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Cisco ME 3800X and 3600X Switch Software Configuration Guide
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Chapter 27 Configuring QoS
Understanding QoS
By choosing different values for the MPLS experimental field, you can mark packets based on their
characteristics, such as rate or type, so that packets have the priority that they require during periods of
congestion.
Figure 27-4
shows an MPLS network that connects two sites of an IP network that belongs to a customer.
Figure 27-4
MPLS Network Connecting Two Customer Sites
PE1 and PE2 are customer-located routers at the boundaries between the MPLS network and the IP
network and are the ingress and egress provider-edge devices. CE1 and CE2 are customer edge devices.
P1 and P2 are service provider routers within the core of the service-provider network.
Packets arrive as IP packets at PE1, the ingress provider-edge router, and PE1 sends the packets to the
MPLS network as MPLS packets. Within the service-provider network, there is no IP precedence field
for the queuing mechanism to look at because the packets are MPLS packets. The packets remain as
MPLS packets until they arrive at PE2, the egress provider-edge router. PE2 removes the label from each
packet and forwards the packets as IP packets.
Service providers can use MPLS QoS to classify packets according to the type, input interface and other
factors by setting (marking) each packet within the MPLS experimental field without changing the IP
precedence or DSCP field. You can use the IP precedence or DSCP bits to specify the QoS for an IP
packet and use the MPLS experimental bits to specify the QoS for an MPLS packet. In an MPLS
network, configure the MPLS experimental field value at PE1 (the ingress router) to set the QoS value
in the MPLS packet.
It is important to assign the correct priority to a packet. The packet priority affects how the packet is
treated during periods of congestion. For example, service providers have service-level agreements with
customers that specify how much traffic the service provider has agreed to deliver. To comply with the
agreement, the customer must not send more than the agreed-upon rate. Packets are considered to be
in-rate or out-of-rate. If there is congestion in the network, out-of-rate packets might be dropped more
aggressively.
MPLS QoS matches only valid MPLS packets. On input, the match is performed before any label
processing on the packet. On output, the match is performed on the final packet after all label operations
are performed. See the
“Configuring MPLS and EoMPLS QoS” section on page 27-53
.
CE1
122012
PE-CLE
PE1
P1
P2
PE-CLE
CE2
PE2
IP
network
MPLS
network
IP
network
MPLS
network
Host A
Host B
Owned by
service provider
Located at customer site
Located at customer site
