Cisco Catalyst 3750 Software Configuration Manual page 731

Chapter 30 Configuring MPLS and EoMPLS
Configuring MPLS and EoMPLS QoS
A service provider might also want to set a QoS value for an MPLS packet to a different value. Instead
of overwriting the value in the IP precedence field that belongs to a customer, the service provider can
set the MPLS experimental field. The IP header remains available for the customer's use, and the QoS
of an IP packet is not changed as the packet travels through the MPLS network.
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 30-6 shows an MPLS network that connects two sites of an IP network that belongs to a customer.
Figure 30-6 MPLS Network Connecting Two Customer Sites
MPLS MPLS
network network
IP IP
network network
Host A Host B
CE1 PE1 P1 P2 PE2 CE2
Owned by
service provider
PE1 and PE2 are at the boundaries between the MPLS network and the IP network and are the ingress
and egress PE 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 PE router. PE2 removes the label from each packet and
forwards the packets as IP packets.
MPLS QoS enables service providers to classify packets according to their 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 priority of a packet 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.
Catalyst 3750 Metro Switch Software Configuration Guide
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