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Bandwidth Management for Downstream Traffic in Edge Networks Overview

The overhead accounting feature enables you to shape traffic based on either frames or cells and assign a byte adjustment value to account for different encapsulations.

This feature is available on MIC and MPC interfaces.

Guidelines for Configuring the Shaping Mode

Frame shaping mode is useful for adjusting downstream traffic with different encapsulations. In frame shaping mode, shaping is based on the number of bytes in the frame, without regard to cell encapsulation or padding overhead. Frame is the default shaping mode on the router.

Cell shaping mode is useful for adjusting downstream cell-based traffic. In cell shaping mode, shaping is based on the number of bytes in cells, and accounts for the cell encapsulation and padding overhead.

When you specify cell mode, the resulting traffic stream conforms to the policing rates configured in downstream ATM switches, reducing the number of packet drops in the Ethernet network.

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.

Guidelines for Configuring Byte Adjustments

When the downstream traffic has different byte sizes per encapsulation, it is useful to configure a byte adjustment value to adjust the frame sizes. For example, you can configure the frame shaping mode and a byte adjustment value to account for differences in Layer 2 protocols for downstream Ethernet traffic.

We recommend that you specify a byte adjustment value that represents the difference between the CPE protocol overhead and B-RAS protocol overhead.

The system rounds up the byte adjustment value to the nearest multiple of 4. For example, a value of 6 is rounded to 8, and a value of –10 is rounded to –8.

You do not need to configure a byte adjustment value to account for the downstream ATM network. However, you can specify the byte value to account for additional encapsulations or decapsulations in the downstream network.

Relationship with Other CoS Features

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

The overhead accounting feature also affects the egress shaping overhead feature that you can configure at the chassis level. We recommend that you use the egress shaping-overhead feature to account for the Layer 2 overhead of the outgoing interface, and use the overhead-accounting feature to account for downstream traffic with different encapsulations and cell-based networks.

When both features are configured together, the total byte adjustment value is equal to the adjusted value of the overhead-accounting feature plus the value of the egress-shaping-overhead feature. For example, if the configured byte adjustment value is 40, and the router internally adjusts the size of each frame by 8, the adjusted overhead accounting value is 48. That value is added to the egress shaping overhead of 24 for a total byte adjustment value of 72.

Setting the Shaping-Rate and Overhead-Accounting Class-of-Service Attributes Based on Access Line Information

You can also use access line parameters in PPPoE discovery packets to set the shaping-rate and overhead-accounting class-of-service attributes on dynamic subscriber interfaces in a broadband access network. This feature is supported on MIC and MPC interfaces on MX Series routers.

The shaping rate is based on the actual-data-rate-downstream attribute.

The overhead accounting value is based on the access-loop-encapsulation attribute and specifies whether the access loop uses Ethernet (frame mode) or ATM (cell mode).

You can mix ANCP and PPPoE vendor-specific tags for dynamically instantiated static and interface sets so that the shaping rate is first set using PPPoE vendor-specific tags and is later adjusted by ANCP. In this case, the shaping rate value overrides the PPPoE value.

The Access Node or DSLAM may forward access line information using several methods. This feature only uses access line information received in Vendor-Specific Point-to-Point Protocol over Ethernet (PPPoE) Tags [TR-101].

When you enable this feature, the values supplied by the PPPoE vendor-specific tags override the parameters that you have configured in the CLI for shaping-rate and overhead-accounting statements at the [edit dynamic-profiles profile-name class-of-service traffic-control-profile] hierarchy level. The shaping rate is based on the actual-data-rate-downstream attribute, and is only overridden if the vs-tag value is less than the configured value.