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.
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:
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; } } } }
The network administrator verifies the adjusted rates.
user@host#show class-of-service traffic-control-profile 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:
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; } } } }
Verify the adjusted rates.
user@host#show class-of-service traffic-control-profile 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.