Ipv4 Multicast Layer 3 Switching Overview; Multicast Layer 3 Switching Cache - Cisco WS-SUP32-GE-3B - Supervisor Engine 32 Software Configuration Manual

Understanding How IPv4 Multicast Layer 3 Switching Works
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IPv4 Multicast Layer 3 Switching Overview

The Policy Feature Card 3B (PFC3B) provides Layer 3 switching for IP multicast flows using the
hardware replication table and hardware Cisco Express Forwarding (CEF), which uses the forwarding
information base (FIB) and the adjacency table on the PFC3B.
The PFC3B supports hardware switching of (*,G) state flows. The PFC3B supports rate limiting of
non-RPF traffic.
Multicast Layer 3 switching forwards IP multicast data packet flows between IP subnets using advanced
application-specific integrated circuit (ASIC) switching hardware, which offloads processor-intensive
multicast forwarding and replication from network routers.
Layer 3 flows that cannot be hardware switched are still forwarded in the software by routers. Protocol
Independent Multicast (PIM) is used for route determination.
The PFC3B uses the Layer 2 multicast forwarding table to determine on which ports Layer 2 multicast
traffic should be forwarded (if any). The multicast forwarding table entries are populated in conjunction
with Internet Group Management Protocol (IGMP) snooping (see
Snooping for IPv4 Multicast

Multicast Layer 3 Switching Cache

This section describes how the PFC3B maintains Layer 3 switching information in hardware tables.
The PFC3B populates the (S,G) or (*,G) flows in the hardware FIB table with the appropriate masks; for
example, (S/32, G/32) and (*/0, G/32). The RPF interface and the adjacency pointer information is also
stored in each entry. The adjacency table contains the rewrite and a pointer to the replication entries. If
a flow matches a FIB entry, the RPF check compares the incoming interface/VLAN with the entry. A
mismatch is an RPF failure, which can be rate limited if this feature is enabled.
The PISA updates its multicast routing table and forwards the new information to the PFC3B whenever
it receives traffic for a new flow. In addition, if an entry in the multicast routing table on the PISA ages
out, the PISA deletes the entry and forwards the updated information to the PFC3B.
The Layer 3 switching cache contains flow information for all active Layer 3-switched flows. After the
switching 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 PFC3B maintains a
list of outgoing interfaces for the IP multicast group. From this list, the PFC3B determines onto which
VLANs traffic from a given multicast flow should be replicated.
These commands affect the Layer 3 switching cache entries:
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Catalyst Supervisor Engine 32 PISA Cisco IOS Software Configuration Guide, Release 12.2ZY
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Understanding How IPv4 Bidirectional PIM Works, page 25-6
Traffic").
When you clear the multicast routing table using the clear ip mroute command, all multicast Layer
3 switching cache entries are cleared.
When you disable IP multicast routing on the PISA using the no ip multicast-routing command,
all multicast Layer 3 switching cache entries on the PFC3B are purged.
When you disable multicast Layer 3 switching on an individual interface basis using the no mls
ipmulticast command, flows that use this interface as the RPF interface are routed only by the PISA
in software.
Chapter 25 Configuring IPv4 Multicast Layer 3 Switching
Chapter 27, "Configuring IGMP
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