Specifications

Multilayer Switching Overview
Introduction to IP Multicast MLS
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• Multicast MLS-Switching Engine (MMLS-SE)—For example, a Catalyst 5000 series switch with
hardware that supports IP multicast MLS. The MMLS-SE provides Layer 3 LAN-switching
services.
• Multicast MLS-Route Processor (MMLS-RP)—Routing platform running Cisco IOS software that
supports IP multicast MLS. The MMLS-RP interacts with the IP multicast routing software and
updates the MLS cache in the MMLS-SE. When you enable IP multicast MLS, the MMLS-RP
continues to handle all non-IP-multicast traffic while off loading IP multicast traffic forwarding to
the MMLS-SE.
Layer 2 Multicast Forwarding Table
The MMLS-SE uses the Layer 2 multicast forwarding table to determine on which ports Layer 2
multicast traffic should be forwarded (if any). The Layer 2 multicast forwarding table is populated by
enabling CGMP, IGMP snooping, or GMRP on the switch. These entries map the destination multicast
MAC address to outgoing switch ports for a given VLAN.
Layer 3 Multicast MLS Cache
The MMLS-SE maintains the Layer 3 MLS cache to identify individual IP multicast flows. Each entry
is of the form {source IP, destination group IP, source VLAN}. The maximum MLS cache size is 128K
and is shared by all MLS processes on the switch (such as IP unicast MLS and IPX MLS). However, if
the total of cache entries exceeds 32K, there is increased probability that a flow will not be switched by
the MMLS-SE and will get forwarded to the router.
The MMLS-SE populates the MLS cache using information learned from the routers participating in IP
multicast MLS. The router and switch exchange information using the multicast MLSP.
Whenever the router receives traffic for a new flow, it updates its multicast routing table and forwards
the new information to the MMLS-SE using multicast MLSP. In addition, if an entry in the multicast
routing table is aged out, the router deletes the entry and forwards the updated information to the
MMLS-SE.
The MLS cache contains flow information for all active multilayer switched flows. After the MLS cache
is populated, multicast packets identified as belonging to an existing flow can be Layer 3 switched based
on the cache entry for that flow. For each cache entry, the MMLS-SE maintains a list of outgoing
interfaces for the destination IP multicast group. The MMLS-SE uses this list to determine on which
VLANs traffic to a given multicast flow should be replicated.
IP Multicast MLS Flow Mask
IP multicast MLS supports a single flow mask, source destination vlan. The MMLS-SE maintains one
multicast MLS cache entry for each {source IP, destination group IP, source VLAN}. The multicast
source destination vlan flow mask differs from the IP unicast MLS source destination ip flow mask in
that, for IP multicast MLS, the source VLAN is included as part of the entry. The source VLAN is the
multicast Reverse Path Forwarding (RPF) interface for the multicast flow.