Ethernet Cos; Figure 14-1 Ip Precedence And Dscp - Cisco ONS 15454 SDH Configuration Manual

Chapter 14 Configuring Quality of Service
Figure 14-1 IP Precedence and DSCP
Bits 0 1 2
DS-Field DSCP
Class Selector
Codepoints
Differentiated Services Code Point
AFC 2474

Ethernet CoS

Ethernet CoS refers to three bits within a four byte IEEE 802.1Q (VLAN) header used to indicate the
priority of the Ethernet frame as it passes through a switched network. The CoS bits in the IEEE 802.1Q
header are commonly referred to as the IEEE 802.1p bits. There are three CoS bits that provide eight
classes, matching the number delivered by IP precedence. In many real-world networks, a packet might
traverse both Layer 2 and Layer 3 domains. To maintain QoS across the network, the IP ToS can be
mapped to the Ethernet CoS and vice versa, for example, in linear or one-to-one mapping, because each
mechanism supports eight classes. Similarly, a set of DSCP values (64 classes) can be mapped into each
of the eight individual Ethernet CoS values.
consists of a 2-byte Ethertype and a 2-byte tag (IEEE 802.1Q tag) on the Ethernet protocol header.
Figure 14-2
CoS
IEEE 802.1Q Tag
3 4 5 6 7
CU
Currently
Unused
Bits (0-2): IP-Precedence Defined
111 (Network Control)
110 (Internetwork Control)
101 (CRITIC/ECP)
100 (Flash Override)
011 (Flash)
101 (Immediate)
001 (Priority)
000 (Routine)
Ethernet Frame and the CoS Bit (IEEE 802.1p)
6
IEEE 802.1p 6
(3 bits)
2
2
CFI
2
VLAN ID
4
Cisco ONS 15454 and Cisco ONS 15454 SDH Ethernet Card Software Feature and Configuration Guide, R8.0
Bits 0
Precedence
RFC 1122
Figure 14-2 shows an IEEE 802.1Q Ethernet frame, which
Destination Address
Source Address
Type=8100
Tag Control Information
Type/Length
MAC DATA
PAD
FCS
1 2 3 4 5
Type of Service
DTR-Bits
RFC 1349
Bits (3-6): Type of Service Defined
0000 (all normal)
1000 (minimize delay)
0100 (maximize throughput)
0010 (maximize reliability)
0001 (minimize monetary cost)
Ethernet CoS
6 7
MBZ
Must
be
zero
14-3