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Configuring an Intermediate IP Multicast Helper

When a multicast-capable internetwork is between two subnets with broadcast-only-capable hosts, you can convert broadcast traffic to multicast at the first hop router, and convert it back to broadcast at the last hop router to deliver the packets to the broadcast clients. Thus, you can take advantage of the multicast capability of the intermediate multicast internetwork. Configuring an intermediate IP multicast helper prevents unnecessary replication at the intermediate routers and can take advantage of multicast fast switching in the multicast internetwork.

See Figure 73 and the example of this feature in the section "IP Multicast Helper Example" later in this chapter.

An extended IP access list controls which broadcast packets are translated, based on the UDP port number.

To configure an intermediate IP multicast helper, the first hop router and the last hop router must be configured. To configure the first hop router, use the following commands beginning in global configuration mode:

After configuring the first hop router, use the following commands on the last hop router beginning in global configuration mode:

IP Multicast Helper Example

Figure 73 illustrates how a helper address on two routers converts traffic from broadcast to multicast and back to broadcast.

Figure 73 IP Multicast Helper Scenario

In this example, a server on the LAN connected to Ethernet interface 0 of Router A is sending a UDP broadcast stream with a source address of 126.1.22.199 and a destination address of 126.1.22.255:4000. The configuration on the first hop router converts the broadcast stream arriving at incoming Ethernet interface 0 destined for UDP port 4000 to a multicast stream. The access list permits traffic being sent from the server at 126.1.22.199 being sent to 126.1.22.255:4000. The traffic is sent to group address 239.254.2.5. The ip forward-protocol command specifies the forwarding of broadcast messages destined for UDP port 4000.

Note This example only displays the configuration related to configuring an IGMP intermediate helper. PIM-SM is used in this example. PIM-SM requires the use of a rendezvous point (RP). For more information about configuring RPs, see the "Configuring Basic IP Multicast" chapter in the Cisco IOS IP Multicast Configuration Guide, Release 12.4.

The second configuration on the last hop router converts the multicast stream arriving at incoming Ethernet interface 1 back to broadcast at outgoing Ethernet interface 2. Again, not all multicast traffic emerging from the multicast cloud should be converted from multicast to broadcast, only the traffic destined for 126.1.22.255:4000.

The configurations for Router A and Router C are as follows:

Router A—First Hop Router Configuration

interface ethernet 0

 ip address 126.1.22.1 255.255.255.0

 ip pim sparse-mode

 ip multicast helper-map broadcast 239.254.2.5 105

access-list 105 permit udp host 126.1.22.199 host 126.1.22.255 eq 4000

ip forward-protocol udp 4000

Router C—Last Hop Router Configuration

interface ethernet 1

 ip address 126.1.26.1 255.255.255.0

 ip pim sparse-mode

 ip multicast helper-map 239.254.2.5 126.1.28.255 105

interface ethernet 2

 ip address 126.1.28.1 255.255.255.0

 ip directed-broadcast

access-list 105 permit udp host 126.1.22.199 any eq 4000

ip forward-protocol udp 4000 

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