to interoperate with each other and have to be compatible with the fiber optic building 
distribution cabling – multimode fiber (MMF) or single mode fiber (SMF).
Note:
Cisco offers a gradual migration path with the support of dual-rate optics, where the same 25 Gbps 
optics can operate at both 10 Gbps and 25 Gbps speeds. With this approach, distribution layer devices can be 
upgraded to 25 Gbps while the access layer still operates at 10 Gbps, and the access layer switches can be 
upgraded over a period of time. 
● 
The distances supported by multimode fiber typically decrease as speeds increase. This means that if 
you are upgrading from 10 Gbps to 25 Gbps between the distribution layer and access layer switches, 
and you have building distribution cabling consisting of multimode fiber (OM2, OM3, or OM4), you have to 
determine if the cabling will support the distances between the main IDF (which may house the 
distribution layer switches) and the floor IDFs (which may house the access layer switches) at the higher 
speeds. Keep in mind losses due to patch cables between the building distribution cabling and the actual 
equipment within the racks. 
Note:
The Cisco 10/25GBASE-CSR Module supports a link length of up to 300/400m over OM3/4 at 10G, and 
up to 300/400m over OM3/4 at 25G – depending upon the multimode fiber quality. It also supports link lengths 
of 82m over OM2 at 10G, and up to 70m over OM2 at 25G. This module requires RS-FEC on the host port for 
full reach operation at 25G. Using BASE-R FEC the module can support 70/100m over OM3/4 and with-out FEC 
it can support 30/50m over OM3/4 at 25G, depending on the multimode fiber quality. For 10G operation FEC is 
not required 
● 
If you have older multimode fiber (OM1), speeds above 10 Gbps may not be supported.
Single mode fiber generally supports both higher transmission rates and longer distances than multimode fiber.