Your software release may not support all the features documented in this module. For the latest feature information and caveats, see the release notes for your platform and software release.
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Enable PIM sparse mode. For information about this procedure, see How to Configure PIM
Configure SSM. For information about this procedure, see Configuring SSM
Before you configure static SSM mapping, you must configure access control lists (ACLs) that define the group ranges to be mapped to source addresses.
Before you can configure and use SSM mapping with DNS look ups, you must be able to add records to a running DNS server. If you do not already have a DNS server running, you need to install one.
Note
You can use a product such as Cisco Network Registrar to add records to a running DNS server.
Restrictions for Configuring SSM
The following are the restrictions for configuring SSM:
To run SSM with IGMPv3, SSM must be supported in the Cisco IOS router, the host where the application is running, and the application itself.
The SSM mapping feature does not have all the benefits of full SSM. Because SSM mapping takes a group join from a host and identifies this group with an application associated with one or more sources, it can only support one such application per group. Full SSM applications can still share the same group as in SSM mapping.
Enable IGMPv3 carefully on the last hop router when you rely solely on SSM mapping as a transition solution for full SSM. When you enable both SSM mapping and IGMPv3 and the hosts already support IGMPv3 (but not SSM), the hosts send IGMPv3 group reports. SSM mapping does not support these IGMPv3 group reports, and the router does not correctly associate sources with these reports.
Existing applications in a network predating SSM do not work within the SSM range unless they are modified to support (S, G) channel subscriptions. Therefore, enabling SSM in a network can cause problems for existing applications if they use addresses within the designated SSM range.
IGMPv3 uses new membership report messages that might not be correctly recognized by older IGMP snooping switches.
Address management is still necessary to some degree when SSM is used with Layer 2 switching mechanisms. Cisco Group Management Protocol (CGMP), IGMP snooping, or Router-Port Group Management Protocol (RGMP) support only group-specific filtering, not (S, G) channel-specific filtering. If different receivers in a switched network request different (S, G) channels sharing the same group, they do not benefit from these existing mechanisms. Instead, both receivers receive all (S, G) channel traffic and filter out the unwanted traffic on input.
Because SSM can re-use the group addresses in the SSM range for many independent applications, this situation can lead to decreased traffic filtering in a switched network. For this reason, it is important to use random IP addresses from the SSM range for an application to minimize the chance for re-use of a single address within the SSM range between different applications. For example, an application service providing a set of television channels should, even with SSM, use a different group for each television (S, G) channel. This setup guarantees that multiple receivers to different channels within the same application service never experience traffic aliasing in networks that include Layer 2 switches.
In PIM-SSM, the last hop router continues to periodically send (S, G) join messages if appropriate (S, G) subscriptions are on the interfaces. Therefore, as long as receivers send (S, G) subscriptions, the shortest path tree (SPT) state from the receivers to the source is maintained, even if the source does not send traffic for longer periods of time (or even never).
The opposite situation occurs with PIM-SM, where (S, G) state is maintained only if the source is sending traffic and receivers are joining the group. If a source stops sending traffic for more than 3 minutes in PIM-SM, the (S, G) state is deleted and only reestablished after packets from the source arrive again through the RPT (rendezvous point tree). Because no mechanism in PIM-SSM notifies a receiver that a source is active, the network must maintain the (S, G) state in PIM-SSM as long as receivers are requesting receipt of that channel.
Information About SSM
The source-specific multicast (SSM) feature is an extension of IP multicast in which datagram traffic is forwarded to receivers from only those multicast sources that the receivers have explicitly joined. For multicast groups configured for SSM, only SSM distribution trees (no shared trees) are created.
This section describes how to configure source-specific multicast (SSM). For a complete description of the SSM commands in this section, refer to the IP Multicast Command Reference, Cisco IOS XE Release 3SE (Catalyst 3850 Switches). To locate documentation for other commands that appear in this chapter, use the command reference master index, or search online.
SSM is a datagram delivery model that best supports one-to-many applications, also known as broadcast applications. SSM is a core networking technology for the Cisco implementation of IP multicast solutions targeted for audio and video broadcast application environments. The switch supports the following components that support SSM implementation:
Protocol independent multicast source-specific mode (PIM-SSM)
PIM-SSM is the routing protocol that supports the implementation of SSM and is derived from PIM sparse mode (PIM-SM).
