Bandwidth Management for Downstream Traffic in Edge Networks Overview

In a subscriber access network, traffic with different encapsulations can be passed downstream to other customer premise equipment (CPE) through the MX Series router. Managing the bandwidth of downstream ATM traffic to Ethernet interfaces can be especially difficult because of the different Layer 2 encapsulations.

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 Trio MPC/MIC interfaces on MX Series routers.

Guidelines for Configuring the Shaping Mode

Frame 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 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 could 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 30 for a total byte adjustment value of 78.

Related Topics

• To configure overhead accounting for static Ethernet interfaces, see Configuring Static Shaping Parameters to Account for Overhead in Downstream Traffic Rates
• To configure overhead accounting for dynamic subscriber access, see Configuring Dynamic Shaping Parameters to Account for Overhead in Downstream Traffic Rates