On August 8, 2014, the Classless Inter-Domain Routing (CIDR) Report, which provides statistics on the global Internet routing table, reported that the global Internet routing table had passed 512,000 routes for the first time in Internet history. When the total number of IPv4 routes exceeds 512,000 on a Cisco Catalyst 6500 Series switch or Cisco 7600 Series routers that run Supervisor 720 (Sup720) engines, the hardware Ternary Content Addressable Memory (TCAM) might report an exception that results in network instability.
On the Catalyst 6500 and Cisco 7600 Series platforms, all of the IPv4/IPv6/MPLS routing information is stored in special high-speed memory called Forwarding Information Base (FIB) TCAM.
When a route is programmed into the Cisco Express Forwarding (CEF) table in the main memory (DRAM), a copy of that route is stored in the hardware FIB TCAM on the Supervisor Policy Feature Card (PFC), as well as any Distributed Forwarding Card (DFC) on the modules.
By default, these platforms are configured as shown here:
IPv4 + MPLS - 512k (default)
IPv6 + IP Multicast - 256k (default)
When the number of IPv4 + MPLS routes exceed 512,000, the system experiences TCAM exception and might report:
%MLSCEF-DFC4-7-FIB_EXCEPTION: FIB TCAM exception, Some entries will be
%MLSCEF-SP-7-FIB_EXCEPTION: FIB TCAM exception, Some entries will be
It is important to be aware that the aforementioned errors are reported only once when the TCAM limit is exceeded. Even if the total number of routes withdraw below the configured limit, the switch remains in the exception state until it is cleared:
Catalyst6509#show mls cef exception status
Current IPv4 FIB exception state = TRUE
Current IPv6 FIB exception state = FALSE
Current MPLS FIB exception state = FALSE
As a result of this TCAM exception condition, connectivity is affected, which might result in elevated CPU usage due to the software switching.