Which statement is correct regarding using the TTL threshold to define the delivery boundaries of
multicast traffic?
A.
If a packet TTL is less than the specified TTL threshold, the packet is forwarded out of the
interface
B.
If a packet TTL is greater or equal to the specified TTL threshold, the packet is forwarded out of
the interface
C.
If a packet TTL is equal to the specified TTL threshold, the packet is dropped
D.
When a multicast packet arrives, the TTL threshold value is decremented by 1. If the resulting
TTL threshold value is greater than or equal to 0, the packet is dropped
router msdp
peer
ttl-threshold
This command prevents any multicast packet whose TTL is below the TTL value (ttl) from
being encapsulated in an SA message sent to the MSDP peer whose IP address is peer-address.
To limit which multicast data packets are sent to an MSDP peer in SA messages, use the Cisco
IOS/IOS XE ip msdp ttl-threshold global configuration command or the Cisco IOS XR ttl-threshold
router MSDP command.
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A TTL threshold problem can be introduced by the encapsulation of the initial multicast packet
of a source in an SA message. Because the multicast packet is encapsulated inside of the
unicast SA message (whose TTL equals 255), its TTL is not decremented as the SA message
travels to the MSDP peer. Furthermore, the total number of hops that the SA message traverses
can be drastically different than the number traversed by a normal multicast packet. This
situation exists because multicast and unicast traffic may follow completely different paths to
the MSDP peer and, therefore, the remote PIM-SM domain. This scenario can result in TTL
thresholds being violated by this encapsulated packet.
The solution to this problem is to configure a TTL threshold that is associated with any
multicast packet that is encapsulated in an SA message sent to a particular MSDP peer.
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