Monday, May 11, 2009

srr-queue commands - part II

In this post i look at the srr-queue shape and share commands, what they do and how they interact.

There are 4 interface queues serviced by SRR. Each queue can be configured for either shaping or sharing, but not both. If shaping is configured then this takes precedence.
( The way i remember this is that shaping comes alphabetically before sharing )

Shaping guarantees a percentage of the bandwidth and limits the traffic to the configured amount. Conversely sharing allocates the bandwidth amongst the sharing queues according to the ratios configured, but does NOT limit it to this level.

Shaped and shared settings are configured using

config-if#srr-queue bandwidth shape {n} {n} {n} {n}
config-if#srr-queue bandwidth share {n} {n} {n} {n}

If the values are not set then the following default values apply

config-if#srr-queue bandwitdh shape 25 0 0 0
config-if#srr-queue bandwitdh share 25 25 25 25

Bandwidth allocation for a 10 mb link can be calculated as follows:-

Bandwidth allocated = 1/25 * BW
Hence for a 10 mb interface BW for queue 1 would be 400kbps

10mb - 400kps = 9.6 mb
Hence for queue 2,3 and 4 BW = 25/(25+25+25) * 9.6 = 3.2 mb

Supposing a lab requirement was to guarantee queue 1 2 mb, queue 2 2 mb and queue 3 and 4 to share the remainder this could be achieved with the following configuration

config-if#srr-queue bandwitdh shape 5 5 0 0
config-if#srr-queue bandwitdh share 0 0 25 25

In the next post i look at how traffic is mapped to the srr queues.

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