Re: IP Router alert option in IGMP messages

On Nov 3, 2006, at 12:19 AM, Princy Elizabeth wrote:

> Hi all,
>
> I had a query on the usage of the IP Router alert option in IGMP  
> messages. I do not understand what exactly is the significance of  
> this option. What does a non-multicast enabled router do on  
> receiving an IGMP 'Join' or a 'Query' with the IP Router alert  
> option set? Does a multicast enabled router actually require this  
> option for recognising IGMP packets in its fast path? Don't most  
> routers look at the IGMP protocol field for identifying IGMP packets?

Most routers have implemented it. (The RA option)
It should be configurable since some hosts have not.
Its most useful for igmpv2 because IGMP host reports are sent to the  
multicast group address
	not a specific address (as in IGMPv3) which the router is a member of.
Yes if the RA is not configured the router looks at the IP protocol  
field, but the
	problem is that the router must listen to the complete multicast  
group range
	just to see if there might be an IGMP packet.  Using RA, the router  
doesn't have to work so hard.

>
> Thanks and regards,
> Princy
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