Brocade Communications Systems Brocate Ethernet Access Switch 6910 - Manual (Page 489)

Brocade 6910 Ethernet Access Switch Configuration Guide 439
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Quality of Service Commands
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• The PHB label is composed of five bits, three bits for per-hop behavior, and two bits for the
color scheme used to control queue congestion. A packet is marked green if it doesn't exceed
the CIR and BC, yellow if it does exceed the CIR and BC, but not the BE, and red otherwise.
• The meter operates in one of two modes. In the color-blind mode, the meter assumes that the
packet stream is uncolored. In color-aware mode the meter assumes that some preceding
entity has pre-colored the incoming packet stream so that each packet is either green, yellow,
or red. The marker (re)colors an IP packet according to the results of the meter. The color is
coded in the DS field [RFC 2474] of the packet.
• The behavior of the meter is specified in terms of its mode and two token buckets, C and E,
which both share the common rate CIR. The maximum size of the token bucket C is BC and the
maximum size of the token bucket E is BE.
The token buckets C and E are initially full, that is, the token count Tc(0) = BC and the token
count Te(0) = BE. Thereafter, the token counts Tc and Te are updated CIR times per second as
follows:
• If Tc is less than BC, Tc is incremented by one, else
• if Te is less then BE, Te is incremented by one, else
• neither Tc nor Te is incremented.
When a packet of size B bytes arrives at time t, the following happens if srTCM is configured to
operate in color-blind mode:
• If Tc(t)-B ≥ 0, the packet is green and Tc is decremented by B down to the minimum value
of 0, else
• if Te(t)-B ≥ 0, the packets is yellow and Te is decremented by B down to the minimum value
of 0,
• else the packet is red and neither Tc nor Te is decremented.
When a packet of size B bytes arrives at time t, the following happens if srTCM is configured to
operate in color-aware mode:
• If the packet has been precolored as green and Tc(t)-B ≥ 0, the packet is green and Tc is
decremented by B down to the minimum value of 0, else
• If the packet has been precolored as yellow or green and if
• Te(t)-B ≥ 0, the packets is yellow and Te is decremented by B down to the minimum value
of 0, else the packet is red and neither Tc nor Te is decremented.
The metering policy guarantees a deterministic behavior where the volume of green packets is
never smaller than what has been determined by the CIR and BC, that is, tokens of a given
color are always spent on packets of that color. Refer to RFC 2697 for more information on
other aspects of srTCM.
Example
This example creates a policy called “rd-policy,” uses the class command to specify the previously
defined “rd-class,” uses the set phb command to classify the service that incoming packets will
receive, and then uses the police srtcm-color-blind command to limit the average bandwidth to
100,000 Kbps, the committed burst rate to 4000 bytes, the excess burst rate to 6000 bytes, to
remark any packets exceeding the committed burst size, and to drop any packets exceeding the
excess burst size.
Console(config)#policy-map rd-policy
Console(config-pmap)#class rd-class