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Campus 3.0 Virtual Switching System Design Guide
Quality of Service
Quality of service (QoS) handling on the Cisco Catalyst 6500 Series switches can be separated into two distinct
areas of responsibility: port-based QoS features and forwarding-engine (PFC or DFC) features. Both areas operate
together to help ensure differentiated servicing of traffic throughout the system.
In a Cisco Virtual Switching System environment, proper QoS handling becomes even more important because of
the following reasons:
●
Control traffic between the two Cisco Virtual Switching System switches (active virtual switch and standby
virtual switch) should be prioritized and not be dropped
●
The existence of the VSL and multichassis Cisco EtherChannel links represents potential points of
congestion that must be properly accounted for
Additionally, the nature of dual-homing logical connections across different chassis and forwarding engines
represents a change in the way features such as policing and marking work. These features need to be properly
accounted for.
VSL as a Congestion Point
From a system-level perspective, the VSL can be viewed as a backplane connection that bonds the two virtual
switch chassis together into a single, logical entity. While provisions have been made to Cisco EtherChannel and
Equal Cost Multipath (ECMP) hashing mechanisms (refer to the section “Cisco EtherChannel Concepts”), under
Equal Cost Multipath (ECMP) hashing mechanisms (refer to the section “Cisco EtherChannel Concepts”), under
certain circumstances (in the case of single-homed connections whether by design or if failures occur) it may be
possible to oversubscribe the links that form the VSL.
VSL should always consist of at least 2 ports of 10 Gigabit Ethernet connections. However, because of hash
inefficiencies, it may be possible to oversubscribe a single VSL member port. Therefore, correct prioritization needs
to occur on the VSL.
Correct prioritization is accomplished by always provisioning the VSL as a link that is in trust-CoS mode,
provisioning that not only maintains the internal differentiated services code point (DSCP) markings set by either
ingress switch but also sets up default receive and transmit queues and properly assigns the appropriate colored
frames to the correct queues (Figure 29).
Figure 29. Virtual Switch Link QoS
The following output shows that Port Channel 2 is configured as a VSL, and it has the QoS configuration of trust
CoS programmed by default. Also note that removing or modifying this trust command is not permitted:
interface Port-channel2
no switchport