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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 13 Configuring CEF for PFC2 and PFC3A
Understanding How Layer 3 Switching Works
Partially and Completely Switched Multicast Flows
Some flows might be partially Layer 3 switched instead of completely Layer 3 switched in these
situations:
•
MSFC2/MSFC3 is configured as a member of the IP multicast group (using the
ip igmp join-group
command) on the RPF interface of the multicast source.
•
MSFC2/MSFC3 is the first-hop router to the source in PIM sparse mode (in this case,
MSFC2/MSFC3 must send PIM-register messages to the rendezvous point).
•
The multicast TTL threshold is configured on an egress interface for the flow.
•
The multicast helper is configured on the RPF interface for the flow, and multicast to broadcast
translation is required.
•
Multicast tag switching is configured on an egress interface.
•
Network Address Translation (NAT) is configured on an interface, and source address translation is
required for the outgoing interface.
Note
CEF for PFC2/PFC3A provides Layer 3 switching when the extended access list deny condition on the
RPF interface specifies something other than the Layer 3 source, Layer 3 destination, or IP protocol (an
example is the Layer 4 port numbers).
For partially switched flows, all multicast traffic belonging to the flow reaches MSFC2/MSFC3 and is
software switched for any interface that is not Layer 3 switched.
Note
All (*,G) flows are always partially Layer 3 switched.
PFC2/PFC3A prevents multicast traffic in flows that are completely Layer 3 switched from reaching
MSFC2/MSFC3, reducing the load on MSFC2/MSFC3. The
show ip mroute
and
show mls ip multicast
commands identify completely Layer 3-switched flows with the text string “RPF-MFD.”
Multicast Fast
Drop (MFD) indicates that from the perspective of MSFC2/MSFC3, the multicast packet is dropped,
because it is switched by the PFC2/PFC3A.
For all completely Layer 3-switched flows, PFC2/PFC3A periodically sends multicast packet and byte
count statistics to MSFC2/MSFC3, because MSFC2/MSFC3 cannot record multicast statistics for
completely switched flows, which it never sees. MSFC2/MSFC3 uses the statistics to update the
corresponding multicast routing table entries and reset the appropriate expiration timers.