27-8
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 27 Configuring QoS
Understanding QoS
the binary form of the DSCP number. When congestion occurs, the drop precedence of a packet
determines the relative importance of the packet within the class. An AF41 class provides the
highest probability of a packet being forwarded from one end of the network to the other.
•
Entering Class Selector (CS) service values of 1 to 7, corresponding to IP precedence bits in the ToS
field of the packet.
•
Using Expedited Forwarding (EF) to specify a low-latency path. This corresponds to a DSCP value
of 46. EF services use priority queuing to preempt lower priority traffic classes.
This display shows the available classification options:
Switch(config-cmap)#
match ip dscp
?
<0-63> Differentiated services codepoint value
af11 Match packets with AF11 dscp (001010)
af12 Match packets with AF12 dscp (001100)
af13 Match packets with AF13 dscp (001110)
af21 Match packets with AF21 dscp (010010)
af22 Match packets with AF22 dscp (010100)
af23 Match packets with AF23 dscp (010110)
af31 Match packets with AF31 dscp (011010)
af32 Match packets with AF32 dscp (011100)
af33 Match packets with AF33 dscp (011110)
af41 Match packets with AF41 dscp (100010)
af42 Match packets with AF42 dscp (100100)
af43 Match packets with AF43 dscp (100110)
cs1 Match packets with CS1(precedence 1) dscp (001000)
cs2 Match packets with CS2(precedence 2) dscp (010000)
cs3 Match packets with CS3(precedence 3) dscp (011000)
cs4 Match packets with CS4(precedence 4) dscp (100000)
cs5 Match packets with CS5(precedence 5) dscp (101000)
cs6 Match packets with CS6(precedence 6) dscp (110000)
cs7 Match packets with CS7(precedence 7) dscp (111000)
default Match packets with default dscp (000000)
ef Match packets with EF dscp (101110)
For more information on DSCP prioritization, see RFC-2597 (AF per-hop behavior), RFC-2598 (EF), or
RFC-2475 (DSCP).
CoS Mapping
The switch uses EVC and EFPs to support VLAN mapping from the customer VLAN-ID (C-VLAN) to
a service-provider VLAN-ID (S-VLAN). See the
“Configuring IEEE 802.1Q Tunneling and Layer 2
Protocol Tunneling using EFPs” section on page 11-17
.
For QoS, you can set the service-provider CoS (S-CoS) from either the customer CoS (C-CoS) or the
customer DSCP (C-DSCP) value. You can map the inner CoS to the outer CoS for any traffic with
traditional 802.1Q tunneling (QinQ). This allows copying the customer CoS into the service provider
network.
By default, the switch supports C-CoS to S-CoS propagation for QinQ. When you configure QinQ, you
can also set the S-CoS from C-DSCP.
Configuring CoS matching on EFPs configured for tunneling:
•
On service instances configured for 802.1Q tunneling, the CoS value of the VLAN tag (inner VLAN
or C-VLAN) received on the interface (C-CoS) is automatically reflected in the tunnel VLAN tag
(outer VLAN or S-VLAN) by default.
•
The set cos policy-map class configuration commands always apply to the outer-most VLAN tag
after processing is complete, that is the S-VLAN-ID. For example, in 802.1Q tunnels, entering a set
cos command changes only the CoS value of the outer tag of the encapsulated packet.
