Chapter 9 Quality of Service (QoS)
VMG8924-B10A and VMG8924-B30A Series User’s Guide
154
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.
4 4 2 010110
010100
010010
010000
5 5 3 011110
011100
011010
011000
<250
6 6 4 100110
100100
100010
100000
5 101110
101000
7 7 6 110000
111000
7
Table 55 Internal 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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