Networking

Multicast TTL-Threshold – Network Rare

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As per books, The TTL of a packet must be higher than the threshold to be forwarded.

From the 12.4 command reference:

ip multicast ttl-threshold

“Only multicast packets with a TTL value greater than the threshold are forwarded out the interface.”

As per guidelines

Below is our topology:

R1—-R2—-R3—-R4

  • PIM-DM is enabled everywhere.
  • R4 has joined 239.0.0.1
  • R1 is sending pings which have 255 TTL when sent from R1.
  • R2 receives the PING, decrements the TTL to 254 before sending to R3.

So if we set TTL threshold to 254 on R2’s interface to R3, it should block it right? No:

R2(config)#int s1/0
R2(config-if)#ip multicast ttl-threshold 254

R1#ping 239.0.0.1   

Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.0.0.1, timeout is 2 seconds:

Reply to request 0 from 192.168.34.4, 164 ms
R1#

The router will still pass packets that have a TTL equal to the threshold if it was the router that decremented the TTL to reach that value. Here we see 255 will fail:

R2(config)#int s1/0
R2(config-if)#ip multicast ttl-threshold 255

R1#ping 239.0.0.1

Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 239.0.0.1, timeout is 2 seconds:
.
R1#

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