IP Quality of Service

MPLS QoS 223

The CEF table shows two equal cost paths taken to reach 222.222.222.3. The local binding

for the prefix is 31, and it distributes to all its LDP neighbors. The remote bindings show

the local label bindings distributed by the LDP adjacent routers. The MPLS router label

switches packets received with a local label with an outgoing label based on the received

remote binding information.

MPLS QoS

QoS is an important component of MPLS. In an MPLS network, QoS information is carried

in the label header’s MPLS CoS field.

Like IP QoS, MPLS QoS is achieved in two main logical steps, as shown in Table 9-2, and

uses the same associated QoS functions. Figure 9-7 depicts the QoS functions used in an

MPLS network.

MPLS uses the same IP QoS functions to provide differentiated QoS for traffic within

an MPLS network. The only real difference is that MPLS QoS is based on the CoS bits

in the MPLS label, whereas IP QoS is based on the IP precedence field in the IP header.

On an ATM backbone with an MPLS-enabled ATM switch, the switch can support MPLS

CoS in two ways:

• Single Label Switched Path (LSP) with Available Bit Rate (ABR) service

• Parallel LSPs with Label Bit Rate (LBR) service

via 210.210.210.18, Hssi1/0, 0 dependencies

traffic share 1

next hop 210.210.210.18, Hssi1/0

valid adjacency

0 packets, 0 bytes switched through the prefix

Listing 9-11 Label Binding Information on Prefix 222.222.222.3 in Router SanFrancisco

SanFrancisco#show mpls ldp bindings

LIB entry: 222.222.222.3/32, rev 8

local binding: label: 31

remote binding: lsr: 222.222.222.2:0, label: 26

remote binding: lsr: 222.222.222.4:0, label: 29

Listing 9-12 Label-Based Forwarding Information on Prefix 222.222.222.3 in Router SanFrancisco

SanFrancisco#show mpls forwarding-table

Local Outgoing Prefix Bytes label Outgoing Next Hop

label label or VC or Tunnel Id switched interface

31 26 222.222.222.3/32 0 Hs1/1 point2point

29 222.222.222.3/32 0 Hs1/0 point2point

Listing 9-10 CEF Entry for Prefix 222.222.222.3 in Router SanFrancisco (Continued)

224 Chapter 9: QoS in MPLS-Based Networks

A single LSP using ATM ABR service can be established through LDP. All MPLS traffic

uses the same ABR LSP, and the differentiation is made on the ingress routers to the ATM

cloud by running WFQ and WRED algorithms on traffic going over an LSP.

Multiple LSPs in parallel can be established through LDP to support traffic with multiple

precedence values. Each established LSP is mapped to carry traffic of certain MPLS CoS

values. The LSPs use the LBR ATM service. LBR is a new ATM service category that relies

on scheduling and discarding in the ATM switch based on WFQ and WRED, respectively,

and hence is more appropriate for IP.

Table 9-2 MPLS QoS

Step

Place of

Application

Applicable

QoS

Functions QoS action

1 Ingress (edge)

router to the

MPLS cloud

Committed

Access Rate

(CAR)

(Option 1) CAR polices traffic on the ingress

router for all incoming IP traffic entering the

MPLS cloud. It sets an IP precedence value

for traffic according to the traffic profile and

policies. The IP packet’s IP precedence value

is copied into the MPLS CoS field.

(Option 2) CAR polices traffic on the ingress

router for all incoming IP traffic entering the

MPLS cloud. It sets an MPLS CoS value for

traffic according to the traffic profile and

contract.

The precedence value in the IP header is left

unchanged end-to-end, unlike Option 1.

2 Entire MPLS

network

Weighted Fair

Queuing (WFQ),

Weighted

Random Error

Detection

(WRED)

Traffic differentiation based on the MPLS CoS

field in the MPLS backbone using the IP QoS

functions WFQ and WRED.