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1.68 CISC-RT-000890

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Information

The Cisco multicast Designated switch (DR) must be configured to set the shortest-path tree (SPT) threshold to infinity to minimalize source-group (S, G) state within the multicast topology where Any Source Multicast (ASM) is deployed.

GROUP ID: V-221141
RULE ID: SV-221141r999750

ASM can have many sources for the same groups (many-to-many). For many receivers, the path via the RP may not be ideal compared with the shortest path from the source to the receiver. By default, the last-hop switch will initiate a switch from the shared tree to a source-specific SPT to obtain lower latencies. This is accomplished by the last-hop switch sending an (S, G) Protocol Independent Multicast (PIM) Join toward S (the source).

When the last-hop switch begins to receive traffic for the group from the source via the SPT, it will send a PIM Prune message to the RP for the (S, G). The RP will then send a Prune message toward the source. The SPT switchover becomes a scaling issue for large multicast topologies that have many receivers and many sources for many groups because (S, G) entries require more memory than (*, G). Hence, it is imperative to minimize the amount of (S, G) state to be maintained by increasing the threshold that determines when the SPT switchover occurs.

Solution

Configure the DR to increase the SPT threshold or set it to infinity to minimalize (S, G) state within the multicast topology where ASM is deployed.

Step 1 : Configure a prefix list or route map to specify the ASM groups. The example below includes all global ASM groups.

SW1(config)# ip prefix-list SPT_GROUPS permit 233.0.0.0/8

Step 2 : Configure the SPT threshold to infinity.

SW1(config)# ip pim spt-threshold infinity group prefix-list SPT_GROUPS
SW1(config)# end

See Also

https://workbench.cisecurity.org/benchmarks/22581

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-6b., CAT|II, CCI|CCI-000366, CSCv7|13.3, Rule-ID|SV-221141r999750_rule, STIG-ID|CISC-RT-000890, Vuln-ID|V-221141

Plugin: Cisco

Control ID: 3ef75308cfdbe9eeb1f14d6bdebf02f30d14c46bc2234f9277bef940e24bb743