Marking Network Traffic

Improved Network Performance

Traffic marking allows you to fine-tune the attributes for traffic on your network. This increased granularity helps single out traffic that requires special handling, and thus, helps to achieve optimal application performance.

Traffic marking allows you to determine how traffic will be treated, based on how the attributes for the network traffic are set. It allows you to segment network traffic into multiple priority levels or classes of service based on those attributes, as follows:

• Traffic marking is often used to set the IP precedence or IP DSCP values for traffic entering a network. Networking devices within your network can then use the newly marked IP precedence values to determine how traffic should be treated. For example, voice traffic can be marked with a particular IP precedence or DSCP and low latency queuing (LLQ) can then be configured to put all packets of that mark into a priority queue. In this case, the marking was used to identify traffic for LLQ.
• Traffic marking can be used to identify traffic for any class-based QoS feature (any feature available in policy-map class configuration mode, although some restrictions exist).
• Traffic marking can be used to assign traffic to a QoS group within a router. The router can use the QoS groups to determine how to prioritize traffic for transmission. The QoS group value is usually used for one of the two following reasons:
  • To leverage a large range of traffic classes. The QoS group value has 100 different individual markings, as opposed to DSCP and Precedence, which have 64 and 8, respectively.
  • If changing the Precedence or DSCP value is undesirable.
• If a packet (for instance, in a traffic flow) needs to be marked to differentiate user-defined QoS services is leaving a router and entering a switch, the router can set the CoS value of the traffic, because the switch can process the Layer 2 CoS header marking. Alternatively, the Layer 2 CoS value of the traffic leaving a switch can be mapped to the Layer 3 IP or MPLS value.
• Weighted random early detection (WRED) uses precedence values or DSCP values to determine the probability that the traffic will be dropped. Therefore, the Precedence and DSCP can be used in conjunction with WRED.

The following is an example of creating a class map to be used for marking network traffic. In this example, a class called class1 has been created. The traffic with a Frame Relay DLCI value of 500 will be put in this class.

Router> enable

Router# configure terminal

Router(config)# class-map class1

Router(config-cmap)# match fr-dlci 500

Router(config-cmap)# end 

In the following example, the table-map (value mapping) command has been used to create and configure a table map called table-map1. This table map will be used to establish a to-from relationship between one traffic-marking value and another.

In table-map1, a traffic-marking value of 0 will be mapped to a value of 1.

Router> enable
Router# configure terminal
Router(config)# table-map
 table-map1 map from 0 to 1

Router(config-tablemap)#
 end

Policy Map Configured to Use set Command

The following is an example of creating a policy map to be used for traffic marking. In this example, a policy map called policy1 has been created, and the set dscpcommand has been configured for class1.

Router> enable
Router# configure terminal
Router(config)# policy-map policy1
Router(config-pmap)# class class1
Router(config-pmap-c)# set dscp 2
Router(config-pmap-c)# end

Policy Map Configured to Use a Table Map

A policy map called policy1 has been created and configured to use table-map1 for setting the precedence value. In this example, the CoS value will be set according to the DSCP value defined in table-map1 created previously.

Router(config)# policy map policy1

Router(config-pmap)# class class-default

Router(config-pmap-c)#
 set cos dscp table table-map1

Router(config-pmap-c)#
 end

Note

As an alternative to configuring the set cos dscp table table-map1 command shown in the example, you could configure the command without specifying the table keyword and the applicable table-map-name argument (that is, you could configure the set cos dscpcommand). When the command is configured without the table keyword and applicable table map name, the values are copied from the specified categories. In this case, the DSCP value is copied and used to set the CoS value. When the DSCP value is copied and used for the CoS value only the first 3 bits (that is, the class selector bits) of the DSCP value will be used to set the CoS value. For example, if the DSCP value is EF (101110), the first 3 bits of this DSCP value will be used to set the CoS value, resulting in a CoS value of 5 (101).

Policy Map Configured to Use a Table Map for Mapping MPLS EXP Values

This section contains an example of a policy map configured to map MPLS experimental (EXP) values. The figure below illustrates the network topology for this configuration example.

