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Oversubscribing Interface Bandwidth on LSQ Interfaces

The term oversubscribing interface bandwidth means configuring shaping rates (peak information rates [PIRs]) so that their sum exceeds the interface bandwidth.

On Channelized IQ PICs, Gigabit Ethernet IQ PICs, and FRF.16 link services IQ (lsq-) interfaces on AS and Multiservices PICs, you can oversubscribe interface bandwidth. The logical interfaces (and DLCIs within an FRF.16 bundle) can be oversubscribed when there is leftover bandwidth. The oversubscription is limited to the configured PIR. Any unused bandwidth is distributed equally among oversubscribed logical interfaces or DLCIs.

For networks that are not likely to experience congestion, oversubscribing interface bandwidth improves network utilization, thereby allowing more customers to be provisioned on a single interface. If the actual data traffic does not exceed the interface bandwidth, oversubscription allows you to sell more bandwidth than the interface can support.

We recommend avoiding oversubscription in networks that are likely to experience congestion. Be careful not to oversubscribe a service by too much, because this can cause degradation in the performance of the router during congestion. When you configure oversubscription, some output queues can be starved if the actual data traffic exceeds the physical interface bandwidth. You can prevent degradation by using statistical multiplexing to ensure that the actual data traffic does not exceed the interface bandwidth.

Note: You cannot oversubscribe interface bandwidth when you configure traffic shaping using the method described in Applying Scheduler Maps and Shaping Rate to DLCIs and VLANs.

When configuring oversubscription for FRF.16 bundle interfaces, you can assign traffic control profiles that apply on a physical interface basis. When you apply traffic control profiles to FRF.16 bundles at the logical interface level, member link interface bandwidth is underutilized when there is a small proportion of traffic or no traffic at all on an individual DLCI. Support for traffic control features on the FRF.16 bundle physical interface level addresses this limitation.

To configure oversubscription of an interface, perform the following steps:

1. Include the shaping-rate statement at the [edit class-of-service traffic-control-profiles profile-name] hierarchy level:
   [edit class-of-service traffic-control-profiles profile-name]shaping-rate (percent percentage | rate);

   Note: When configuring oversubscription for FRF.16 bundle interfaces on a physical interface basis, you must specify shaping-rate as a percentage.

   On LSQ interfaces, you can configure the shaping rate as a percentage.

   On IQ and IQ2 interfaces, you can configure the shaping rate as an absolute rate from 1000 through 160,000,000,000 bits per second.

   Alternatively, you can configure a shaping rate for a logical interface and oversubscribe the physical interface by including the shaping-rate statement at the [edit class-of-service interfaces interface-name unit logical-unit-number] hierarchy level. However, with this configuration approach, you cannot independently control the delay-buffer rate, as described in Step 2.

   Note: For channelized and Gigabit Ethernet IQ interfaces, the shaping-rate and guaranteed-rate statements are mutually exclusive. You cannot configure some logical interfaces to use a shaping rate and others to use a guaranteed rate. This means there are no service guarantees when you configure a PIR. For these interfaces, you can configure either a PIR or a committed information rate (CIR), but not both. This restriction does not apply to Gigabit Ethernet IQ2 PICs or link services IQ (LSQ) interfaces on AS or Multiservices PICs. For LSQ and Gigabit Ethernet IQ2 interfaces, you can configure both a PIR and a CIR on an interface. For more information about CIRs, see Configuring Guaranteed Minimum Rate on LSQ Interfaces.

2. Optionally, you can base the delay buffer calculation on a delay-buffer rate. To do this, include the delay-buffer-rate statement at the [edit class-of-service traffic-control-profiles profile-name] hierarchy level:

   Note: When configuring oversubscription for FRF.16 bundle interfaces on a physical interface basis, you must specify delay-buffer-rate as a percentage.

   [edit class-of-service traffic-control-profiles profile-name]delay-buffer-rate (percent percentage | rate);

   The delay-buffer rate overrides the shaping rate as the basis for the delay-buffer calculation. In other words, the shaping rate or scaled shaping rate is used for delay-buffer calculations only when the delay-buffer rate is not configured.

