Multicast Stub Routing and IGMP Helper

Multicast stub routing can be used for remote sites that are connected to the main site using low bandwidth links. When you use PIM dense mode these links might become overburdened with multicast traffic because of the flood and prune behavior of PIM dense mode. PIM sparse mode can also utilize the link because of rendezvous point announcements. The remote router has to accept RP discovery messages, build the RP cache and maintain states in the multicast routing table for all its local receivers.

When we configure a router as “multicast stub” then it will not process any PIM or IGMP messages. Instead it will forward all these messages to the router on the main site. Let me give you an example topology:

Above we have a small network with 4 routers. R1 and R2 are on the “remote office” and R3 and R4 are on the “main site”. Between R2 and R3 is a WAN serial link with limited capacity.

We will configure this network so that R1 is the receiver, R4 is the source and R2 will become our multicast stub router. Let’s get started!

R1,R2,R3,R4:
(config)#router ospf 1
(config-router)#network 0.0.0.0 255.255.255.255 area 0

Let’s enable multicast routing on all routers:

R1,R2,R3,R4:
(config)#ip multicast-routing

Now we will configure R3, the router on the “main site”…

R3(config)#ip pim rp-address 192.168.23.3
R3(config)#interface serial 0/0
R3(config-if)#ip pim sparse-mode 
R3(config-if)#ip pim neighbor-filter 1
R3(config-if)#interface fastethernet0/0
R3(config-if)#ip pim sparse-mode

R3(config)#access-list 1 deny host 192.168.23.2 
R3(config)#access 1 permit any

I will configure R3 as the RP. We have to enable PIM on the serial0/0 interface because our router won’t forward multicast traffic out of a non-PIM enabled interface. We don’t want R2 and R3 to become PIM neighbors because this would allow R2 to prune multicast traffic using PIM. Let’s configure R2 now:

R2(config)#interface fastEthernet 0/0
R2(config-if)#ip igmp helper-address 192.168.23.3
R2(config-if)#ip pim dense-mode
R2(config-if)#exit
R2(config)#interface serial0/0
R2(config-if)#ip pim dense-mode

The IGMP helper-address command is used to forward all IGMP traffic towards R3 instead of dealing with it ourselves. We will use PIM dense mode because it allows the router to flood ANY multicast traffic that we receive from R2 towards our clients.

Now let’s configure R1 as the receiver and R4 to send some multicast packets to see if our configuration works:

R1(config)#interface fastEthernet 0/0
R1(config-if)#ip igmp join-group 239.1.1.1

As soon as we join the group you will see this on R3:

R3#show ip igmp groups 239.1.1.1
IGMP Connected Group Membership
Group Address    Interface                Uptime    Expires   Last Reporter   Group Accounted
239.1.1.1      Serial0/0               00:00:20  00:02:39  192.168.23.2 

The IGMP join has been forwarded from R2 towards R3. Now let’s send some traffic:

R4#ping 239.1.1.1 repeat 9999

Type escape sequence to abort.
Sending 9999, 100-byte ICMP Echos to 239.1.1.1, timeout is 2 seconds:

Reply to request 0 from 192.168.12.1, 4 ms
Reply to request 1 from 192.168.12.1, 16 ms
Reply to request 2 from 192.168.12.1, 4 ms

Our pings are working, now let’s check the multicast routing table of R2 and R3:

R3#show ip mroute 239.1.1.1
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
       L - Local, P - Pruned, R - RP-bit set, F - Register flag,
       T - SPT-bit set, J - Join SPT, M - MSDP created entry,
       X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
       U - URD, I - Received Source Specific Host Report,
       Z - Multicast Tunnel, z - MDT-data group sender,
       Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
 Timers: Uptime/Expires
 Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 239.1.1.1), 00:04:36/stopped, RP 192.168.23.3, flags: SJC
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    Serial0/0, Forward/Sparse, 00:04:36/00:02:06

(192.168.34.4, 239.1.1.1), 00:02:34/00:02:57, flags: T
  Incoming interface: FastEthernet0/0, RPF nbr 0.0.0.0
  Outgoing interface list:
    Serial0/0, Forward/Sparse, 00:02:34/00:02:06

As you can see R3 is forwarding it towards R2 using PIM sparse mode.

R2#show ip mroute 239.1.1.1
IP Multicast Routing Table
Flags: D - Dense, S - Sparse, B - Bidir Group, s - SSM Group, C - Connected,
       L - Local, P - Pruned, R - RP-bit set, F - Register flag,
       T - SPT-bit set, J - Join SPT, M - MSDP created entry,
       X - Proxy Join Timer Running, A - Candidate for MSDP Advertisement,
       U - URD, I - Received Source Specific Host Report,
       Z - Multicast Tunnel, z - MDT-data group sender,
       Y - Joined MDT-data group, y - Sending to MDT-data group
Outgoing interface flags: H - Hardware switched, A - Assert winner
 Timers: Uptime/Expires
 Interface state: Interface, Next-Hop or VCD, State/Mode

(*, 239.1.1.1), 00:05:12/stopped, RP 0.0.0.0, flags: DC
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    Serial0/0, Forward/Dense, 00:05:12/00:00:00
    FastEthernet0/0, Forward/Dense, 00:05:12/00:00:00

(192.168.34.4, 239.1.1.1), 00:03:10/00:02:57, flags: T
  Incoming interface: Serial0/0, RPF nbr 192.168.23.3
  Outgoing interface list:
    FastEthernet0/0, Forward/Dense, 00:03:10/00:00:00