GE ML1200 Switch User 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 ML1200 Managed Field Switch 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 ML1200 Managed Field
Switch, 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 ML1200 Managed Field Switch 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 ML1200 Managed Field
Switchs 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 ML1200 Managed Field Switch 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
priority queue)
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
that port
Data +FCS
ToS byte
IP precedence
3 bits