Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
182
Token Bucket
The t oken bucket algorit hm uses t okens in a bucket to control when traffic can be t ransm it t ed. The
bucket st ores t okens, each of which represents one byte. The algorit hm allows burst s of up to b
byt es which is also t he bucket size, so t he bucket can hold up t o b t okens. Tokens are generated
and added into t he bucket at a constant rat e. The following shows how tokens work wit h packets:
• A packet can be t ransm itt ed if the number of t okens in the bucket is equal t o or great er than the
size of t he packet (in bytes) .
• After a packet is transm it t ed, a num ber of tokens corresponding t o t he packet size is rem oved
from the bucket .
• I f t her e are no t okens in t he bucket, t he Device st ops transm itt ing until enough tokens are
generat ed.
• I f not enough tokens are available, t he Device treats t he packet in either one of t he following
ways:
I n t raffic shaping:
• Holds it in t he queue unt il enough t okens are available in the bucket .
I n t raffic policing:
• Drops it.
• Transm it s it but adds a DSCP m ark. The Device m ay drop t hese m arked packets if the network
is over loaded.
Configure t he bucket size t o be equal t o or less t han t he am ount of the bandwidth t hat t he interface
can support . I t does not help if you set it to a bucket size over t he interface’s capability. The sm aller
the bucket size, the lower t he data t ransm ission rate and that m ay cause outgoing packets t o be
dropped. A larger transm ission rate requires a big bucket size. For exam ple, use a bucket size of 10
kbyt es to get the t ransm ission rate up to 10 Mbps.
Single Rate Three Color Marker
The Single Rat e Three Color Marker (srTCM, defined in RFC 2697) is a t ype of traffic policing t hat
identifies packet s by com paring t hem t o one user- defined rat e, t he Comm it ted I nform at ion Rate
( CI R) , and t wo burst sizes: the Com m it t ed Burst Size (CBS) and Excess Burst Size ( EBS) .
6 6 4 100110
100100
100010
100000
5 101110
101000
7 7 6 110000
111000
7
Table 50 I nt ernal Layer2 and Layer3 QoS Mapping
PRIORITY
QUEUE
LAYER 2 LAYER 3
IEEE 802.1P USER
PRIORITY
(ETHERNET
PRIORITY)
TOS (IP
PRECEDENCE)
DSCP
IP PACKET
LENGTH (BYTE)
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