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QoS with MPLS in an AVVID-Enabled Network

This chapter discusses the ability of MPLS VPNs to support QoS functionality for AVVID, whether the
MPLS backbone is an SP-owned or customer-owned network. It includes the following:

Overview

Considerations for MPLS VPN QoS

Implementing MPLS VPN QoS

Overview

Traditionally, VPNs for site-to-site connectivity have been provided as either Layer 2 networks (such as
a Frame Relay PVC), as IPSec tunnels across Layer a 3 network (such as the Internet), or as a
combination of the two. MPLS was designed to provide for a more robust Layer 3 VPN offering,
allowing for overlapping address space, additional hierarchy of routing, and traffic engineering.

MPLS is a new forwarding mechanism in which packets are forwarded based on labels. Labels can
correspond to IP destination networks (equal to traditional IP forwarding) or to other parameters, such
as QoS or source address.

Unlike in Layer 3 networks, with MPLS only edge routers must perform a routing lookup. Although all
the devices within the network participate in the routing architecture (control plane), the core routers
switch packets based on simple label lookups and swap labels, instead of the traditional IP address
lookup in a forwarding database. A core router can be an MPLS-capable traditional Layer 3 router or a
Layer 2 device, such as an ATM switch that is enabled to participate in the MPLS control plane. With
MPLS, ATM switches forward cells based on a knowledge of the dynamic Layer 3 architecture of the
network, rather than on static ATM PVC configurations.