configured. The timeout indicates how long the router will keep the current mapping before querying the DNS
server for this group. The timeout is derived from the cache time of the DNS RR entry and can be configured
for each group/source entry on the DNS server. You can configure this time for larger values if you want to
minimize the number of DNS queries generated by the router. Configure this time for a low value if you want
to be able to quickly update all routers with new source addresses.
Refer to your DNS server documentation for more information about configuring DNS RRs.Note
To configure DNS-based SSM mapping in the software, you must configure a few global commands but no
per-channel specific configuration is needed. There is no change to the configuration for SSM mapping if
additional channels are added. When DNS-based SSM mapping is configured, the mappings are handled
entirely by one or more DNS servers. All DNS techniques for configuration and redundancy management
can be applied to the entries needed for DNS-based SSM mapping.
Related Topics
Configuring DNS-Based SSM Mapping(CLI), on page 271
Verifying SSM Mapping Configuration and Operation, on page 276
Configuring Basic IP Multicast Routing, on page 290
Configuring Static Traffic Forwarding with SSM Mapping (CLI), on page 274
SSM Mapping Benefits
•
The SSM Mapping feature provides almost the same ease of network installation and management as a
pure SSM solution based on IGMPv3. Some additional configuration is necessary to enable SSM mapping.
•
The SSM benefit of inhibition of DoS attacks applies when SSM mapping is configured. When SSM
mapping is configured the only segment of the network that may still be vulnerable to DoS attacks are
receivers on the LAN connected to the last hop router. Since those receivers may still be using IGMPv1
and IGMPv2, they are vulnerable to attacks from unwanted sources on the same LAN. SSM mapping,
however, does protect those receivers (and the network path leading towards them) from multicast traffic
from unwanted sources anywhere else in the network.
•
Address assignment within a network using SSM mapping needs to be coordinated, but it does not need
assignment from outside authorities, even if the content from the network is to be transited into other
networks.
How to Configure SSM and SSM Mapping
Configuring Source Specific Multicast
Follow these steps to configure SSM:
Before You Begin
If you want to use an access list to define the Source Specific Multicast (SSM) range, configure the access
list before you reference the access list in the ip pim ssm command.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
OL-32598-01 267
Configuring SSM
How to Configure SSM and SSM Mapping
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