From CCIE Study Wiki
The Multicast Helper feature allows a multicast network to deliver traffic between broadcast-only hosts.
Theory
When a multicast network sits in-between two hosts who need to communicate via broadcasts, the multicast helper feature can enable the communication. The multicast helper converts the broadcast traffic from the sender to multicast at the first hop router, and will convert the multicast traffic back to broadcast packets at the last hop router to deliver the traffic to the broadcast receiver.
The multicast/broadcast conversions are handled by the ip multicast helper-map command. The command will reference the multicast group number that traffic is being converted to/from, as well as an access-list. The access-list will usually be built to permit a UDP port that the broadcast traffic that needs to be translated is being sent on.
In addition to the helper-map, the processing of directed broadcasts must be allowed on each router with the ip directed-broadcast command. You must also have the ip forward-protocol udp port command configured on both routers, with the port number being the broadcast port referenced by the ACL.
Commands
- ip multicast helper-map {group-address broadcast-address | broadcast multicast-address} acl [ttl value] (interface) - Command to configure a multicast/broadcast conversion on the interface. An access list will need to be configured and referenced as well. You also have the option of specifying a TTL value for the packets.
- ip multicast helper-map broadcast multicast-address acl (interface) - Specific version of the helper map to be used on the first hop router.
- ip multicast helper-map group-address broadcast-address acl (interface) - Specific version of the helper map to be used on the last hop router.
- ip directed-broadcast (interface) - Enables directed broadcasts on the interface
- ip forward-protocol udp [port] (interface) - Forces broadcasts matching the port number to be examined by the cpu.
Default Settings
Verification
Troubleshooting, Tips, and Tricks
- The “ip forward-protocol” command is only necessary to force the udp traffic to be process switched. This process switching can cause CPU overhead if high traffic levels are reached by the broadcast traffic.
- On the last hop router, the ip multicast helper-map command automatically introduces ip igmp join-group group-address on that interface.
- The TTL value of these packets may need to be tweaked if your network is configured for TTL scoping.
Online Resources
- An older version of the Cisco IOS Network Protocol configuration guide has info on configuring the multicast helper.
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