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Manuel Muetsch
Beginner

ip option "Router Alert" set in IGMP query beginning IOS 15.1SY version

Dear Support Community,

troubleshooting a multicast case I hit on the ip option "Router Alert".

It looks like Cisco has implemented the ip option "Router Alert" for sending igmp querys beginning with IOS 15.1SY version, see attached Wireshark Trace.

I didn't find any prove about that in any Release Note, but had several hits with google at least for IOS 15.1SY version for the checking of this ip option.

With the global command "ip igmp snooping check rtr-alert-option" we are able to force our router to accept only igmp packets with router-alert option.

But I could not found any command to disable setting this ip option.

Is there some kind of hidden command to disable setting this router-alert ip option?

It looks like that the multicast clients have trouble with this ip option and do send an igmp membership report out of time (not within the maximum response time).

Regards,

Manuel

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