Cisco Cisco Nexus 5010 Switch Guia Do Desenho
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If you want to change this behavior and help ensure that all multicast traffic is queued based on the outgoing
interface list and not based on the traffic class, you can define a new traffic classification criteria that matches
multicast traffic only and define multicast optimize on this traffic class.
The multicast optimize feature can be enabled on only one class, so if you want to enable it for a class other than
class-default, you have to disable it on that class.
For instance, you can define the following classification and network QoS policy:
policy-map type qos new-classification-policy
class-map type qos class-ip-multicast
set qos-group 2
class type qos class-default
set qos-group 0
policy-map type network-qos new-nq-policy
class type network-qos class-ip-multicast
multicast-optimize
class type network-qos class-default
no multicast-optimize
By defining this classification and network policy, you can help ensure that all multicast traffic is queued based on
the outgoing interface list instead of the traffic class (for example, CoS). With the configuration shown here, one of
the six QoS groups is used for the multicast classification, and one is used by class-default, thus leaving four QoS
groups for further classification.
Multicast on Cisco Nexus 5000 Series
The Cisco Nexus 5000 Series are Layer 2 switches, and consequently Internet Group Management Protocol
(IGMP) snooping governs the multicast forwarding behavior of these switches. Well-known multicast best practices
apply in the case of the Cisco Nexus 5000 Series, summarized at the following link:
IGMP Snooping Theory of Operations
Without IGMP snooping, Layer 2 forwarding of multicast frames would result in the flooding of every multicast
frame. This flooding then would degrade the network performance by turning a switch into the equivalent of a hub,
consuming bandwidth on network links, and degrading server performance.
IGMP snooping is the feature that allows Layer 2 frames to be forwarded only to the switch ports on which
receivers are subscribed to a given multicast group. By constraining the forwarding of multicast frames to only the
hosts that need to receive them, the link bandwidth on the other hosts is not unnecessarily consumed, and the
CPU of the hosts that did not subscribe does not have to drop unneeded frames.
Basic IGMP Operations
If no host sent any IGMP report, a multicast frame sent to a Layer 2 switch (configured for IGMP snooping) would
just be dropped (this is true for all multicast frames except the link-local frames). For a multicast frame to be
forwarded, either IGMP snooping must be disabled, and as a result the frame is flooded, or IGMP must be enabled,
and hosts have to explicitly join multicast groups if they expect to receive traffic.