Automatic Priority Queue Assignment; Token Bucket - ZyXEL Communications VMG3312-T20A User Manual

IP Precedence
Similar to IEEE 802.1p prioritization at layer-2, you can use IP precedence to prioritize packets in a layer-3
network. IP precedence uses three bits of the eight-bit ToS (Type of Service) field in the IP header. There
are eight classes of services (ranging from zero to seven) in IP precedence. Zero is the lowest priority
level and seven is the highest.

Automatic Priority Queue Assignment

If you enable QoS on the VMG, the VMG can automatically base on the IEEE 802.1p priority level, IP
precedence and/or packet length to assign priority to traffic which does not match a class.
The following table shows you the internal layer-2 and layer-3 QoS mapping on the VMG. On the VMG,
traffic assigned to higher priority queues gets through faster while traffic in lower index queues is
dropped if the network is congested
Table 55 Internal Layer2 and Layer 3 QoS Mapping
ETHERNET PRIORITY
PRIORITY
(802.1P)
4 001, 010
3 000, 011
2 100
2 101
1 (Highest) 110, 111

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 VMG stops transmitting until enough tokens are generated.
• If not enough tokens are available, the VMG 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 VMG 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.
Chapter 10 Quality of Service (QoS)
IP PRECEDENCE
000XXX, 001XXX
010XXX, 011XXX
100XXX
101XXX
110XXX, 111XXX
VMG3312-T20A User's Guide
160
IP PACKET LENGTH
(BYTE)
>1100
250-1100
<250