Internet Group Management Protocol version 3 (IGMPv3)
SSM and Internet Standard Multicast (ISM)
The current IP multicast infrastructure in the Internet and many enterprise intranets is based on the PIM-SM protocol and Multicast Source Discovery Protocol (MSDP). These protocols have the limitations of the Internet Standard Multicast (ISM) service model. For example, with ISM, the network must maintain knowledge about which hosts in the network are actively sending multicast traffic.
The ISM service consists of the delivery of IP datagrams from any source to a group of receivers called the multicast host group. The datagram traffic for the multicast host group consists of datagrams with an arbitrary IP unicast source address (S) and the multicast group address (G) as the IP destination address. Systems receive this traffic by becoming members of the host group. Membership in a host group simply requires signaling the host group through IGMP version 1, 2, or 3.
In SSM, delivery of datagrams is based on (S, G) channels. In both SSM and ISM, no signaling is required to become a source. However, in SSM, receivers must subscribe or unsubscribe to (S, G) channels to receive or not receive traffic from specific sources. In other words, receivers can receive traffic only from (S, G) channels to which they are subscribed, whereas in ISM, receivers need not know the IP addresses of sources from which they receive their traffic. The proposed standard approach for channel subscription signaling uses IGMP and includes modes membership reports, which are supported only in IGMP version 3.
SSM IP Address Range
SSM can coexist with the ISM service by applying the SSM delivery model to a configured subset of the IP multicast group address range. Cisco IOS software allows SSM configuration for the IP multicast address range of 224.0.0.0 through 239.255.255.255. When an SSM range is defined, existing IP multicast receiver applications do not receive any traffic when they try to use an address in the SSM range (unless the application is modified to use an explicit (S, G) channel subscription).
SSM Operations
An established network, in which IP multicast service is based on PIM-SM, can support SSM services. SSM can also be deployed alone in a network without the full range of protocols required for interdomain PIM-SM (for example, MSDP, Auto-RP, or bootstrap router [BSR]) if only SSM service is needed.
If SSM is deployed in a network already configured for PIM-SM, only the last-hop routers support SSM. Routers that are not directly connected to receivers do not require support for SSM. In general, these not-last-hop routers must only run PIM-SM in the SSM range and might need additional access control configuration to suppress MSDP signalling, registering, or PIM-SM shared tree operations from occurring within the SSM range.
Use the ip pim ssm global configuration command to configure the SSM range and to enable SSM. This configuration has the following effects:
For groups within the SSM range, (S, G) channel subscriptions are accepted through IGMPv3 include-mode membership reports.
PIM operations within the SSM range of addresses change to PIM-SSM, a mode derived from PIM-SM. In this mode, only PIM (S, G) join and prune messages are generated by the router, and no (S, G) rendezvous point tree (RPT) or (*, G) RPT messages are generated. Incoming messages related to RPT operations are ignored or rejected, and incoming PIM register messages are immediately answered with register-stop messages. PIM-SSM is backward-compatible with PIM-SM unless a router is a last-hop router. Therefore, routers that are not last-hop routers can run PIM-SM for SSM groups (for example, if they do not yet support SSM).
No MSDP source-active (SA) messages within the SSM range are accepted, generated, or forwarded.
SSM Mapping
In a typical set-top box (STB) deployment, each TV channel uses one separate IP multicast group and has one active server host sending the TV channel. A single server can send multiple TV channels, but each to a different group. In this network environment, if a router receives an IGMPv1 or IGMPv2 membership report for a particular group, the report addresses the well-known TV server for the TV channel associated with the multicast group.
When SSM mapping is configured, if a router receives an IGMPv1 or IGMPv2 membership report for a particular group, the router translates this report into one or more channel memberships for the well-known sources associated with this group.
When the router receives an IGMPv1 or IGMPv2 membership report for a group, the router uses SSM mapping to determine one or more source IP addresses for the group. SSM mapping then translates the membership report as an IGMPv3 report and continues as if it had received an IGMPv3 report. The router then sends PIM joins and continues to be joined to these groups as long as it continues to receive the IGMPv1 or IGMPv2 membership reports, and the SSM mapping for the group remains the same.
