20-5
Cisco 10000 Series Router Quality of Service Configuration Guide
OL-7433-09
Chapter 20 Configuring Quality of Service for MPLS Traffic
MPLS QoS
MPLS QoS Implementation
When precedence-based weighted random early detection (WRED) is configured on an output policy
map and outgoing packets are MPLS packets, the router drops the MPLS packets based on the three EXP
bits in the MPLS label, instead of using the three bits of the IP precedence field in the underlying IP
packets.
When DSCP-based WRED is configured on an output policy map and outgoing packets are MPLS
packets, the router drops the MPLS packets based on the three EXP bits in the MPLS label, instead of
using the six bits of the DSCP field in the underlying IP packets. The router left shifts the three EXP bits
and makes it six bits. For example, if the value of the EXP bits is 5 (binary 101), the router converts them
to binary 101000 (makes it looks like six DSCP bits), and drops packets based on this value.
When configuring the set and police commands in a traffic class, regardless of whether it is an input or
output policy map, the police command is processed later than the set command. This means that the
values implemented by the police command override the values set by the set command. The value can
be IP precedence, DSCP, qos-group, MPLS experimental imposition, discard-class, or ATM CLP bit.
Discard-class is a number between 0 and 7; qos-group is a number between 0 and 63.
Restrictions and Limitations for MPLS QoS
The router does not support the set mpls experimental imposition topmost command.
Configuring MPLS QoS on the Ingress Label Switching Router
A label switching router (LSR) is an ingress provider edge (PE) router, a provider (P) router, or a
penultimate-hop provider router. Setting the MPLS EXP field is only valid for packets that arrive on a
non-MPLS interface of the LSR and leave on an MPLS interface. Therefore, only input service policies
can cause the MPLS EXP bits to be set when the packet goes out an MPLS interface. If the packet arrives
on an MPLS interface, setting the MPLS EXP field has no affect.
The IP header of an outbound IP packet determines the packet’s QoS. For general information, see the
Cisco IOS Quality of Service Solutions Configuration Guide.
The MPLS EXP field in the topmost label of an outbound MPLS packet determines the packet’s QoS.
For general information, see the MPLS Class of Service manual.
To configure MPLS QoS on the ingress LSR, perform the following configuration tasks to configure the
ingress label switching router:
• Classifying IP Packets Using a Class Map, page 20-6
• Setting the MPLS EXP Field Using a Policy Map, page 20-7
• Attaching an MPLS QoS Service Policy to an Interface, page 20-8
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