Cisco Systems A9014CFD manual Classification, 24-7

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 24-3shows 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

Layer 2 IEEE 802.1Q and IEEE 802.1p Frame

Preamble

Start frame

delimiter

DA

SA

Type

TAG

2 Bytes

PT

Data

FCS

3 bits used for CoS

(IEEE 802.1p user priority)

PRI

CFI

VLAN ID

Layer 3 IPv4 Packet

Version

length

ToS

1 Byte

Len

ID

Offset TTL

Proto FCS IP-SA

IP-DA

Data

7		6	5	4	3	2	1	0
	IP precedence						Flow control

for DSCP

DSCP

Standard IPv4:

MSBs called IP precedence

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	Cisco ASR 901 Series Aggregation Services Router Software Configuration Guide
OL-23826-09		24-7