Campus lan and Wireless lan solution Design Guide

Uplink queuing 
The actual uplink oversubscription ratio that you implement between your access and distribution layer switches 
is dependent upon your business requirements. Uplink ratios up to 20:1 between the access and distribution 
layer switches, and 4:1 between the distribution layer and core switches have been implemented in networks. 
The higher the oversubscription ratio, the higher the probability that temporary or transient congestion of the 
uplink may occur if multiple devices transmit or receive simultaneously. 
Catalyst 9000 Series switches dynamically allocate buffer space across switch ports and queues in order to 
optimize the utilization of the existing buffer space. However, switch buffer space is a finite quantity that 
depends on the model of the Catalyst 9000 Series switch platform (specifically the version of the UADP ASIC 
and the number of UADP ASICS within the switch platform). During periods of temporary congestion the buffers 
on individual switch ports and queues may become exhausted, resulting in packet loss.
It is recommended to implement queuing on switches, where oversubscription of ports can result in transient 
congestion, which can cause packet loss. Queuing ensures that when congestion occurs, your higher priority 

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traffic which is more susceptible to packet loss – such as voice traffic – is given preferential treatment over 
lower priority traffic. This preserves the quality of experience for your business critical applications, such as 
voice. 
Access layer variations 
In some situations, such as for IoT and for multi-dwelling unit (MDU) deployments, the access layer is often 
augmented with additional cascaded switches. For Cisco Software-Defined Access (described later) access 
extension deployments these access devices may be extended node switches. For MDU deployments the 
devices may be small distributed access switches or gigabit passive optical network (GPON) optical network 
termination devices. Network designs for these and other variations of the access layer are outside the scope of 
this guide.