Multicast - helper-address

Overview

The ip multicast helper-address command can be useful when you have a need to send broadcast traffic between two layer 2 or more layer 2 domains. For instance a legacy application that pushes lots of information to clients such as a stock ticker program.. The steps are outlined below.

1. Configure PIM multicast along path between source and target broadcast LAN(s)
2. Configure ip forward-protocol   on ingress router
3. Configure ip muticast helper-address on ingress router interface facing the destination L2 network
4. Configure ip forward-protocol   on egress router
5. Configure ip muticast helper-address  on egress router's interface(s) facing the source.
6. Configure ip directed-broadcast  on ingress router's interface facing the destination L2 network
7. Configure ip broadcast-address  on ingress router interface facing the destination L2 network

Concepts tested

• Configuring PIM SM mode
• Forwarding broadcast traffic via multicast
• Verification of configuration

Topology

Lab Tasks

• Configure PIM SM across the paths between nodes R1, R2, R4, and R5. R3 should not be a multicast path.
• R2 should be the BSR and RP
• Configure R4 so that udp traffic from R5 on port 49000 is broadcast on to the 120.0.0.0/24 subnet
• Use group 239.1.1.1 and DNS broadcast to test

GNS3 configuration file, requires IOS v15 for the 7200 router: Link


Solution

R1(config)#ip multicast-routing
R1(config)#int g0/0
R1(config-if)#ip pim sparse-mode
R1(config-if)#exit
R1(config)#int loopback 0
R1(config-if)#ip pim sparse-mode
R1(config-if)#exit

R2(config)#ip multicast-routing
R2(config)#int g0/0
R2(config-if)#ip pim sparse-mode
R2(config-if)#exit
R2(config)#int g1/0
R2(config-if)#ip pim sparse-mode
R2(config-if)#exit
R2(config)#ip pim rp-candidate loopback 0
R2(config)#ip pim bsr-candidate loopback 0

R2(config)#ip access-list extended ALLOWED_IPS

R2(config-ext-nacl)#permit udp any any eq 49000

R2(config-ext-nacl)#permit udp any any eq 53

R2(config)#ip forward-protocol udp 49000
R2(config)#int g1/0

R2(config-if)#ip multicast helper-map 239.1.1.1 120.0.0.255 ALLOWED_IPS

R2(config)#int g0/0

R2(config-if)#ip directed-broadcast

R2(config-if)#ip broadcast-address 120.0.0.255

R3(config)#ip multicast-routing

R3(config)#int g0/0

R3(config-if)#ip pim sparse-mode

R3(config-if)#exit

R3(config)#int g1/0

R3(config-if)#ip pim sparse-mode

R3(config-if)#exit

R4(config)#ip multicast-routing

R4(config)#int g0/0

R4(config-if)#ip pim sparse-mode

R4(config-if)#exit

R4(config)#int g1/0

R4(config-if)#ip pim sparse-mode

R4(config-if)#exit

R4(config)#int g2/0

R4(config-if)#ip pim sparse-mode

R4(config-if)#exit

R4(config)#ip access-list extended ALLOWED_IPS

R4(config-ext-nacl)#permit udp any any eq 49000

R4(config-ext-nacl)#permit udp any any eq 53

R4(config)#ip forward-protocol udp 49000

R4(config-if)#ip multicast helper-map broadcast 239.1.1.1 ALLOWED_IPS

R5(config)#ip multicast-routing

R5(config)#

R5(config)#int g0/0

R5(config-if)#ip pim sparse-mode

R5(config-if)#exit

Verification

R5(config)#ip domain lookup

R4(config)#int g0/0

R4(config-if)#no ip mfib cef input

R4(config-if)#no ip mfib cef output

R4(config-if)#int g2/0

R4(config-if)#no ip mfib cef input

R4(config-if)#no ip mfib cef output

R4(config)#access-list 100 permit udp any any eq 53

R4#debug ip mfib pak 239.1.1.1

R4#deb ip pack detail 100

R2(config)#int g0/0

R2(config-if)#no ip mfib cef input

R2(config-if)#no ip mfib cef output

R2(config-if)#int g1/0

R2(config-if)#no ip mfib cef input

R2(config-if)#no ip mfib cef output

R2(config)#access-list 100 permit udp any any eq 53

R4:

R4#debug ip packet detail 100

R4#debug ip mfib pak 239.1.1.1

*Dec  8 08:57:12.407: IP: s=120.0.45.5 (GigabitEthernet2/0), d=255.255.255.255, len 60, rcvd 2

*Dec  8 08:57:12.407:     UDP src=50743, dst=53

*Dec  8 09:43:26.099: MFIBv4(0x0): Pkt (120.0.45.5,239.1.1.1) from GigabitEthernet2/0 (PS) accepted for forwarding

*Dec  8 09:43:26.103: MFIBv4(0x0): Pkt (120.0.45.5,239.1.1.1) from GigabitEthernet2/0 (PS) sent on GigabitEthernet0/0

R2:

R2#debug ip mfib pak 239.1.1.1

*Dec  8 09:44:54.859: MFIBv4(0x0): Pkt (120.0.45.5,239.1.1.1) from GigabitEthernet1/0 (PS) accepted for forwarding

*Dec  8 09:44:54.863: MFIBv4(0x0): Pkt (120.0.45.5,239.1.1.1) from GigabitEthernet1/0 (PS) sent on GigabitEthernet0/0

R2#sh ip mroute 239.1.1.1

(*, 239.1.1.1), 00:41:35/00:02:32, RP 2.2.2.2, flags: SJCL

  Incoming interface: Null, RPF nbr 0.0.0.0

  Outgoing interface list:

    GigabitEthernet0/0, Forward/Sparse, 00:16:41/00:02:33

    GigabitEthernet1/0, Forward/Sparse, 00:36:26/00:02:32

(120.0.45.5, 239.1.1.1), 00:02:47/00:00:12, flags: LJT

  Incoming interface: GigabitEthernet1/0, RPF nbr 120.0.24.4

  Outgoing interface list:

    GigabitEthernet0/0, Forward/Sparse, 00:02:47/00:02:33