Configuring an Intermediate IP Multicast Helper Between Broadcast-Only Networks

Last Updated: January 3, 2012

When a multicast-capable internetwork is between two subnets with broadcast-only-capable hosts, you can convert broadcast traffic to IP multicast traffic at the first hop router and convert it back to broadcast traffic at the last hop router to deliver the packets to the broadcast clients. You can thus take advantage of the multicast capability of an intermediate IP multicast helper. Configuring an intermediate IP multicast helper allows the transport of broadcast packets across an IP multicast-enabled network, thereby preventing unnecessary replication at the intermediate routers.

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Prerequisites for Configuring an Intermediate IP Multicast Helper Between Broadcast-Only Networks

  • You understand the concepts documented in the " IP Multicast Technology Overview " module.
  • You have IP multicast configured in your network environment and your IP multicast network is between broadcast-only networks. See the " Configuring Basic IP Multicast " module for more information about configuring IP multicast.

Information About Configuring an Intermediate IP Multicast Helper Between Broadcast-Only Networks

Intermediate IP Multicast Helper Capability

An intermediate IP multicast helper allows the transport of broadcast packets across an IP multicast-enabled network, thereby preventing unnecessary replication at the intermediate routers.

When configuring an intermediate IP multicast helper between broadcast-only networks, you must configure the first hop router to convert broadcast traffic to IP multicast traffic and the last hop router to convert IP multicast traffic back to broadcast traffic.

How to Configure an Intermediate IP Multicast Helper Between Broadcast-Only Networks

Configuring the First Hop Router to Convert Broadcast Traffic to IP Multicast Traffic

Perform this task to convert broadcast traffic to IP multicast traffic on the first hop router. The first hop router is on the border between the broadcast-only network and IP multicast network.

Before You Begin
  • This task assumes that you have an IP multicast network configured between two broadcast-only networks.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    access-list access-list-number {deny | permit} udp {any | [host] source-address source-wildcard} [operator [port]] {any | [host] destination-address destination-wildcard} [operator [port]]

4.    interface type number

5.    ip multicast helper-map broadcast group-address access-list

6.    exit

7.    ip forward-protocol udp [port]

8.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
access-list access-list-number {deny | permit} udp {any | [host] source-address source-wildcard} [operator [port]] {any | [host] destination-address destination-wildcard} [operator [port]]


Example:

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

 

Creates an extended IP access list to control which UDP broadcast packets are translated.

 
Step 4
interface type number


Example:

Router(config)# interface gigabitethernet 0/0/0

 

Enters interface configuration mode.

  • For the type and number arguments, select an incoming interface on the first hop router that is receiving broadcast-only traffic
 
Step 5
ip multicast helper-map broadcast group-address access-list


Example:

Router(config-if)# ip multicast helper-map broadcast 239.254.2.5 105

 

Allows IP multicast routing in a multicast-capable internetwork between two broadcast-only internetworks.

  • In the configuration on the first hop router,the ip multicast helper-map command is used with the broadcast keyword and group-address argument to specify the traffic to be converted from broadcast to multicast. The multicast group address specified for the group-address argument is the address to which the converted traffic will be directed.
  • For the access-list argument, specify the name or number of the access list created in Step 3 of this task.
 
Step 6
exit


Example:

Router(config-if)# exit

 

Exits interface configuration mode and returns to global configuration mode.

 
Step 7
ip forward-protocol udp [port]


Example:

Router(config)# ip forward-protocol udp 4000

 

Configures the forwarding of UDP broadcast messages destined for the specified port.

 
Step 8
end


Example:

Router(config)# end

 

Exits global configuration mode and enters privileged EXEC mode.

 

Configuring the Last Hop Router to Convert the IP Multicast Traffic Back to Broadcast Traffic

Perform this task to convert the IP multicast traffic back to broadcast traffic on the last hop router. The last hop router is on the border between the intermediate IP multicast network and broadcast-only network.

Before You Begin
  • This task assumes that you have an IP multicast network configured between two broadcast-only networks.

SUMMARY STEPS

1.    enable

2.    configure terminal

3.    access-list access-list-number {deny | permit} udp {any | [host] source-address source-wildcard} [operator [port]] {any | [host] destination-address destination-wildcard} [operator [port]]

4.    interface type number

5.    ip multicast helper-map group-address broadcast-address access-list

6.    exit

7.    interface type number

8.    ip directed-broadcast

9.    exit

10.    ip forward-protocol udp [port]

11.    end


DETAILED STEPS
  Command or Action Purpose
Step 1
enable


Example:

Router> enable

 

Enables privileged EXEC mode.

