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Chapter 10 Quality of Service (QoS)
Table 53 Internal Layer2 and Layer3 QoS Mapping
PRIORITY
QUEUE
6
7
Token Bucket
The token bucket algorithm uses tokens in a bucket to control when traffic can be transmitted. The
bucket stores tokens, each of which represents one byte. The algorithm allows bursts of up to b
bytes which is also the bucket size, so the bucket can hold up to b tokens. Tokens are generated
and added into the bucket at a constant rate. The following shows how tokens work with packets:
• A packet can be transmitted if the number of tokens in the bucket is equal to or greater than the
size of the packet (in bytes).
• After a packet is transmitted, a number of tokens corresponding to the packet size is removed
from the bucket.
• If there are no tokens in the bucket, the Device stops transmitting until enough tokens are
generated.
• If not enough tokens are available, the Device treats the packet in either one of the following
ways:
In traffic shaping:
• Holds it in the queue until enough tokens are available in the bucket.
In traffic policing:
• Drops it.
• Transmits it but adds a DSCP mark. The Device may drop these marked packets if the network
is overloaded.
Configure the bucket size to be equal to or less than the amount of the bandwidth that the interface
can support. It does not help if you set it to a bucket size over the interface's capability. The smaller
the bucket size, the lower the data transmission rate and that may cause outgoing packets to be
dropped. A larger transmission rate requires a big bucket size. For example, use a bucket size of 10
kbytes to get the transmission rate up to 10 Mbps.
Single Rate Three Color Marker
The Single Rate Three Color Marker (srTCM, defined in RFC 2697) is a type of traffic policing that
identifies packets by comparing them to one user-defined rate, the Committed Information Rate
(CIR), and two burst sizes: the Committed Burst Size (CBS) and Excess Burst Size (EBS).
182
LAYER 2
LAYER 3
IEEE 802.1P USER
PRIORITY
TOS (IP
(ETHERNET
PRECEDENCE)
PRIORITY)
6
4
5
7
6
7
IP PACKET
DSCP
LENGTH (BYTE)
100110
100100
100010
100000
101110
101000
110000
111000
VMG1312-B10B / VMG1312-B30B Series User's Guide
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