Enterprise QoS Solution Reference Network Design Guide

Version 3.3
WAN Aggregator QoS Design
CHAPTER
This chapter discusses WAN QoS considerations and designs, including the following:
Slow-speed (£ 768 kbps) WAN link design
Medium-speed (768 kbps to T1/E1 speed) WAN link design
Enterprise QoS Solution Reference Network Design Guide
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High-speed (> T1/E1 speed) WAN link design
Additionally, these designs are applied to specific Layer 2 WAN media, including the following:
Leased lines
Frame Relay
ATM
ATM-to-Frame Relay Service Interworking
ISDN
A fundamental principle of economics states that the more scarce a resource is the more efficiently it
should be managed. In an enterprise network infrastructure, bandwidth is the prime resource and also is
the scarcest (and, likewise, most expensive) over the WAN. Therefore, the case for efficient bandwidth
optimization using QoS technologies is strongest over the WAN, especially for enterprises that are
converging their voice, video, and data networks.
The design principles described in this chapter apply primarily to Layer 2 WANs, such as leased lines,
Frame Relay, and ATM (including ATM-to-Frame Relay Service Interworking). However, many service
providers use these Layer 2 WAN technologies to access Layer 3 VPN services. Therefore, many of the
design principles and examples presented in this chapter also apply to such VPN access scenarios.
This chapter provides design guidance for enabling QoS over the WAN. It is important to note that the
recommendations in this chapter are not autonomous. They are critically dependent on the
recommendations discussed in Chapter 2, “Campus QoS Design.”
Where Is QoS Needed over the WAN?
Within typical WAN environments, routers play one of two roles: a WAN aggregator or a branch router.
In some very complex WAN models, enterprises might have distributed WAN aggregators to cover
regional branches, but the role of such middle-tier routers is not significantly different from that of a
WAN aggregator located at a campus edge. This chapter focuses on WAN edge
recommendations—primarily for WAN aggregator routers, but these correspondingly apply to the WAN
edge designs of branch routers. QoS policies required on WAN edges are shown in Figure 3-1.
3-1