Enterprise QoS Solution Reference Network Design Guide

Chapter 2 Campus QoS Design
Version 3.3
Enterprise QoS Solution Reference Network Design Guide
QoS Design Overview
It is quite rare under normal operating conditions for campus networks to suffer congestion. And if
congestion does occur, it is usually momentary and not sustained, as at a WAN edge. However, critical
applications like VoIP still require service guarantees regardless of network conditions.
The only way to provide service guarantees is to enable queuing at any node that has the potential for
congestion—regardless of how rarely, in fact, this may occur. The potential for congestion exists in
campus uplinks because of oversubscription ratios and speed mismatches in campus downlinks (for
example, GigabitEthernet to FastEthernet links). The only way to provision service guarantees in these
cases is to enable queuing at these points.
Queuing helps to meet network requirements under normal operating conditions, but enabling QoS
within the campus is even more critical under abnormal network conditions such as DoS/worm attacks.
During such conditions, network traffic may increase exponentially until links are fully utilized. Without
QoS, the worm-generated traffic drowns out applications and causes denial of service through
unavailability. Enabling QoS policies within the campus, as detailed later in this chapter, maintains
network availability by protecting and servicing critical applications such as VoIP and even Best Effort
traffic.
The intrinsic interdependencies of network QoS, High Availability and security are clearly manifest in
such worse-case scenarios.
So where is QoS required in campus?
Access switches require the following QoS policies:
Appropriate (endpoint-dependant) trust policies, and/or classification and marking policies
Policing and markdown policies
Queuing policies.
Distribution and core switches require the following QoS policies:
DSCP trust policies
Queuing policies
Optional per-user microflow policing policies (only on supported platforms)
These recommendations are summarized in Figure 2-2.
2-3