mldv1_unsolicited_report_interval

file: /proc/sys/net/ipv6/conf/interface/mldv1_unsolicited_report_interval
variable: net.ipv6.conf.interface.mldv1_unsolicited_report_interval
Official reference

The interval in milliseconds in which the next unsolicited MLDv1 report retransmit will take place. Default: 10000 (10 seconds)

Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.

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