TTL Scoping

Thu, 30 Aug 2018 | Routing TCP IP

One method for establishing boundaries to limit the scope of multicast traffic is to set a special filter on outgoing interfaces that checks the TTL value of all multicast packets Only packets whose TTL value, after the normal decrement performed by the router, exceeds a configured threshold are forwarded All other multicast packets are dropped

Figure 5-22 shows an example On this router, a multicast packet arrives on interface E2 with a TTL of 13 The router decrements the packet's TTL to 12 Interface EO has a multicast TTL threshold of 0, which is the default, no multicast packets are blocked based on their TTL Therefore, a copy of the packet is forwarded out EO Likewise, a copy of the packet is forwarded out interface El, because its TTL threshold is set to 5, which is less than the packet's TTL However, the packet is not forwarded out E3 That interface's TTL threshold is 30, meaning that only packets whose TTL value is greater than 30 can be forwarded

Figure 5-22 Multicast Packets Are Forwarded Only Out Downstream Interfaces Whose TTL Threshold Is Less Than the Outgoing Packet's TTL