Specifications

Configuring IP Multicast Multilayer Switching
IP Multicast MLS Configuration Examples
10
• Switch B and Switch C perform Layer 2 switching functions only.
Operation Before IP Multicast MLS Example
Without IP multicast MLS, when Server A (on VLAN 10) sends traffic destined for IP multicast group
G1, Switch B forwards the traffic (based on the Layer 2 multicast forwarding table entry) to Host A on
VLAN 10 and to Switch A. Switch A forwards the traffic to the Router A and Router B subinterfaces in
VLAN 10.
Router A receives the multicast traffic on its incoming subinterface for VLAN 10, checks the multicast
routing table, and replicates the traffic to the outgoing subinterface for VLAN 20. Router B receives the
multicast traffic on its incoming interface for VLAN 10, checks the multicast routing table, and
replicates the traffic to the outgoing subinterface for VLAN 30.
Switch A receives the traffic on VLANs 20 and 30. Switch A forwards VLAN 20 traffic to the
appropriate switch ports (in this case, to Host C), based on the contents of the Layer 2 multicast
forwarding table. Switch A forwards the VLAN 30 traffic to Switch C.
Switch C receives the VLAN 30 traffic and forwards it to the appropriate switch ports (in this case,
Hosts D and E) using the multicast forwarding table.
Operation After IP Multicast MLS Example
After IP multicast MLS is implemented, when Server A sends traffic destined for multicast group G1,
Switch B forwards the traffic (based on the Layer 2 multicast forwarding table entry) to Host A on
VLAN 10 and to Switch A.
Switch A checks its Layer 3 multicast MLS cache and recognizes that the traffic belongs to a multicast
MLS flow. Switch A does not forward the traffic to the router subinterfaces in VLAN 10 (assuming a
completely switched flow). Instead, Switch A replicates the traffic on the appropriate outgoing
interfaces (VLANs 20 and 30).
VLAN 20 traffic is forwarded to Host C and VLAN 30 traffic is forwarded to Switch C (based on the
contents of the Layer 2 multicast forwarding table). The switch performs a packet rewrite for the
replicated traffic so that the packets appear to have been routed by the appropriate router subinterface.
Switch C receives the VLAN 30 traffic and forwards it to the appropriate switch ports (in this case,
Hosts D and E) using the multicast forwarding table.
If not all the router subinterfaces are eligible to participate in IP multicast MLS, the switch must forward
the multicast traffic to the router subinterfaces in the source VLAN (in this case, VLAN 10). In this
situation, on those subinterfaces that are ineligible, the routers perform multicast forwarding and
replication in software in the usual manner. On those subinterfaces that are eligible, the switch performs
multilayer switching.
Note On both MMLS-RPs, no user-configured IP multicast MLS management interface is specified.
Therefore, the VLAN 1 subinterface is used by default.
Router A (MMLS-RP) Configuration
ip multicast-routing
interface fastethernet1/0.1
encapsulation isl 1
ip address 172.20.1.1 255.255.255.0
interface fastethernet1/0.10
encapsulation isl 10
ip address 172.20.10.1 255.255.255.0