Cisco WS-X6708-10G-3CXL= - 10 Gigabit Ethernet Module Manual de usuario descargar pdf (Pagina 53)

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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 also 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 also need to be properly

accounted for.

VSL as a Congestion Point

In terms of 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. Although provisions have been made to Cisco EtherChannel and

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, but 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 38).

Figure 38. 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

no ip address

switch virtual link 2

mls qos trust cos

end

vss#conf t

Enter configuration commands, one per line. End with CNTL/Z.

vss(config)#int po2

1 2 ... 48 49 50 51 52 53 54 55 56 57 58 ... 61 62

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Cisco WS-X6708-10G-3CXL= - 10 Gigabit Ethernet Module Manual de usuario Pagina 53