  • Enter your password if prompted.
 
Step 2
configure terminal


Example:

Router# configure terminal

 

Enters global configuration mode.

 
Step 3
access-list access-list-number {deny | permit} udp {any | [host] source-address source-wildcard} [operator [port]] {any | [host] destination-address destination-wildcard} [operator [port]]


Example:

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

 

Creates an extended IP access list to control which UDP broadcast packets are translated.

 
Step 4
interface type number


Example:

Router(config)# interface gigabitethernet 1/0/0

 

Enters interface configuration mode.

  • For the type and number arguments, select an incoming interface on the last hop router that is receiving IP multicast traffic.
 
Step 5
ip multicast helper-map group-address broadcast-address access-list


Example:

Router(config-if)# ip multicast helper-map 239.254.2.5 126.1.28.255 105

 

Allows IP multicast routing in a multicast-capable internetwork between two broadcast-only internetworks.

  • In the configuration on the last hop router,the ip multicast helper-map command is used with the group-addressand broadcast-address arguments to specify the traffic to be converted from IP multicast to broadcast. The multicast group address specified for the group-address argument is the address of the traffic to be converted from IP multicast to broadcast. The broadcast address specified for the broadcast-address argument is the address to which the broadcast traffic will be sent.
  • For the access-list argument, specify the name or number of the access list created in Step 3 of this task.
 
Step 6
exit


Example:

Router(config-if)# exit

 

Exits interface configuration and returns to global configuration mode.

 
Step 7
interface type number


Example:

Router(config)# interface gigabitethernet 2/0/0

 

Enters interface configuration mode.

  • For the type and number arguments, select an outgoing interface on the last hop router that is facing the destination broadcast-only subnet.
 
Step 8
ip directed-broadcast


Example:

Router(config-if)# ip directed-broadcast

 

Enables the translation of a directed broadcast to physical broadcasts.

 
Step 9
exit


Example:

Router(config-if)# exit

 

Exits interface configuration and returns to global configuration mode.

 
Step 10
ip forward-protocol udp [port]


Example:

Router(config)# ip forward-protocol udp 4000

 

Configures the forwarding of UDP broadcast messages destined for the specified port.

 
Step 11
end


Example:

Router(config)# end

 

Exits global configuration mode and enters privileged EXEC mode.

 

Configuration Examples for an Intermediate IP Multicast Helper Between Broadcast-Only Networks

Configuring an Intermediate IP Multicast Helper Between Broadcast-Only Networks Example

This example shows how to configure an intermediate IP multicast helper between broadcast-only networks. The topology used for this example is illustrated in the figure below.

Figure 1 IP Multicast Helper Example Topology


In this example, a server on the LAN connected to Gigabit Ethernet interface 0/0/0 of Router A is sending a UDP broadcast traffic 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 traffic arriving at incoming Gigabit Ethernet interface 0/0/0 destined for UDP port 4000 to IP multicast traffic. 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 primarily displays the configuration related to configuring an intermediate IP multicast helper. Protocol Independent Multicast-Sparse Mode (PIM-SM) is the multicast protocol 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 module.

The configuration on the last hop router converts the IP multicast traffic at incoming Gigabit Ethernet interface 1/0/0 back to broadcast at outgoing Gigabit Ethernet interface 2/0/0. Again, not all multicast traffic emerging from the multicast network 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 gigabitethernet 0/0/0
 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 gigabitethernet 1/0/0
 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 gigabitethernet 2/0/0
 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

Additional References

Related Documents

Related Topic

Document Title

Basic IP multicast concepts, configuration tasks, and examples

" Configuring Basic IP Multicast " module

Overview of the IP multicast technology area

" IP Multicast TechnologyOverview " module

IP multicast commands: complete command syntax, command mode, command history, command defaults, usage guidelines, and examples

Cisco IOS IP Multicast Command Reference

Standards

Standard

Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

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MIBs

MIB

MIBs Link

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To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs

RFCs

RFC

Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

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Feature Information for Configuring an Intermediate IP Multicast Helper Between Broadcast-Only Networks

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.

Table 1 Feature Information for Configuring an IP Multicast Helper Between Broadcast-Only Networks

Feature Name

Releases

Feature Information

This table is intentionally left blank because no features were introduced or modified in this module since Cisco IOS Release 12.2(1). This table will be updated when feature information is added to this module.

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Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

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