Cisco 3845 - Security Bundle Router Software Manual page 752

Configuring IP Multicast Routing
Static SSM Mapping
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. Then you can map the groups permitted by those ACLs to sources by using the ip igmp
static ssm-map 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.
DNS-Based SSM Mapping
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 (see
Figure 33-3
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.
Cisco ME 3800X and 3600X Switch Software Configuration Guide
33-18
Figure
DNS-Based SSM-Mapping
Source
(S, G) Join
(S, G) Join
IGMPv2 membership report
STB host 1 STB host 2
33-3).
DNS response
Reverse DNS lookup
STB host 3
Chapter 33 Configuring IP Multicast Routing
DNS server
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