How to Plan Forwarding Behavior

Forwarding behavior determines the priority and drop precedence of traffic flows that are about to be forwarded onto the network. You can choose two major forwarding behaviors: prioritizing the flows of a class in relationship to other traffic classes or dropping the flows entirely.

The diffserv model uses the marker to assign the chosen forwarding behavior to traffic flows. IPQoS offers the following marker modules.

• dscpmk, which you use to mark the DS field of an IP packet with a DS codepoint.

• dlcosmk, which you use to mark the VLAN tag of a datagram with a class of service (CoS) value.

The suggestions in this section refer specifically to IP packets. If your IPQoS system includes a VLAN device, you can use the dlcosmk marker to mark forwarding behaviors for datagrams. For more information, refer to Using the dlcosmk Marker With VLAN Devices.

To prioritize IP traffic, you need to assign a DS codepoint to each packet. The dscpmk marker marks the DS field of the packet with the DS codepoint. You choose the DS codepoint for a class from a group of well-known codepoints that are associated with the forwarding behavior type. These well-known codepoints are 46 (101110) for the EF PHB and a range of codepoints for the AF PHB. For overview information on DS codepoints and forwarding, refer to Traffic Forwarding on an IPQoS-Enabled Network.

The next steps assume that you have defined classes and filters for the QoS policy. Though you often use the meter with the marker to control traffic, you can use the marker alone to define a forwarding behavior.

1. Review the classes that you have created thus far and the priorities that you have assigned to them.

   Not all traffic classes need to be marked.

2. Assign the EF per-hop behavior to the class with the highest priority.

   The EF PHB guarantees that packets with the EF DS codepoint 46 (101110) are released onto the network before packets that are marked with any AF PHBs. Use the EF PHB for your highest-priority traffic. For more information about EF, refer to Expedited Forwarding (EF) PHB.

3. Assign forwarding behaviors to classes that have traffic to be metered.

   Traffic is generally metered for the following reasons:

   • An SLA guarantees packets of this class greater or lesser service when the network is heavily used.

   • A class with a lower priority might have a tendency to flood the network.

   You use the marker with the meter to provide differentiated services and bandwidth management to these classes. For example, the following table shows a portion of a QoS policy that defines a class for a popular games application that generates a high level of traffic.

   Class	Priority	Filters	Selectors	Rate	Forwarding?
   games_app	9	games_in	sport 6080
   		games_out	dport 6081	meter=tokenmt committed rate=5000000  committed burst =5000000  peak rate =10000000  peak burst=15000000  green precedence=continue processing  yellow precedence=mark yellow PHB  red precedence=drop	green =AF31  yellow=AF42  red=drop

   The forwarding behaviors assign low-priority DS codepoints to games_app traffic that conforms to its committed rate or is below the peak rate. When games_app traffic exceeds peak rate, the QoS policy indicates that packets from games_app are to be dropped. A list of all AF codepoints is in table Table 6–2.

4. Assign DS codepoints to the remaining classes in agreement with the priorities that you have assigned to them.

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