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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
OL-25303-03
Chapter 1 Implementing IPv6 Multicast
Information About Implementing IPv6 Multicast
• Provides hooks to allow clients residing on the RP to read traffic statistics on demand. (dMFIB does
not periodically upload these statistics to the RP.)
The combination of dMFIB and MRIB subsystems also allows the switch to have a "customized" copy
of the MFIB database in each line card and to transport MFIB-related platform-specific information from
the RP to the line cards.
IPv6 Multicast VRF Lite
The IPv6 Multicast VRF Lite feature provides IPv6 multicast support for multiple virtual
routing/forwarding contexts (VRFs). The scope of these VRFs is limited to the switch in which the VRFs
are defined.
This feature provides separation between routing and forwarding, providing an additional level of
security because no communication between devices belonging to different VRFs is allowed unless it is
explicitly configured. The IPv6 Multicast VRF Lite feature simplifies the management and
troubleshooting of traffic belonging to a specific VRF.
IPv6 Multicast Process Switching and Fast Switching
A unified MFIB is used to provide both fast switching and process switching support for PIM-SM and
PIM-SSM in IPv6 multicast. In process switching, the Route Processor must examine, rewrite, and
forward each packet. The packet is first received and copied into the system memory. The switch then
looks up the Layer 3 network address in the routing table. The Layer 2 frame is then rewritten with the
next-hop destination address and sent to the outgoing interface. The RP also computes the cyclic
redundancy check (CRC). This switching method is the least scalable method for switching IPv6
packets.
IPv6 multicast fast switching allows switches to provide better packet forwarding performance than
process switching. Information conventionally stored in a route cache is stored in several data structures
for IPv6 multicast switching. The data structures provide optimized lookup for efficient packet
forwarding.
In IPv6 multicast forwarding, the first packet is fast-switched if the PIM protocol logic allows it. In IPv6
multicast fast switching, the MAC encapsulation header is precomputed. IPv6 multicast fast switching
uses the MFIB to make IPv6 destination prefix-based switching decisions. In addition to the MFIB, IPv6
multicast fast switching uses adjacency tables to prepend Layer 2 addressing information. The adjacency
table maintains Layer 2 next-hop addresses for all MFIB entries.
The adjacency table is populated as adjacencies are discovered. Each time an adjacency entry is created
(such as through ARP), a link-layer header for that adjacent node is precomputed and stored in the
adjacency table. Once a route is determined, it points to a next hop and corresponding adjacency entry.
It is subsequently used for encapsulation during switching of packets.
A route might have several paths to a destination prefix, such as when a switch is configured for
simultaneous load balancing and redundancy. For each resolved path, a pointer is added for the
adjacency corresponding to the next-hop interface for that path. This mechanism is used for load
balancing across several paths.
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