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Rewriting Packet Headers to Ensure Forwarding Behavior

 

As packets enter or exit a network, edge routers might be required to alter the class-of-service (CoS) settings of the packets. Rewrite rules set the value of the CoS bits within the packet’s header. Each rewrite rule reads the current forwarding class and loss priority information associated with the packet, locates the chosen CoS value from a table, and writes this CoS value into the packet header.

In effect, the rewrite rule performs the opposite function of the behavior aggregate (BA) classifier used when the packet enters the routing device. As the packet leaves the routing platform, the final CoS action is generally the application of a rewrite rule.

You configure rewrite rules to alter CoS values in outgoing packets on the outbound interfaces of an edge router to meet the policies of a targeted peer. This allows the downstream routing device in a neighboring network to classify each packet into the appropriate service group.

In addition, you often need to rewrite a given marker (IP precedence, Differentiated Services code point [DSCP], IEEE 802.1p, or MPLS EXP settings) at the inbound interfaces of an edge router to accommodate BA classification by core devices.

Figure 1 shows a flow of packets through four routing devices. Router A rewrites the CoS bits in incoming packet to accommodate the BA classification performed by Routers B and C. Router D alters the CoS bits of the packets before transmitting them to the neighboring network.

Figure 1: Packet Flow Across the Network
Packet Flow Across the Network

For every incoming packet, the ingress classifier decodes the ingress CoS bits into a forwarding class and packet loss priority (PLP) combination. The egress CoS information depends on which type of rewrite marker is active, as follows:

  • For Multiprotocol Label Switching (MPLS) EXP and IEEE 802.1 rewrite markers, values are derived from the forwarding class and PLP values in rewrite rules. MPLS EXP and IEEE 802.1 markers are not preserved because they are part of the Layer 2 encapsulation.

  • For IP precedence and DiffServ code point (DSCP) rewrite markers, the marker alters the first three bits on the type-of-service (ToS) byte while leaving the last three bits unchanged.

To configure CoS rewrite rules, you define the rewrite rule and apply it to an interface. Include the following statements at the [edit class-of-service] hierarchy level: