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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 48 Configuring IP Multicast Routing Configuring IP Multicast Routing
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 48-4).
Figure 48-4 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
Refer to your DNS server documentation for more information about configuring DNS resource records,
and go to this URL for additional information on SSM mapping:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00801a6d6f.
html
STB host 1 STB host 2 STB host 3
DNS response DNS server
Reverse DNS lookup
IGMPv2 membership report
(S, G) Join
(S, G) Join
Source
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