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Why does my NBG6515 send a multicast general query packet with source IP address in AP mode?


Why does my NBG6515 send a multicast general query packet with source IP address in AP mode?


The NBG6515 supports the multicast snooping function and it is not the querier. Therefore, it can't use its IP address to send the packet and will use IP address as the source IP.

This is the standard policy which is mentioned in RFC4541-IGMP snooping and following are the reasons why the devices use IP address as the source address.

Page 3, 2.1.1. 1)
b) The arrival port for IGMP Queries (sent by multicast routers) where the source address is not
The address represents a special case where the switch is proxying IGMP Queries for faster network convergence, but is not itself the Querier. The switch does not use its own IP address (even if it has one), because this would cause the Queries to be seen as coming from a newly elected Querier. The address is used to indicate that the Query packets are NOT from a multicast router.

Page 4, 2.1.1
4) An IGMP snooping switch should be aware of link layer topology changes caused by Spanning Tree operation. When a port is enabled or disabled by Spanning Tree, a General Query may be sent on all active non-router ports in order to reduce network convergence time. Non-Querier switches should be aware of whether the Querier is in IGMPv3 mode. If so, the switch should not spoof any General Queries unless it is able to send an IGMPv3 Query that adheres to the most recent information sent by the true Querier. In no case should a switch introduce a spoofed IGMPv2 Query into an IGMPv3 network, as this may create excessive network disruption.
 If the switch is not the Querier, it should use the 'all-zeros' IP Source Address in these proxy queries (even though some hosts may elect to not process queries with a IP Source Address). When such proxy queries are received, they must not be included in the Querier election process.


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