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.

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