Multicast group 224.0.1.40 !

[illusion_rox]

I came by in a forum and this question really attracted me, my topology is completely based on dense mode, then why the routers join this group by default, if they dont will the topology will ever run into any problem if in future sparse mode is configured ?

[shrikar.dange]

Hi,

AFAIK the 224.0.1.40 is a reserved multicast group for cisco's Rp descovery. All cisco routers are members of this grup by default and listen to this group for Cisco RP discovery messages advertised by mapping agent even if it is not configured.I dont think there would be any problems if you change your topology to Sparse mode. But you should take in to account all the pros and cons.

HTH,

regards,

Shri :)

[illusion_rox]

Dear Shri, the question is why they are joining this group automatically ?? what is the reason behind it ?, i really appreciate ur feedback but i didnt addressed my question at all :-)

Kindly guide me

[Jason Fraioli]

What exactly are you looking for in an answer? It looks like your question has been answered already.

All cisco routers are members of this grup by default and listen to this group for Cisco RP discovery messages advertised by mapping agent even if it is not configured.

[illusion_rox]

Dear Jason, what you have written is the fact/observation right ? i am asking the reason for it ? what is the reason that cisco implemented it ? is there any reason behind this behaviour ? whats the use of making them members of this group when they are all configured in Dense mode ?

[markausten]

Auto-RP groups rely on dense mode to distribute RP information. All multicast Routers must join 224.0.1.40 to receive RP information from the mapping agents. These routers have no Auto-RP configuration so do not know if it's needed, due to this they join automatically.

[illusion_rox]

Dear Mark, if a router is configured for sparse-mode it knows that to learn RP it must listen to 224.0.1.40, so i think it can join it when it needs, i see no reason that it should join this group automatically when it doesnt need it. I hope cisco can clear what is the reason behind this automatic behaviour !

[Mohammad Rahman]

Hi Sri,

            The RP discovery IPAddress is 224.0.0.40 not 224.0.1.40. Please clear me.

Thanks,

Mostafa