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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
Understanding the Forwarding Decisions
CEF for PFC2/PFC3A provides Layer 3 switching that is based on the following:
•
Entries in the ACL ternary content addressable memory (TCAM) for policy-based routing decisions
•
Entries in the NetFlow table for TCP intercept and reflexive ACL forwarding decisions (see the
“Understanding the NetFlow Statistics” section on page 13-11
)
•
Entries in the FIB and adjacency table for all other forwarding decisions
Enter the
show mls entry
command to display information about the entries that are used to make
forwarding decisions. CEF for PFC2/PFC3A makes a forwarding decision for each packet and sends the
rewrite information for each packet to the egress port, where the rewrite occurs when the packet is
transmitted from the switch.
Understanding the FIB
The FIB resides in a separate TCAM. The adjacency table is stored separately in DRAM. The NetFlow
table is stored separately in DRAM. The FIB, the adjacency table, and the NetFlow table do not compete
with any other features for storage space.
The FIB is conceptually similar to a routing table. It maintains a mirror image of the forwarding
information that is contained in the unicast and multicast routing tables on MSFC2/MSFC3. When
routing or topology changes occur in the network, the unicast and multicast routing tables on
MSFC2/MSFC3 are updated and those changes are reflected in the FIB. The FIB maintains next-hop
address information that is based on the information in the routing tables on MSFC2/MSFC3. The FIB
supports 256,000 entries, which includes 16,000 IP multicast entries (128,000 IP multicast entries on
MSFC3). With reverse path forwarding (RPF) check enabled, the number of IP entries doubles (with
Supervisor Engine 720, the number of IP entries remain the same).
FIB lookup uses the following criteria:
•
Destination IP address for IP unicast
•
Destination IPX network for IPX unicast
•
Source and destination IP address for IP unicast with RPF check
•
Source and destination IP address for IP multicast with RPF check
Note
Because the FIB mirrors the unicast and multicast routing tables on MSFC2/MSFC3, any commands on
MSFC2/MSFC3 that change the unicast or multicast routing tables affect the FIB. Forwarding entries
cannot be cleared from the Supervisor Engine 2 or Supervisor Engine 720 command-line interface (CLI).
In switches with redundant supervisor engines and MSFC2s/MSFC3s, the designated MSFC2/MSFC3
supports the FIB on the active Supervisor Engine 2 or Supervisor Engine 720. The routing protocols on
the nondesignated MSFC2/MSFC3 send information to the routing protocols on the designated
MSFC2/MSFC3.
Enter the
show mls entry cef
command to display the following:
•
Module number of the MSFC that is supporting the FIB
•
FIB entry type (receive, connected, resolved, drop, wildcard, or default)
•
Destination address (IP address or IPX network)
•
Destination mask
•
Next-hop address (IP address or IPX network)
