Broadcast/Multicast Conversion

Broadcast/Multicast Conversion

Assume that you have an application on a host that does not support IP multicast, only IP unicast and broadcast. Further assume that the application wants to send to a receiver or multiple receivers on a different subnet. We have seen in Chapter 2, “Internet Protocol (IP) Addresses,” that this is not possible, at least not yet. Using IP unicast only allows the sender to send to one host, and IP broadcast only allows the sender to send to hosts on the same subnet. What we need is a way to turn a broadcast into a multicast for delivery to the receivers. Now if the receivers cannot receive multicast traffic, then the multicast stream would need to be converted back to a broadcast stream on the receiving subnet (see Figure 9-3).

Figure 9-3: A broadcast-to-multicast-to-broadcast conversion is needed to enable a non-mulitcast sender to send to a non-multicast receiver.

To enable the broadcast-to-multicast conversion and the multicast-to-broadcast conversion, use the following interface configuration command on the router attached to the sender, or first hop router:

ip multicast helper-map broadcast multicast-address extended-acl

broadcast

Specifies the traffic is being converted from broadcast to multicast.

multicast-address

Multicast group address of the traffic that is to be converted to broadcast traffic.

extended-acl

IP extended access list used to determine which broadcast packets are to be converted to multicast. Based on the UDP port number.

Use the following form of the command on the router attached to the receiver or last hop router:

ip multicast helper-map group-address IP-broadcast-address extended-acl

group-address

Multicast group address of traffic to be converted to broadcast traffic.

IP-broadcast-address

IP broadcast address to which broadcast traffic is sent.

extended-acl

IP extended access list used to determine which multicast packets are to be converted to broadcast. Based on the UDP port number.

For the network in Figure 9-3, the first hop and last hop routers would have the configuration listed below:

Router A—First Hop Router.

interface Ethernet 0

ip directed-broadcast

ip multicast helper-map broadcast 239.1.2.3 100

ip pim dense-mode

access-list 100 permit any any udp 2000

access-list 100 deny any any udp

ip forward-protocol udp 2000

Router D Last Hop Router

interface ethernet 0

ip directed-broadcast

ip igmp join-group 239.1.2.3

ip multicast helper-map 239.1.2.3 172.16.1.255 100

ip pim dense-mode

access-list 100 permit any any udp 2000

access-list 100 deny any any udp

ip forward-protocol udp 2000

As configured, router A translates broadcasts to udp port 2000 to the multicast address 239.1.2.3, while router D translates traffic for multicast group 239.1.2.3 to the IP broadcast address for the subnet. The command ip igmp join-group on the last hop router is automatically configured when the ip multicast helper-map command is used. The ip forward-protocol command is necessary to disable fast-switching, which does not perform the conversion from broadcast to multicast and multicast to broadcast.