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Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09
Chapter 24 Configuring QoS
Understanding QoS
• Devices in the management network (network connected to Fast Ethernet port) cannot be accessed
from any other port. If the default route is configured on Cisco ASR 901 to fast ethernet interface
(Fa0/0), all the routed packets will be dropped. However, this configuration could keep CPU busy
and affect overall convergence.
Classification
Classification distinguishes one kind of traffic from another by examining the fields in the packet header.
When a packet is received, the router examines the header and identifies all key packet fields. A packet
can be classified based on the DSCP, the CoS, or the IP precedence value in the packet, or by the VLAN
ID. Figure 24-3 shows the classification information carried in a Layer 2 or a Layer 3 IP packet header,
using six bits from the deprecated IP type of service (ToS) field to carry the classification information.
• On ports configured as Layer 2 IEEE 802.1Q trunks, all traffic is in 802.1Q frames except for traffic
in the native VLAN. Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that
carries the CoS value, called the User Priority bits, in the three most-significant bits, and the VLAN
ID value in the 12 least-significant bits. Other frame types cannot carry Layer 2 CoS values.
Layer 2 CoS values range from 0 to 7.
• Layer 3 IP packets can carry either an IP precedence value or a DSCP value. QoS supports the use
of either value because DSCP values are backward-compatible with IP precedence values.
IP precedence values range from 0 to 7. DSCP values range from 0 to 63.
• Output remarking is based on the Layer 2 or Layer 3 marking type, marking value and packet type.
Figure 24-3 QoS Classification Layers in Frames and Packets
141151
Layer 2 IEEE 802.1Q and IEEE 802.1p Frame
Preamble
Start frame
delimiter
DA
Len
SA Type
TAG
2 Bytes
PT Data FCS
Layer 3 IPv4 Packet
Version
length
ToS
1 Byte
ID Offset TTL Proto FCS IP-SA IP-DA
PRI
7
6 5 4 3 2 1 0
CFI VLAN ID
Data
3 bits used for CoS
(IEEE 802.1p user priority)
IP precedence
DSCP
Standard IPv4:
MSBs called IP precedence
Flow control
for DSCP