Figure 2

Network Topology for Mapping MPLS EXP Value

For this configuration example, traffic arrives at the input interface (an Ethernet 1/0 interface) of the ingress label edge router (LER). The precedence value is copied and used as the MPLS EXP value of the traffic when the MPLS label is imposed. This label imposition takes place at the ingress LER.

The traffic leaves the ingress LER through the output interface (an Ethernet 2/0 interface), traverses through the network backbone into the MPLS cloud, and enters the egress LER.

At the input interface of the egress LER (an Ethernet 3/0 interface), the MPLS EXP value is copied and used as the QoS group value. At the output interface of the egress LER (an Ethernet 4/0 interface), the QoS group value is copied and used as the precedence value.

To accomplish configuration described above, three separate policy maps were required--policy1, policy2, and policy3. Each policy map is configured to convert and propagate different traffic-marking values.

The first policy map, policy1, is configured to copy the precedence value of the traffic and use it as the MPLS EXP value during label imposition.

Router(config)# policy-map policy1

Router(config-pmap)# class class-default

Router(config-pmap-c)#
 set mpls experimental imposition precedence

Router(config-pmap-c)#
 end

When the traffic leaves the LER through the output interface (the Ethernet 2/0 interface), the MPLS EXP value is copied from the precedence value during MPLS label imposition. Copying the MPLS EXP value from the precedence value ensures that the MPLS EXP value reflects the appropriate QoS treatment. The traffic now proceeds through the MPLS cloud into the egress LER.

A second policy map called policy2 has been configured to copy the MPLS EXP value in the incoming MPLS traffic to the QoS group value. The QoS group value is used for internal purposes only. The QoS group value can be used with output queueing on the output interface of the egress router. The QoS group value can also be copied and used as the precedence value, as traffic leaves the egress LER through the output interface (the Ethernet 4/0 interface).

Router(config)# policy-map policy2

Router(config-pmap)# class class-default

Router(config-pmap-c)#
 set qos-group mpls experimental topmost 

Router(config-pmap-c)#
 end

A third policy map called policy3 has been configured to copy the internal QoS group value (previously based on the MPLS EXP value) to the precedence value. The QoS group value will be copied to the precedence value as the traffic leaves the egress LER through the output interface.

Router(config)# policy-map policy3

Router(config-pmap)# class class-default

Router(config-pmap-c)#
 set precedence qos-group

Router(config-pmap-c)#
 end

Configuring these policy maps as shown (and attaching them to interfaces as shown in Example Attaching the Policy Map to an Interface), causes the appropriate quality of service treatment to be preserved for the traffic as the traffic progresses along an IP network, through an MPLS cloud, and back again into an IP network.

Note

This configuration could also have been accomplished by first creating a table map (used to map one value to another) and then specifying the table keyword and table-map-name argument in each of the set commands (for example, set precedence qos-group table tablemap1). In the MPLS configuration example, a table map was not created, and the set commands were configured without specifying the table keyword and table-map-name argument (for example, set precedence qos-group). When the set commands are configured without specifying the table keyword and table-map-name argument, the values are copied from the specified categories. In this case, the QoS group value was copied and used to set the precedence value. When the DSCP value is copied and used for the MPLS EXP value, only the first 3 bits (that is, the class selector bits) of the DSCP value will be used to set the MPLS value.

The following is an example of attaching the policy map to the interface. In this example, the policy map called policy1 has been attached in the input direction of the Serial4/0 interface.

Router> enable
Router# configure terminal
Router(config)# interface serial4/0
Router(config-if)# service-policy input policy1 
Router(config-if)# end

The following is an example of configuring QoS when using IPsec VPNs. In this example, the crypto map command specifies the IPsec crypto map (mymap 10) to which the qos pre-classify command will be applied.

Router> enable
Router# configure terminal
Router(config)# crypto map mymap 10
 
Router(config-crypto-map)# qos pre-classify
Router(config-crypto-map)# exit