Description

This article provides information about the the calculation of the egress shaping overhead in CoS.

Symptoms

Information about the the calculation of the egress shaping overhead in CoS.

Solution

 

Junos provides the ability to control how much overhead to count with the traffic-manager statement and options. By default, the overhead of 24 bytes (20 bytes for the header, plus 4 bytes of cyclical redundancy check [CRC]), is added to egress shaping statistics. You can configure the system to adjust the number of bytes to add to a packet to determine the shaped session packet length by adding more bytes (up to 124) of overhead. You can also subtract bytes for egress shaping overhead (up to -63 bytes).

The following example adds 12 more bytes of overhead to the egress shaping statistics:

user@router# set chassis fpc 0 pic 0 traffic-manager egress-shaping-overhead 12
As configured above, the L2 headers (DA/SA + Vlan tags) are automatically a part of the shaping calculation.

Overhead accounting feature :
user@router# set class-of-service traffic-control-profiles profile-name overhead-accounting
This feature also affects the egress shaping overhead feature that can bed configured at the chassis level. It is recommended to use the egress-shaping-overhead feature to account for the Layer 2 overhead of the outgoing interface and the overhead-accounting feature to account for downstream traffic with different encapsulations and cell-based networks.

When both of the 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.

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