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Study Tips

The following bullets review important BCMSN exam preparation points of this chapter. The bullets only briefly highlight the main points of this chapter related to the BCMSN exam. Consult the text of this chapter for additional information regarding these topics. Table 9-3 lists important commands to review for the BCMSN exam:

  • In describing CEF-based MLS in a few sentences, CEF-based MLS is the Layer 3 forwarding mechanism deployed currently on all shipping Catalyst switches. CEF-based MLS consists of two logical devices: a control plane and a data plane. In CEF, the control plane is generally referred to as the software-switching engine or Layer 3 forwarding engine. In addition, the data plane carries several common names, including the hardware-switching component, "in hardware," hardware forwarding engine, and ASICs.
  • The control plane is responsible for building the IP routing table and ARP table. The IP routing table and ARP table ultimately build the IP CEF and CEF adjacency table. The CEF table contains prefixes of subnets, hosts, and so on, which are used to quickly search for an adjacency index. The index into the adjacency table provides the next-hop address, interface, and Layer 3 rewrite information.
  • The control plane downloads its CEF and adjacency table to the data plane (hardware forwarding engine). The hardware forwarding engine consists of hardware components including TCAM that enable high-speed switching and routing of frames. All changes, updates, additions, and so on to the CEF and adjacency table are controlled by the Layer 3 forwarding engine.
  • With the Catalyst 6500 family of switches, the control plane and data plane are easily distinguishable (MSFC and PFC, respectively). However, the other Catalyst switches do not have separate components from an end-user standpoint, yet these switches still have a distinct control plane and data plane.
  • Layer 3 switching (or multilayer switching) is the term used to describe the ability of a Cisco Catalyst switch to perform Layer 3 routing in hardware (hardware-switching) using high-speed hardware components.
  • Layer 3 switching is beneficial in the campus network because it provides versatility, high-availability, large port density, and high performance.
  • Current shipping Layer 3 switches use CEF to populate IP routing tables and Layer 2 rewrite information in hardware.
  • The terms CEF-based MLS or just plain CEF are used to describe the routing architecture of Catalyst switches.
  • No specific configuration is needed for CEF-based MLS because it is enabled by default.
  • ARP throttling is a feature that prevents ARP-based DoS attacks on Catalyst switches. The feature works by installing a throttling adjacency in the hardware CEF table such that hardware-switching drops subsequent identical ARP requests for a specific time until the respective ARP response is received.
  • With CEF, when a router is connected directly to several hosts, the FIB table on the router maintains a prefix for the subnet rather than for the individual host prefixes. The subnet prefix points to a glean adjacency. When packets need to be forwarded to a specific host, the adjacency database is gleaned for the specific prefix.
  • With CEF, punt adjacencies in TCAM on Catalyst switch hardware are used to send packets to the Layer 3 engine (for example, MSFC) that require special handling or that are used for special situations where the hardware does not yet support the packet flow in question (for example, NAT).
  • Catalyst switches support two methods for CEF-based MLS: centralized switching and distributing switching.
  • Centralized switching describes hardware-switching using a centralized hardware-switching engine found on the Supervisor Engine of a Cisco Catalyst switch. With centralized switching, data to be routed or switched must pass through the switching fabric of the Catalyst switch to the centralized hardware-switching engine for processing. After the forwarding decision has been made and the packet rewritten with new Layer 2 information, the centralized switching engine forwards the packet to the correct destination port and VLAN.
  • The Catalyst 4500 family of switches is based on centralized switching.
  • Distributed switching describes the hardware-switching method used by Catalyst switches that utilize hardware-switching engines on individual line cards and port groups. With distributed switching, individual line cards determine the routing destination and rewrite information for a packet. Therefore, with distributed switching line cards, a Catalyst switch is able to route a packet between source and destination without sending the packet to the Supervisor Engine for processing. In addition, the packet might never leave the line card if the source and destination of the line card are on the same switch.
  • Review the "Sample CEF-Based MLS Operation" section and Figure 9-8, which illustrates the path of a packet during CEF-based MLS operation.
  • Review the "CEF-Based MLS Troubleshooting Methodology" section of this chapter for details on how to approach CEF-based MLS troubleshooting.

Table 9-3. Commands to Review



debug ip cef

Enables debugging with IP CEF. Recommended practice is to use this debug option with optional parameters including ACLs to limit output. In addition, use this debug command in production only in coordination with a Cisco TAC engineer.

show adjacency detail

Displays the IP CEF Layer 2 rewrite information and statistical information from the perspective of the Layer 3 engine.

show arp

Displays the ARP table contents. This command is the first step in troubleshooting adjacency issues.

show ip cef

Displays the IP CEF information from the Layer 3 engine perspective. Displays the prefix, the next hop, and the next-hop interface (outgoing interface).

show ip route

Displays the IP routing table. This command is the first step in troubleshooting IP routing or IP CEF issues.

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