Foundry NetIron M2404C and M2404F Metro Access Switches
Configuring QoS (Rev. 03)
Overview
© 2008 Foundry Networks, Inc.
Page 5 of 62
Sorting Packets for QoS Handling
Packet Sorting by 802.1p Priority Values
The devices support the standard 802.1p priority bits that are part of a tagged Ethernet packet. The
802.1p bits can be used to prioritize the packet.
When a packet arrives at the device, the device examines the 802.1p priority field and assigns the
packet to a specific QoS queue for transmission. The 802.1p priority field is located directly
following the 802.1Q type field, and preceding the 802.1Q VLAN ID, as shown in
Figure 2
.
Figure 2: 802.1p Priority Header Fields
When the device detects ingress traffic that contains 802.1p prioritization information, the traffic is
mapped to various hardware queues on the egress port of the device. The transmitting hardware
queue determines the bandwidth management and priority characteristics used when transmitting
packets.
The default mapping of each 802.1p priority value to QoS priority is shown in
Table 5
. To change
the default configuration use the
qos map
command in Global Configuration mode.
By default, 802.1p priority information is not replaced or manipulated, and the information
observed on ingress is preserved when the packet is transmitted. This behavior is not affected by
the switching or routing configuration of the device. However, the device is capable of inserting
and/or overwriting 802.1p priority information when it transmits an 802.1Q tagged frame. The
802.1p priority information that is transmitted is determined by the hardware queue that is used
when transmitting the packet.
NOTE
The device does not change the VLAN Priority Tag (VPT) for a switched packet
that comes with an 802.1Q tag, since it assumes that the sender of the packet has
already determined the VPT.
VPT of packets received without a tag can be set using the
qos priority
command in
the Interface Configuration Mode.
Packet Sorting by DiffServ Values
The header of every IP packet contains a field for IP Type of Service (ToS). The device uses this
ToS field to determine the type of service provided to the packet.
The application software can use ToS/DiffServ values to sort packets into QoS queues. Individual
ToS values, or ranges of values, are mapped to 802.1p priority values. Packets are sorted into QoS
queues based on this derived priority value.
Figure 3
shows the ToS fields in the IP packet header.