   For LSQ interfaces, if you do not configure a delay-buffer rate, the guaranteed rate (CIR) is used to assign buffers. If you do not configure a guaranteed rate, the shaping rate (PIR) is used in the undersubscribed case, and the scaled shaping rate is used in the oversubscribed case.

   On LSQ interfaces, you can configure the delay-buffer rate as a percentage.

   On IQ and IQ2 interfaces, you can configure the delay-buffer rate as an absolute rate from 1000 through 160,000,000,000 bits per second.

   The actual delay buffer is based on the calculations described in the Junos OS Class of Service Configuration Guide. For an example showing how the delay-buffer rates are applied, see Examples: Oversubscribing an LSQ Interface.

   Configuring large buffers on relatively low-speed links can cause packet aging. To help prevent this problem, the software requires that the sum of the delay-buffer rates be less than or equal to the port speed.

   This restriction does not eliminate the possibility of packet aging, so you should be cautious when using the delay-buffer-rate statement. Though some amount of extra buffering might be desirable for burst absorption, delay-buffer rates should not far exceed the service rate of the logical interface.

   If you configure delay-buffer rates so that the sum exceeds the port speed, the configured delay-buffer rate is not implemented for the last logical interface that you configure. Instead, that logical interface receives a delay-buffer rate of zero, and a warning message is displayed in the CLI. If bandwidth becomes available (because another logical interface is deleted or deactivated, or the port speed is increased), the configured delay-buffer-rate is reevaluated and implemented if possible.

   If you do not configure a delay-buffer rate or a guaranteed rate, the logical interface receives a delay-buffer rate in proportion to the shaping rate and the remaining delay-buffer rate available. In other words, the delay-buffer rate for each logical interface with no configured delay-buffer rate is equal to:

   (remaining delay-buffer rate * shaping rate) / (sum of shaping rates)

   The remaining delay-buffer rate is equal to:

   (interface speed) – (sum of configured delay-buffer rates)
3. To assign a scheduler map to the logical interface, include the scheduler-map statement at the [edit class-of-service traffic-control-profiles profile-name] hierarchy level:
   [edit class-of-service traffic-control-profiles profile-name]scheduler-map map-name;

   For information about configuring schedulers and scheduler maps, see the Junos OS Class of Service Configuration Guide.

4. Optionally, you can enable large buffer sizes to be configured. To do this, include the q-pic-large-buffer statement at the [edit chassis fpc slot-number pic pic-number] hierarchy level:
   [edit chassis fpc slot-number pic pic-number]q-pic-large-buffer;

   If you do not include this statement, the delay-buffer size is more restricted. We recommend restricted buffers for delay-sensitive traffic, such as voice traffic. For more information, see the Junos OS Class of Service Configuration Guide.

5. To enable scheduling on logical interfaces, include the per-unit-scheduler statement at the [edit interfaces interface-name] hierarchy level:
   [edit interfaces interface-name ]per-unit-scheduler;

   When you include this statement, the maximum number of VLANs supported is 768 on a single-port Gigabit Ethernet IQ PIC. On a two-port Gigabit Ethernet IQ PIC, the maximum number is 384.

6. To enable scheduling for FRF.16 bundles physical interfaces, include the no-per-unit-scheduler statement at the [edit interfaces interface-name] hierarchy level:
   [edit interfaces interface-name]no-per-unit-scheduler;
7. To apply the traffic-scheduling profile to the logical interface, include the output-traffic-control-profile statement at the [edit class-of-service interfaces interface-name unit logical-unit-number] hierarchy level:
   [edit class-of-service interfaces interface-name unit logical-unit-number]output-traffic-control-profile profile-name;

   You cannot include the output-traffic-control-profile statement in the configuration if any of the following statements are included in the logical interface configuration: scheduler-map, shaping-rate, adaptive-shaper, or virtual-channel-group.

   For a table that shows how the bandwidth and delay buffer are allocated in various configurations, see the Junos OS Class of Service Configuration Guide.

Examples: Oversubscribing an LSQ Interface