Table of Contents
Advertisement
Configuring Layer 3 QoS
Overview of Layer 3 Switching QoS
Extensive QoS features are built into the Layer 3 Services module architecture to ensure QoS policy
enforcement and queuing of the ingress port and weighted round-robin (WRR) scheduling at the egress
port.
QoS on the Gigabit Ethernet interfaces is based on IP precedence for partitioning traffic into multiple
classes of service. IP precedence uses the three type-of-service (ToS) precedence bits in the IP header
to specify class of service (CoS) assignment for each packet.
You can map IP precedence into adjacent technologies (for example, tag switching or ATM) to support
end-to-end network QoS policies. This mapping enables you to establish service classes with no
changes to existing applications and with no complicated network signaling requirements.
The system obtains IP precedence information from the IP header ToS field. For an incoming IP packet,
the first two (most significant) bits of the ToS field determine the delay priority. The Gigabit Ethernet
interfaces on the Catalyst 4000 Layer 3 Services module recognize four queue classes (Q-0 to Q-3),
which are described in Table 11.
Table 11
IP Precedence
Bits
0 0 0
0 0 1
0 1 0
0 1 1
1 0 0
1 0 1
1 1 0
1 1 1
The Catalyst 4000 Layer 3 Services module can read the precedence field and switch the packet
accordingly, but it cannot reclassify traffic. The edge router or switch is expected to set the precedence
field according to its local policy.
The Layer 3 Services module queues packets based on the delay priority and the target next-hop
interface.
You cannot apply QoS configurations to an EtherChannel, but you can configure QoS on
Note
the individual EtherChannel member ports.
Overview of Scheduling and WRR
Frame scheduling becomes more important when an outgoing interface is congested. To handle this
situation, you can assign weights to each of the different queues. By assigning weights to the different
queues, you can allocate bandwidth to higher priority applications (using IP precedence) while still
Installation and Configuration Note for the Catalyst 4000 Layer 3 Services Module
68
Queue Classes
Queue
Delay Priority
Selected
0 0
Q-0
0 0
Q-0
0 1
Q-1
0 1
Q-1
1 0
Q-2
1 0
Q-2
1 1
Q-3
1 1
Q-3
78-10164-03
Advertisement
Table of Contents
