Technical Documentation

Example: Configuring Static Shaping Parameters to Account for Overhead in Downstream Traffic Rates

This topic describes two scenarios for which you can configure static shaping parameters to account for packet overhead in a downstream network.

Figure 1 shows the sample network that the examples reference.

Figure 1: Sample Network Topology for Downstream Traffic

Image g017442.gif

Managing Traffic with Different Encapsulations

In this example, the MX Series router shown in Figure 1 sends stacked VLAN frames to the DSLAM, and the DSLAM sends single-tagged VLAN frames to the residential gateway.

To accurately shape traffic at the residential gateway, the MX Series router must account for the different frame sizes. The difference between the stacked VLAN (S-VLAN) frames sent by the router and the single-tagged VLAN frames received at the residential gateway is a 4-byte VLAN tag. The residential gateway receives frames that are 4 bytes less.

To account for the different frame sizes, the network administrator configures the frame shaping mode with –4 byte adjustment:

  1. The network administrator configure the traffic shaping parameters and attaches them to the interface.

    Enabling the overhead accounting feature affects the resulting shaping rate, guaranteed rate, and excess rate parameters, if they are configured.

    [edit]class-of-service {traffic-control-profiles {tcp-example-overhead-accounting-frame-mode {shaping-rate 10m;shaping-rate-priority-high 4m;guaranteed-rate 2m;excess-rate percent 50;overhead-accounting frame-mode bytes -4;}}interfaces {ge-1/0/0 {output-traffic-control-profile tcp-example-overhead-accounting-frame-mode;}}}}
  2. The network administrator verifies the adjusted rates.

    Traffic control profile: tcp-example-overhead-accounting-frame-mode, Index: 61785
    Shaping rate: 10000000
    Shaping rate priority high: 4000000
    Excess rate 50
    Guaranteed rate: 2000000
    Overhead accounting mode: Frame Mode
    Overhead bytes: —4

Managing Downstream Cell-Based Traffic

In this example, the DSLAM and residential gateway shown in Figure 1 are connected through an ATM cell-based network. The MX Series router sends Ethernet frames to the DSLAM, and the DSLAM sends ATM cells to the residential gateway.

To accurately shape traffic at the residential gateway, the MX Series router must account for the different physical network characteristics.

To account for the different frame sizes, the network administrator configures the cell shaping mode with –4 byte adjustment:

  1. Configure the traffic shaping parameters and attach them to the interface.

    Enabling the overhead accounting feature affects the resulting shaping rate, guaranteed rate, and excess rate parameters, if they are configured.

    [edit]class-of-service {traffic-control-profiles {tcp-example-overhead-accounting-cell-mode {shaping-rate 10m;shaping-rate-priority-high 4m;guaranteed-rate 2m;excess-rate percent 50;overhead-accounting cell-mode;}}interfaces {ge-1/0/0 {output-traffic-control-profile tcp-example-overhead-accounting-cell-mode;}}}}
  2. Verify the adjusted rates.

    Traffic control profile: tcp-example-overhead-accounting-cell-mode, Index: 61785
    Shaping rate: 10000000
    Shaping rate priority high: 4000000
    Excess rate 50
    Guaranteed rate: 2000000
    Overhead accounting mode: Cell Mode
    Overhead bytes: 0

    To account for ATM segmentation, the MX Series router adjusts all of the rates by 48/53 to account for ATM AAL5 encapsulation. In addition, the router accounts for cell padding, and internally adjusts each frame by 8 bytes to account for the ATM trailer.


Published: 2010-04-15