Brocade Communications Systems Brocate Ethernet Access Switch 6910 - Manual (Page 491)

Brocade 6910 Ethernet Access Switch Configuration Guide 441
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Quality of Service Commands
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Command Usage
• You can configure up to 16 policers (i.e., class maps) for ingress ports.
• The committed-rate and peak-rate cannot exceed the configured interface speed, and the
committed-burst and peak-burst cannot exceed 16 Mbytes.
• The trTCM as defined in RFC 2698 meters a traffic stream and processes its packets based on
two rates – Committed Information Rate (CIR) and Peak Information Rate (PIR), and their
associated burst sizes - Committed Burst Size (BC) and Peak Burst Size (BP).
• 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 red if it exceeds the PIR.
Otherwise it is marked either yellow or green depending on whether it exceeds or doesn't
exceed the CIR.
The trTCM is useful for ingress policing of a service, where a peak rate needs to be enforced
separately from a committed rate.
• 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, P and C,
which are based on the rates PIR and CIR, respectively. The maximum size of the token bucket
P is BP and the maximum size of the token bucket C is BC.
• The token buckets P and C are initially (at time 0) full, that is, the token count Tp(0) = BP and
the token count Tc(0) = BC. Thereafter, the token count Tp is incremented by one PIR times per
second up to BP and the token count Tc is incremented by one CIR times per second up to BC.
When a packet of size B bytes arrives at time t, the following happens if trTCM is configured to
operate in color-blind mode:
• If Tp(t)-B < 0, the packet is red, else
• if Tc(t)-B < 0, the packet is yellow and Tp is decremented by B, else
• the packet is green and both Tp and Tc are decremented by B.
When a packet of size B bytes arrives at time t, the following happens if trTCM is configured to
operate in color-aware mode:
• If the packet has been precolored as red or if Tp(t)-B < 0, the packet is red, else
• if the packet has been precolored as yellow or if Tc(t)-B < 0, the packet is yellow and Tp is
decremented by B, else
• the packet is green and both Tp and Tc are decremented by B.
• The trTCM can be used to mark a IP packet stream in a service, where different, decreasing
levels of assurances (either absolute or relative) are given to packets which are green, yellow,
or red. Refer to RFC 2698 for more information on other aspects of trTCM.