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In the previous multicast routing protocols example, PIM Examples, we have talked about ASM (Any Source Multicast) like PIM Dense Mode, PIM Sparse Mode etc. This means that, the multicast receivers do not care about the multicast source. Any multicast source can be accepted by the users. This was Any Source Multicast (ASM). In this lesson, we will focus on PIM-SSM (Source Specific Multicast). We will learn, What is PIM-SSM and How to Configure PIM-SSM on Cisco Routers.
PIM Source Specific Multicast is a multicast method in which, receivers receive multicast traffic from a specific multicast source as its name implies. In other words, for Source Specific Multicast, source discovery responsibility shifts from multicast network to multicast receiver.
PIM-SSM (Source Specific Multicast) is based on PIM-SM but with some differences. PIM messages and PIM timers remain unchanged for PIM Source Specific Multicast. But this time Randevous-Point related jobs will no longer be used. We also do not need Auto-RP and Bootstrap anymore.
In PIM-SSM, multicast receivers can request traffic from a specific source within the multicast group. In other ASM (Any Source Multicast) modes, this was not possible. In ASM, any source can be accepted by the receiver as its name implies.
PIM-SSM (Source Specific Multicast) requires IGMPv3. The relationship between multicast receivers and the source is determined by this last version of IGMP, IGMPv3. This is a requirement for Source Specific Multicast.
PIM-SSM is based on PIM Sparse Mode as we have mentioned before. So, Source Specific Multicast mechanism uses Neighbor Discovery, DR election and SPT Building like PIM Sparse Mode.
In this method, there is no Shared Tree, there is no Randevous-Point (RP). Only Shortest Path Tree is built from receiver to the multicast source. Because, the router which receive IGMPv3 join request, knows how to reach that specific multicast source without getting help from a Randevous-Point (RP).
There is a specific Class D IP range that are used for Source Specific Multicast only. These range is of 232.0.0.0/8. This specific range was reserved by IANA (Internet Assigned Numbers Authority). We can use other ranges for SSM but other multicast modes can’t use this range for their jobs. In other words, we can say that, all of 232.0.0.0/8 IPs are SSM but not all SMM use of 232.0.0.0/8.
We have learned what is PIM-SSM and how does it works. Now, it is time to configure Source Specific Multicast on Cisco routers. For our Cisco PIM-SSM Configuration Example, we will use the below simple topology.
Here firstly, we will configure interface ip addresses and static routes between these routers for IP reachability. After the configuration of all routers for layer 3 reachability, we will configure PIM-SSM on all routers.
Firstly, let’s start with interface IP address configurations.
You can also check Packet Tracer Labs Page For More Configuration Examples.
We will configure the interface IP addresses as given above on all the routers.
Router1(config)# interface fastEthernet 0/0
Router1(config-if)# ip address 10.0.0.1 255.255.255.0
Router1(config-if)# no shutdown
Router1(config-if)# exit
Router2(config)# interface fastEthernet 0/0
Router2(config-if)# ip address 10.0.0.2 255.255.255.0
Router2(config-if)# no shutdown
Router2(config-if)# interface fastEthernet 0/1
Router2(config-if)# ip address 20.0.0.2 255.255.255.0
Router2(config-if)# no shutdown
Router2(config-if)# exit
Router3(config)# interface fastEthernet 0/0
Router3(config-if)# ip address 20.0.0.1 255.255.255.0
Router3(config-if)# no shutdown
Router3(config-if)# exit
After IP address configurations, let’s write static routes for layer 3 reachability. We will write static routes on Router 1 and Router3.
Router1(config)# ip route 20.0.0.0 255.255.255.0 10.0.0.2
Router3(config)# ip route 10.0.0.0 255.255.255.0 20.0.0.2
For multicast routing process, we need to enable it on all routers. To do this we will use “ip multicast-routing” command on all the routers under global configuration mode.
Router1(config)# ip multicast-routing
Router2(config)# ip multicast-routing
Router3(config)# ip multicast-routing
As we have mentioned in PIM-SSM lesson, PIM-SSM is PIM Sparse Mode based. There are some differences between them but there are also many similarities. So, we will enable PIM Sparse mode under the router interfaces with “ip pim sparse-mode” command. You can also use “ip sparse-dense-mode” command. Both of them will satisfy our need.
Router1(config)# interface fastEthernet 0/0
Router1(config-if)# ip pim sparse-mode
Router1(config-if)# exit
Router2(config)# interface fastEthernet 0/0
Router2(config-if)# ip pim sparse-mode
Router2(config-if)# interface fastEthernet 0/1
Router2(config-if)# ip pim sparse-mode
Router2(config-if)# exit
Router3(config)# interface fastEthernet 0/0
Router3(config-if)# ip pim sparse-mode
Router3(config-if)# exit
Now, it is time to configure PIM-SSM on all the routers. To do this, we will use “ip pim ssm default” command. The default SSM range is 232.0.0.0/8 range. So, if we do not configure any different range, the default SSM range will be used.
To configure a specific SSM range, you can use “ip pim ssm range access-list” command. Here, you should define the range with an access-list before to use here.
In this example, we will prefer to use the default SSM range.
Router1(config)# ip pim ssm default
Router2(config)# ip pim ssm default
Router3(config)# ip pim ssm default
PIM-SSM is used with IGMPv3. For this reason, we will configure IGMPv3 under source and receiver router interfaces. To do this, we will use “ip igmp version 3” command. The default igmp version is version 2. So, we should specify igmp version 3.
Router1(config)# interface fastEthernet 0/0
Router1(config-if)# ip igmp version 3
Router3(config)# interface fastEthernet 0/0
Router3(config-if)# ip igmp version 3
After configuring PIM-SSM on routers. Now, we can verify our configuration. To do this, we will use two Cisco commands. These are “show ip igmp groups detail” and “show ip mroute”.
Here, “show ip igmp groups detail”command will shows the (S,G) channel subscription through IGMPv3. And “show ip mroute” command will show that if a multicast group supports SSM and if a source-specific host was received.
In this lesson, we have learned how to configure Cisco PIM-SSM on a simple topology. We have enabled multicast routing, PIM Sparse mode, then we have configured PIM-SSM globally and IGMPv3 on the receiver and source interfaces.
To learn on multicast routing mode, you can check the other PIM modes and their configuration examples.
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