ip directed-broadcast with multicast

[o-ziltener]

Hello

does a router(in my case a msfc2) replicate a single mulitcast stream (MC-source is not directed connected) to all Vlans with "ip directed-broadcast" enabled? If yes, will do the msfc this in software and could cause higher cpu?

many thanks for helping

Oliver

[Richard Burts]

Oliver

I am not sure that I really understand your question. But the command ip directed-braodcast has nothing to do with replicating multicast.

First lets deal with the ip directed-broadcast command. To understand this command think of a router interface configured with ip address 172.16.14.1 255.255.255.0. What would happen if the router received a packet whose destination address was 172.16.14.255? Should the router send it out the interface as a broadcast (with the destination MAC set to the broadcast address)? If the interface is configured with ip directed-broadcast then the answer is yes the router should forward the broadcast. If the interface is configured with no ip directed-broadcast (which is now the default) then no the router should not forward the packet.

So the directed-broadcast command deals with broadcast packets not with multicast.

Now lets consider what happens if the MSFC receives a multicast packet (other than the multicast to addresses 224.0.0.x which are reserved for the local link). If the MSFC is not configured with ip multicast routing the MSFC will do nothing with the packet. If the MSFC is configured for ip multicast routing and if multicast routing is enabled on the interface on which the packet arrived, then the MSFC will replicate and forward the packet on any interface which is in the forwarding table for that multicast address. While that may require some overhead on the MSFC I do not believe that forwarding a single multicast stream would cause enough overhead to be a problem.

HTH

Rick

[o-ziltener]

Hey Rick

thanks for answering. My question is based on the point, that multicast is a special form of broadcast.

Of course, the router is configured global to forward multicast and every vlan interface has the cmds "ip pim dense-mode", ip cgmp and ip directed-broadcast. And if the router send out a broadcast on those interface, it could do it also with a destination of 239.0.0.255, or not?

best regards

Oliver

[Richard Burts]

Oliver

When we think in terms of layer 2 then I agree that we can think of broadcast as a special form of multicast (single frame with multiple receiving stations). If we think in terms of layer 3 then broadcast and multicast operate very differently.

If multicast routing is enabled globally and if every interface is configured with ip pim dense-mode and with ip cgmp then certainly the MSFC will forward multicast packets in accordance with the multicast distribution tree.

If each interface is configured with ip directed-broadcasts then each interface would forward a directed broadcast onto the local segment. However 239.0.0.255 is not a broadcast address. The multicast address space (224.0.0.0 through 239.255.255.255) does not have the same concept of broadcast address that the unicast address space has. So 239.0.0.255 is a particular multicast group and not a broadcast address.

HTH

Rick