SSM mapping enables the last hop router to determine the source addresses either by a statically configured table on the router or through a DNS server. When the statically configured table or the DNS mapping changes, the router leaves the current sources associated with the joined groups.
With static SSM mapping, you can configure the last hop router to use a static map to determine the sources that are sending to groups. Static SSM mapping requires that you configure ACLs to define group ranges. After configuring the ACLs to define group ranges, you can then map the groups permitted by those ACLs to sources by using the ip igmp ssm-map static global configuration command.
You can configure static SSM mapping in smaller networks when a DNS is not needed or to locally override DNS mappings. When configured, static SSM mappings take precedence over DNS mappings.
You can use DNS-based SSM mapping to configure the last hop router to perform a reverse DNS lookup to determine sources sending to groups. When DNS-based SSM mapping is configured, the router constructs a domain name that includes the group address and performs a reverse lookup into the DNS. The router looks up IP address resource records and uses them as the source addresses associated with this group. SSM mapping supports up to 20 sources for each group. The router joins all sources configured for a group.
Figure 1. DNS-Based SSM Mapping. The following figure displays DNS-based SSM mapping.
The SSM mapping mechanism that enables the last hop router to join multiple sources for a group can provide source redundancy for a TV broadcast. In this context, the last hop router provides redundancy using SSM mapping to simultaneously join two video sources for the same TV channel. However, to prevent the last hop router from duplicating the video traffic, the video sources must use a server-side switchover mechanism. One video source is active, and the other backup video source is passive. The passive source waits until an active source failure is detected before sending the video traffic for the TV channel. Thus, the server-side switchover mechanism ensures that only one of the servers is actively sending video traffic for the TV channel.
To look up one or more source addresses for a group that includes G1, G2, G3, and G4, you must configure these DNS records on the DNS server:
G4.G3.G2.G1 [multicast-domain] [timeout] IN A source-address-1 IN A source-address-2 IN A source-address-n
See your DNS server documentation for more information about configuring DNS resource records.
For a complete description of the source-specific multicast (SSM) commands in this section, see the IP Multicast Command Reference, Cisco IOS XE Release 3SE (Catalyst 3850 Switches). To locate documentation for other commands that appear in this chapter, use the command reference master index, or search online.
Selects an interface that is connected to hosts on which IGMPv3 can be enabled, and enters the interface configuration mode.
The specified interface must be one of the following:
A routed port—A physical port that has been configured as a Layer 3 port by entering the no switchport interface configuration command. You will also need to enable IP PIM sparse-dense-mode on the interface, and join the interface as a statically connected member to an IGMP static group. For a configuration example, see Example: Interface Configuration as a Routed Port
An SVI—A VLAN interface created by using the interface vlanvlan-id global configuration command. You will also need to enable IP PIM sparse-dense-mode on the VLAN, join the VLAN as a statically connected member to an IGMP static group, and then enable IGMP snooping on the VLAN, the IGMP static group, and physical interface. For a configuration example, see Example: Interface Configuration as an SVI
These interfaces must have IP addresses assigned to them.
Step 4
ip pim {sparse-mode | sparse-dense-mode}
Example:
Switch(config-if)# ip pim sparse-dense-mode
Enables PIM on an interface. You must use either sparse mode or sparse-dense mode.
Step 5
ip igmp version 3
Example:
Switch(config-if)# ip igmp version 3
Enables IGMPv3 on this interface. The default version of IGMP is set to Version 2.
Configuring Source Specific Multicast Mapping
The Source Specific Multicast (SSM) mapping feature supports SSM transition when supporting SSM on the end system is impossible or unwanted due to administrative or technical reasons. You can use SSM mapping to leverage SSM for video delivery to legacy STBs that do not support IGMPv3 or for applications that do not use the IGMPv3 host stack.
5.Repeat Step 4 to configure additional static SSM mappings, if required.
6.end
7.show running-config
8.copy running-config startup-config
DETAILED STEPS
Command or Action
Purpose
Step 1
configureterminal
Example:
Switch# configure terminal
Enters the global configuration mode.
