CHAPTER 14: QUALITY OF SERVICE
MULTILINK ML2400 ETHERNET COMMUNICATIONS SWITCH – INSTRUCTION MANUAL
FIGURE 14–2: IP Precedence ToS Field in an IP Packet Header
The three most significant bits (correlating to binary settings 32, 64, and 128) of the Type of
Service (ToS) field in the IP header constitute the bits used for IP precedence. These bits are
used to provide a priority from 0 to 7 for the IP packet.
Because only three bits of the ToS byte are used for IP precedence, you need to
differentiate these bits from the rest of the ToS byte.
The MultiLink family of switches has the capability to provide QoS at Layer 2. At Layer 2, the
frame uses Type of Service (ToS) as specified in IEEE 802.1p. ToS uses 3 bits, just like IP
precedence, and maps well from Layer 2 to layer 3, and vice versa.
The switches have the capability to differentiate frames based on ToS settings. With two
queues present - high or low priority queues or buffers in MultiLink family of switches,
frames can be placed in either queue and serviced via the weight set on all ports. This
placement of queues, added to the weight set plus the particular tag setting on a packet
allows each queue to have different service levels.
MultiLink QoS implementations provide mapping of ToS (or IP precedence) to Class of
Service (CoS). A CoS setting in an Ethernet Frame is mapped to the ToS byte of the IP
packet, and vice versa. A ToS level of 1 equals a CoS level of 1. This provides end-to-end
priority for the traffic flow when MultiLink switches are deployed in the network.
Not all packets received on a port have high priority. IGMP and BPDU packets have high
priority by default.
The MultiLink family of switches has the capability to set the priorities based on three
different functions. They are
• Port QoS: assigns a high priority to all packets received on a port, regardless of the
type of packet.
• TAG QoS: if a packet contains a tag, the port on which the packet was received
then looks to see at which level that tag value is set. Regardless of the tag value, if
there is a tag, that packet is automatically assigned high priority (sent to the high
• ToS QoS: (Layer 3) when a port is set to ToS QoS, the most significant 6-bits of the
IPv4 packet (which has 64 bits) are used. If the 6 bits are set to ToS QoS for the
specific port number the packet went to, that packet is assigned high priority by
QUALITY OF SERVICE