Content

| 7.3 |  | Routing Multicast Traffic |
| 7.3.3 |  | Managing delivery of multicast packets |

In a campus network, limiting high-bandwidth traffic to specific LANs or regions within the network is essential to containing or eliminating unnecessary consumption of resources. One method of containing multicast traffic is to impose constraints on the forwarding of that traffic by using the Time-To-Live (TTL) field in the IP packet.

As in unicast routing, the multicast TTL field controls the lifetime of the packet. The function of TTL is to prevent packets from being looped forever because of routing errors. However, the TTL field in multicasting also carries the concept of a “threshold".

Multicast-enabled routers have a TTL threshold assigned to each interface. Those packets with a TTL greater than the interface threshold are forwarded, and those packets with a TTL equal to or less than the interface threshold are discarded. The packet TTL is compared with the interface threshold first. The router then decrements the packet TTL as the packet is sent out the interface.

A multicast packet with a TTL of less than 16 is restricted to the same department, or site, and should not be forwarded across an interface to other sites in the same region. Defining the scope of a “site” or “region” is the responsibility of the network administrator.

A multicast router will forward a multicast packet across an interface only if the TTL field in the IP header is greater than the TTL threshold assigned to the interface. If the TTL field in the IP header of the packet is equal to or less than the TTL threshold assigned to the interface, the packet is discarded. If the interface has no assigned TTL threshold, the packet is forwarded.

In Figure , the interfaces on the router have been configured with the following TTL thresholds:

- E1: TTL Threshold = 16
- E2: TTL Threshold = 0 (none)
- E3: TTL Threshold = 64

An incoming multicast packet for Group XYZ is received on Interface E0 with a TTL of 24. The outgoing interface list for Group XYZ contains Interfaces E1, E0, and S2. The TTL threshold check is performed on each outgoing interface, as follows:

- E1: TTL (24) > TTL Threshold (16). FORWARD
- E2: TTL (24) > TTL Threshold (0). FORWARD
- E3: TTL (24) < TTL Threshold (64). DROP

The TTL is then decremented to 23 by normal router IP packet processing.