Multicast Layer 3 Switching Cache; Ipv4 Multicast Layer 3 Switching Overview - Cisco 7604 Configuration Manual

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

The Policy Feature Card (PFC) 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 PFC. In systems with Distributed Forwarding Cards (DFCs),
IP multicast flows are Layer 3 switched locally using Multicast Distributed Hardware Switching
(MDHS). MDHS uses local hardware CEF and replication tables on each DFC to perform Layer 3 switching
and rate limiting of reverse path forwarding (RPF) failures locally on each DFC-equipped switching module.
The PFC and the DFCs support hardware switching of (*,G) state flows. The PFC and the DFCs support
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 PFC and the DFCs all use 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
IGMP Snooping for IPv4 Multicast

Multicast Layer 3 Switching Cache

This section describes how the PFC and the DFCs maintain Layer 3 switching information in hardware
tables.
The PFC and DFC populate 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 MSFC updates its multicast routing table and forwards the new information to the PFC whenever it
receives traffic for a new flow. In addition, if an entry in the multicast routing table on the MSFC ages
out, the MSFC deletes the entry and forwards the updated information to the PFC. In systems with DFCs,
flows are populated symmetrically on all DFCs and on the PFC.
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 PFC maintains a list
of outgoing interfaces for the IP multicast group. From this list, the PFC determines onto which VLANs
traffic from a given multicast flow should be replicated.
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
28-2
Layer 3-Switched Multicast Packet Rewrite, page 28-3
Partially and Completely Switched Flows, page 28-4
Non-RPF Traffic Processing, page 28-5
Multicast Boundary, page 28-7
Understanding How IPv4 Bidirectional PIM Works, page 28-7
Chapter 28
Traffic").
Configuring IPv4 Multicast Layer 3 Switching
Chapter 30, "Configuring
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