Specifications

Multilayer Switching Overview

Introduction to IP Multicast MLS

The IP multicast MLS feature provides high-performance, hardware-based, Layer 3 switching of IP

multicast traffic for routers connected to LAN switches.

An IP multicast flow is a unidirectional sequence of packets between a multicast source and the members

of a destination multicast group. Flows are based on the IP address of the source device and the

destination IP multicast group address.

IP multicast MLS switches IP multicast data packet flows between IP subnets using advanced, ASIC

switching hardware, thereby off loading processor-intensive, multicast packet routing from network

routers.

The packet forwarding function is moved onto the connected Layer 3 switch whenever a supported path

exists between a source and members of a multicast group. Packets that do not have a supported path to

reach their destinations are still forwarded in software by routers. Protocol Independent Multicast (PIM)

is used for route determination.

IP Multicast MLS Network Topology

IP multicast MLS requires specific network topologies to function correctly. In each of these topologies,

the source traffic is received on the switch, traverses a trunk link to the router, and returns to the switch

over the same trunk link to reach the destination group members. The basic topology consists of a switch

and an internal or external router connected through an ISL or 802.1Q trunk link.

Figure 67 shows this basic configuration before and after IP multicast MLS is deployed (assuming a

completely switched flow). The topology consists of a switch, a directly connected external router, and

multiple IP subnetworks (VLANs).

The network in the upper diagram in Figure 67 does not have the IP multicast MLS feature enabled.

Note the arrows from the router to each multicast group in each VLAN. In this case, the router must

replicate the multicast data packets to the multiple VLANs. The router can be easily overwhelmed with

forwarding and replicated multicast traffic if the input rate or the number of outgoing interfaces

increases.

As shown in the lower diagram in Figure 67, this potential problem is prevented by having the switch

hardware forward the multicast data traffic. (Multicast control packets are still moving between the

router and switch.)