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ScreenOS Cookbook by Sunil Wadhwa, Joe Kelly, Ken Draper, David Delcourt, Vik Davar, Stefan Brunner

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14.5. Troubleshoot QoS

Problem

Traffic shaping is not doing what you thought you configured, and you need more background on how traffic shaping works on ScreenOS.

Solution

Check settings and bucket utilization on the policy:

	get policy id 1009

Check settings and bucket utilization on the interface:

	get traffic-shaping interface e3

Discussion

Traffic shaping is implemented in ScreenOS via a dual-token bucket model (see Figure 14-1). One bucket is used to control GBW, while tokens overspill into a second bucket for shared bandwidth in excess of all GBW. Tokens are added to the GBW bucket at the rate the GBW was configured. Any traffic falling within GBW takes tokens out of the GBW bucket. Any unused tokens overflow into a second bucket for shared bandwidth. If a packet is transmitted, it takes tokens in the amount of that traffic out of the GBW and shared buckets. Only if tokens are left in the buckets can a packet be transmitted. In ScreenOS, a token has the size of 1 byte.

The two buckets relate to the concept of one bucket measuring guarantees and another serving as a spillover for shared bandwidth. Each policy maintains a GBW and an MBW bucket, as shown in Figure 14-2, where both are used for measuring traffic on that policy. Only GBW buckets spill over to shared buckets.

Leftover bandwidth is shared among queues but also among dependent interfaces. Dependent interfaces are logical subinterfaces, or a virtual interface such as a tunnel or loopback interface. A tunnel interface can send ...

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