Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010, Configuration Manual

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Operational QoS Shaping Mode for ATM Interfaces Overview
The E Series router enables you to shape ATM traffic based on either frames or cells.
The default frame shaping mode provides compatibility with previous versions of
the E Series software. When you use cell shaping mode to configure the shaping or
policing rate, the resulting traffic stream conforms exactly to the policing rates
configured in downstream ATM switches. Using cell shaping also reduces the number
of packet drops in the ATM network.
ATM policing is sensitive to cell delay variation tolerance (CDVT). If the cells on a
particular VC or VP arrive too closely spaced, an ATM switch might drop cells.
However, the cell scheduler reduces CDVT by ensuring cell spacing. The router enables
you to use techniques such as WRR on the HRR scheduler to achieve the proper
packet scheduling. You use the SAR scheduler in series with the HRR scheduler to
even out cell bursts into smoother per-VC and per-VP traffic profiles that bound CDVT.
You accomplish this by using the
qos-shaping-mode cell command to configure the
QoS shaping mode, and the qos-mode-port low-cdv command to configure the port
queuing mode.
The QoS shaping mode also determines how QoS statistics are reported. Frame
shaping reports QoS statistics such as transmitted bytes and dropped bytes based
on bytes within frames. Cell shaping reports the statistics in bytes within cells and
also accounts for cell encapsulation and padding overhead.
ERX7xx Models, ERX14xx Models, and the ERX310 Router
The ERX7xx models, ERX14xx models, and the ERX310 router use an operational
shaping mode that is based on the following two commands:
■
The QoS shaping mode you set with the qos-shaping-mode command on port
0 and on the specific port
■
The port queuing mode you set with the qos-mode-port command on port 0
The router uses the following rules to determine the operational shaping mode used
for a port:
1.
If the specific port has a QoS shaping mode configured, the operational shaping
mode for that port is the same as the QoS shaping mode.
2.
If the specific port has no QoS shaping mode configured, the operational shaping
mode is the same as the QoS shaping mode for port 0, if one is configured.
3.
If both the specific port and port 0 have no QoS shaping mode configured, the
operational shaping mode is based on the port 0 queuing mode. If the port 0
queuing mode (set by the
qos-mode-port command) is low-cdv, the operational
shaping mode is cell; otherwise the operational shaping mode is frame.
Table 17 on page 165 lists the possible combinations of the two commands and the
resultant operational shaping mode.
164 ■
Per-Packet Queuing on the SAR Scheduler Overview
JUNOSe 11.1.x Quality of Service Configuration Guide