Step 2
ip igmp ssm-map enable
Example:
Switch(config)# ip igmp ssm-map enable
Enables SSM mapping for groups in the configured SSM range.
Note
By default, this command enables DNS-based SSM mapping.
Step 3
no ip igmp ssm-map query dns
Example:
Switch(config)# no ip igmp ssm-map dns
(Optional) Disables DNS-based SSM mapping.
Note
Disable DNS-based SSM mapping if you only want to rely on static SSM mapping. By default, the ip igmp ssm-map global configuration command enables DNS-based SSM mapping.
Step 4
ip igmp ssm-map staticaccess-list source-address
Example:
Switch(config)# ip igmp ssm-map static 11 172.16.8.11
Configures static SSM mapping.
The ACL supplied for access-list defines the groups to be mapped to the source IP address entered for the source-address.
Note
You can configure additional static SSM mappings. If additional SSM mappings are configured and the router receives an IGMPv1 or IGMPv2 membership report for a group in the SSM range, the switch determines the source addresses associated with the group by using each configured ip igmp ssm-map static command. The switch associates up to 20 sources per group.
Step 5
Repeat Step 4 to configure additional static SSM mappings, if required.
—
Step 6
end
Example:
Switch(config)# end
Returns to privileged EXEC mode.
Step 7
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 8
copy running-config startup-config
Example:
Switch# copy running-config
startup-config
(Optional) Saves your entries in the configuration file.
To configure DNS-based SSM mapping, you need to create a DNS server zone or add records to an existing zone. If the routers that are using DNS-based SSM mapping are also using DNS for other purposes, you should use a normally configured DNS server. If DNS-based SSM mapping is the only DNS implementation being used on the router, you can configure a false DNS setup with an empty root zone or a root zone that points back to itself.
Configuring Static Traffic Forwarding with SSM Mapping
Use static traffic forwarding with SSM mapping to statically forward SSM traffic for certain groups.
SUMMARY STEPS
1.configureterminal
2.interfacetype number
3.ip igmp static-groupgroup-addresssource ssm-map
4.end
5.show running-config
6.copy running-config startup-config
DETAILED STEPS
Command or Action
Purpose
Step 1
configureterminal
Example:
Switch# configure terminal
Enters the global configuration mode.
Step 2
interfacetype number
Example:
Switch(config)# interface
gigabitethernet 1/0/1
Selects an interface on which to statically forward traffic for a multicast group using SSM mapping, and enters interface configuration mode.
The specified interface must be one of the following:
A routed port—A physical port that has been configured as a Layer 3 port by entering the no switchport interface configuration command. You will also need to enable IP PIM sparse-dense-mode on the interface, and join the interface as a statically connected member to an IGMP static group. For a configuration example, see Example: Interface Configuration as a Routed Port
An SVI—A VLAN interface created by using the interface vlanvlan-id global configuration command. You will also need to enable IP PIM sparse-dense-mode on the VLAN, join the VLAN as a statically connected member to an IGMP static group, and then enable IGMP snooping on the VLAN, the IGMP static group, and physical interface. For a configuration example, see Example: Interface Configuration as an SVI
These interfaces must have IP addresses assigned to them.
Note
Static forwarding of traffic with SSM mapping works with either DNS-based SSM mapping or statically configured SSM mapping.
Step 3
ip igmp static-groupgroup-addresssource ssm-map
Example:
Switch(config-if)# ip igmp
static-group 239.1.2.1 source
ssm-map
Configures SSM mapping to statically forward a (S, G) channel from the interface.
Use this command if you want to statically forward SSM traffic for certain groups. Use DNS-based SSM mapping to determine the source addresses of the channels.
Step 4
end
Example:
Switch(config-if)# end
Returns to privileged EXEC mode.
Step 5
show running-config
Example:
Switch# show running-config
Verifies your entries.
Step 6
copy running-config startup-config
Example:
Switch# copy running-config
startup-config
(Optional) Saves your entries in the configuration file.
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The source-specific multicast (SSM) feature is an extension of IP multicast in which datagram traffic is forwarded to receivers from only those multicast sources that the receivers have explicitly joined. For multicast groups configured for SSM, only SSM distribution trees (no shared trees) are created.