Marking at Layer 2
CoS uses the three 802.1p bits in the 802.1Q trunking tag to mark traffic.
These three bits have eight possible values, ranging between zero and seven.
IP Precedence uses three bits in the IP header, so it has the same range of
values as does CoS. Table 4-1 lists the values and their standard meanings.
Table 4-1 IP Precedence and CoS Values
IP Precedence/CoS Name
7 Network
6 Internet
5 Critical
4 Flash-override
3 Flash
2 Immediate
1 Priority
0 Routine
When frames enter a switch, the Layer 2 header is stripped off. The switch
maps the CoS value to an internal DSCP value as the packet moves through
it. This DSCP value is then translated back to a CoS value if the packet is
sent over another trunk link. There are default values for the mappings
between and CoS and DSCP, but they can also be configured.
MPLS labels have a three-bit field, called the MPLS experimental (MPLS
EXP) field, which has the same eight possible values as CoS and IP
Precedence. By default, any IP Precedence value is copied into this field and
becomes the Layer 2 marking for MPLS traffic. Service providers alternatively
can set these bits independently, thus marking the traffic within their
network without changing their customer’s Layer 3 marking. The value of
MPLS EXP bits is preserved through the MPLS network.
Table 4-2 lists the eight Layer 2 markings, and some suggested applications
for them.
Table 4-2 Layer 2 Markings and Applications
CoS Value Application
7 Reserved
6 Reserved
5 Voice Bearer
4 Video
3 Call Signaling
2 High-Priority Data
1 Medium-Priority Data
0 Best-Effort Data