Руководство По Проектированию для Cisco Cisco Nexus 5010 Switch
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Note: For more information about IGMP scalability, please refer to the “Multicast Scalability” section later in this
chapter.
chapter.
In some deployments, server pairs or other similar devices need to see the same copy of the traffic. If these
devices do not respond to IGMP queries, instead of disabling IGMP, which is strongly discouraged because it
causes flooding and consumes bandwidth, you can configure static IGMP entries with the following command:
ip igmp snooping static-group <multicast IP address> interface ethernet 1/1
ethernet 1/2
ethernet 1/2
Multicast Routers
Multicast routers (mrouters) actively query VLANs for potential receivers. To do this, they send IGMP queries, and
a Layer 2 switch forwards general queries from mrouters to all ports on the VLAN.
A Layer 2 switch intercepts the IGMP reports from the hosts and sends them (in an aggregated fashion) to the
multicast router.
Also when a host originates a multicast frame, the switch must send it to all the hosts that subscribed to the
multicast group as well as to the multicast router. Unknown IP multicast traffic is sent to mrouter ports, and if there
are no mrouter ports, the traffic is dropped.
The switch knows where the multicast router is located (that is, to which ports it must forward multicast frames and
IGMP reports) by snooping the following traffic:
●
IGMP membership query sent to 01-00-5e-00-00-01
●
PIMv1 hello sent to 01-00-5e-00-00-02
●
PIMv2 hello sent to 01-00-5e-00-00-0d
●
Distance Vector Multicast Routing Protocol (DVMRP) probes sent to 01-00-5e-00-04
●
Multicast Open Shortest Path First (MOSPF) message sent to 01-00-5e-00-05 or 06
If a switch does not receive any of these messages, no mrouter ports get programmed in the layer 2 forwarding
table, and as a result a multicast stream is only forwarded to the layer 2 ports which subscribed to the multicast
group. If no IGMP report was previously received by the switch or no PIM, query packet, the multicast frame is
simply not forwarded to any port.
If you need to configure the mrouter port manually, you can use the following command:
ip igmp snooping mrouter interface <interface> vlan <vlan-id>
IGMP versions
Several versions of IGMP exist: versions1, 2, and 3. All versions support PIM Sparse Mode (PIM-SM) and
bidirectional PIM (BiDir PIM). IGMPv3 also supports Source-Specific Multicast (SSM), allowing a host to join a
group for a specific source only (S,G).
The destination IP address for the multicast reports and queries varies depending on the IGMP version. With
IGMPv3, hosts send the reports to the specific multicast address 224.0.0.22. IMGPv1 and v2 send the report to the
same IP address as for the group to which the report refers (for example, for a join message to 239.1.0.1, the
report is sent to 239.1.0.1).
IGMP queries from the router are addressed to 224.0.0.1 in IGMPv3, or in the case of group-specific queries, to the
group IP address.