Information Technology Reference
In-Depth Information
In this output (see the accompanying note), the CRC error counter indicates the number of CRC errors
of the CPCS-PDU frame. Both commands were typed on the same router; because no CrcErrors can be
seen on the display of statistics for PVC 0/36, we can assume that the input errors of the sh int were due
to another PVC.
The output of sh atm pvc might vary depending on the card functionality and code feature.
The example shown on this page uses the PA-A3 with 12.1 IOS code version.
Note
As a last remark, one input error does not always mean one packet loss. The cell discarded by the ATM
provider could be the last one of the frame; therefore, the cell discarded had this special bit set to 1. The
only way for the destination to find the frame boundaries is to check this bit. As a result, the destination
router concatenates at reassembly time all cells that it receives until it finds a cell with this bit set to 1.
If the last cell of a frame is discarded, two CPCS-PDU frames will be lost and will result in only one
CRC and length error.
Understanding the Basics of Traffic Shaping and Policing
Two basic terms will be used in this chapter: traffic shaping and policing . Traffic shaping refers to an
action done by the source of the ATM traffic. Policing refers to actions done by the ATM switches,
usually at the provider's side.
Traffic shaping is the action of adapting the cell flow to a specific traffic contract. This is illustrated in
Figure 22-4.
Figure22-4 Traffic Shaping—Adapting the Cell Flow to a Specific Traffic Contract
Router
Traffic shaping
takes place
Policing is the action of checking whether the cell flow respects a specific traffic contract. This is
illustrated in Figure 22-5.
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