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The following are the prerequisites for configuring source-specific multicast (SSM) and SSM mapping:
Note |
You can use a product such as Cisco Network Registrar to add records to a running DNS server. |
The following are the restrictions for configuring 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.
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:
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 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).
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:
In IGMPv3, hosts signal membership to last hop routers of multicast groups. Hosts can signal group membership with filtering capabilities with respect to sources. A host can either signal that it wants to receive traffic from all sources sending to a group except for some specific sources (called exclude mode), or that it wants to receive traffic only from some specific sources sending to the group (called include mode).
IGMPv3 can operate with both Internet Standard Multicast (ISM) and Source Specific Multicast (SSM). In ISM, both exclude and include mode reports are applicable. In SSM, only include mode reports are accepted by the last-hop router. Exclude mode reports are ignored.
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.
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.
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.
The following procedure describes how to configure static SSM mapping.
3.
no ip igmp ssm-map query dns
4.
ip igmp ssm-map static
access-list source-address
5. Repeat Step 4 to configure additional static SSM mappings, if required.
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.
4.
ip domain multicast
domain-prefix
5. ip name-server server-address1 [ server-address2... server-address6]
6. Repeat Step 5 to configure additional DNS servers for redundancy, if required.
Command or Action | Purpose | |||
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Step 1 |
configure terminal Example: Switch# configure terminal |
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Step 2 |
ip igmp ssm-map enable Example: Switch(config)# ip igmp ssm-map enable |
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Step 3 |
ip igmp ssm-map query dns Example: Switch(config)# ip igmp ssm-map query dns |
(Optional) Enables DNS-based SSM mapping. By default, the ip igmp ssm-map command enables DNS-based SSM mapping. Only the no form of this command is saved to the running configuration.
|
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Step 4 |
ip domain multicast
domain-prefix Example: Switch(config)# ip domain multicast ssm-map.cisco.com |
(Optional) Changes the domain prefix used by the switch for DNS-based SSM mapping. |
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Step 5 |
ip name-server
server-address1 [
server-address2...
server-address6] Example: Switch(config)# ip name-server 172.16.1.111 172.16.1.2 |
Specifies the address of one or more name servers to use for name and address resolution. |
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Step 6 | Repeat Step 5 to configure additional DNS servers for redundancy, if required. | |||
Step 7 |
end Example: Switch(config)# end |
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Step 8 |
show running-config Example: Switch# show running-config |
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Step 9 |
copy running-config startup-config Example: Switch# copy running-config startup-config |
Use static traffic forwarding with SSM mapping to statically forward SSM traffic for certain groups.
Command or Action | Purpose | |||
---|---|---|---|---|
Step 1 |
configure terminal Example: Switch# configure terminal |
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Step 2 |
interface
type 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.
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Step 3 |
ip igmp static-group
group-address
source 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. |
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Step 4 |
end Example: Switch(config-if)# end |
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Step 5 |
show running-config Example: Switch# show running-config |
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Step 6 |
copy running-config startup-config Example: Switch# copy running-config startup-config |
Command |
Purpose |
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Displays the (S, G) channel subscription through IGMPv3. |
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Displays whether a multicast group supports SSM service or whether a source-specific host report was received. |
Command |
Purpose |
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Displays the sources that SSM mapping uses for a particular group. |
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show ip igmp groups [ group-name | group-address | interface-type interface-number] [ detail] |
Displays the multicast groups with receivers that are directly connected to the router and that were learned through IGMP. |
Displays the default domain name, the style of name lookup service, a list of name server hosts, and the cached list of hostnames and addresses. |
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Displays the IGMP packets received and sent and IGMP host-related events. |
You can configure the following for your IP multicast configuration:
Related Topic | Document Title |
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For complete syntax and usage information for the commands used in this chapter. |
Catalyst 2960-XR Switch IP Multicast Command Reference |
Standard/RFC | Title |
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— |
— |
MIB | MIBs Link |
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All supported MIBs for this release. |
To locate and download MIBs for selected platforms, Cisco IOS releases, and feature sets, use Cisco MIB Locator found at the following URL: |
Description | Link |
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Release | Modification |
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Cisco IOS 15.0(2)EX1 |
This feature was introduced. |