Multicast routers bombarded with data

[mkaften]

We have a Lan with several multicast routers. Each router handles different multicast groups sent by remote sources to the hosts on the Lan. All the routers are connected to a Cisco 3560G switch. The routers range from 7204s to 2621XM, and even a 6503 switch. We found that each router is constantly bombarded with multicast data from each other router, even though the router did not join any groups. Obviously, we'd rather not have this happen, because it causes each router's CPU to rise to very uncomfortable levels. If I bring a new router onto the Lan, as soon as I enable multicast, the data starts hitting the router's interface. In an attempt to stop the traffic, I protected each router port, so that (supposedly) the router will not be able to communicate with any other protected port. This doesn't help. Why is this happening, and is there any way to stop it from happening? Thanks very much.

[Richard Burts]

It is the nature of multicast that the Cisco router interface will receive and process every multicast packet. I am not sure that IGMP snooping would resolve this but it is the first thing that comes to mind.

The other suggestion would be the possibility of re-design of the network. If there are so many routers and so many multicast groups then it suggests that it is a pretty large vlan/subnet. What if you created several vlans on the switch and reduced the number of multicast groups per vlan.

HTH

Rick

Sent from Cisco Technical Support iPhone App

[mkaften]

Thanks for the suggestions. IGMP snooping is on, so obviously it will not help--although I'm not sure why. The routers are not requesting any data at all from each other.

We have hosts that receive parts of the data set from several different routers, and having to create 3 or 4 vlans requiring a separate NIC on each host is a solution, just not a convenient one.

I'm still seeking an answer to my main question: WHY is a router receiving multicast data from another router when it is not requesting this data? Once I understand this, maybe I can come up with my own solution.

[Mohamed Sobair]

Hi,

This is normal behaviour, and I would assume you are using Pim Dense Mode multicast.

To optimize the multicast traffic and reduces unnecessary bandwidth, I would suggest using Pim Sparse Mode instead. which then it only forward multicast traffic to the recievers when there is a Pim Join Request from the DR site.

Regards,

Mohamed

[mkaften]

Dense or sparse mode, I don't understand why this is "normal". Again, I have a router that is not requesting any data at all--it has joined no groups. It merely has multicast-routing enabled. The switch is using IGMP snooping. So why is the router being bombarded with data, not PIM messages or IGMP joins, but DATA, from other routers? Even if a dense-mode router floods the switch with data, it will not be forwarded to switch ports where the host has not issued a join message, correct? And if the switch ports are protected, why should the router see any data being sent from another protected port?

[mkaften]

To elaborate. All multicast data on the network has been requested. Just not by other routers. There are hosts requesting all available data. So sparse-mode would produce the same data on the network as dense-mode. The problem is that all the routers are seeing the data of all the other routers. IGMP snooping is supposed to take care of this. My gut tells me that there's nothing I can do, but I just want to understand why.