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Cisco Nexus 5000: Multicast Traffic not passing across the switch






Multicast involves both a method of delivery and discovery of senders and receivers of multicast data, which is transmitted on IP multicast addresses called groups. A multicast address that includes a group and source IP address is often referred to as a channel. The routers in the network listen for receivers to advertise their interest in receiving multicast data from selected groups. The routers then replicate and forward the data from sources to the interested receivers. Multicast data for a group is transmitted only to those LAN segments with receivers that requested it. Because multicast traffic is destined for an arbitrary group of hosts, the router uses reverse path forwarding (RPF) to route data to active receivers for the group. When receivers join a group, a path is formed either toward the source (SSM mode) or the RP (ASM mode). The path from a source to a receiver flows in the reverse direction from the path that was created when the receiver joined the group.





When the server sends the multicast packet to the address in range of, these don’t get delivered across the Nexus switch. This has been proved by running tcpdumps on the server and client. Both devices are connected to the same Nexus switch running version 5.0(3)N1(1c) and the interfaces (10G GEM) are configured as simple “switchport access vlan 211”.



IGMP Snooping


Internet Group Management Protocol (IGMP) snooping streamlines multicast traffic handling for VLANs. By examining (snooping) IGMP membership report messages from interested hosts, multicast traffic is limited to the subset of VLAN interfaces on which the hosts reside. Using the interface information, IGMP snooping can reduce bandwidth consumption in a multi-access LAN environment to avoid flooding the entire VLAN. The IGMP snooping feature tracks which ports are attached to multicast-capable routers to help it manage the forwarding of IGMP membership reports. The IGMP snooping software responds to topology change notifications.

When there is no multicast router in the VLAN to originate the queries, you must configure an IGMP snooping querier to send membership queries. When an IGMP snooping querier is enabled, it sends out periodic IGMP queries that trigger IGMP report messages from hosts that want to receive IP multicast traffic. IGMP snooping listens to these IGMP reports to establish appropriate forwarding.





Check the switch configuration. If you don't have PIM enabled, and hence no IGMP querier, then you will need to configure an IGMP snooping querier for the VLAN. When a host joins a multicast group via IGMP it will periodically report that it is still interested in receiving traffic destined to the multicast group, something it does by way of an IGMP Report message. The trigger for a host to send an IGMP report is an IGMP Query, which is usually originated by the PIM router.

When you don't have a PIM router enabled, there are no IGMP queries and so no IGMP Reports from the hosts, with the result that the switch considers there are no longer any hosts interested in the multicast stream.



Sample Configuration


vlan 211
  ip igmp snooping querier


The IP address in the above is any free IP address in correct vlan, in our case its vlan 211.



Related Information


Cisco Nexus 5000 Series NX-OS Multicast Routing Configuration Guide

Nexus 5596UP multicast to