CHAPTER
3-81
Cisco IOS XR Troubleshooting Guide for the Cisco CRS-1 Router
OL-21483-02
3
Troubleshooting Forwarding
Cisco Express Forwarding (CEF) is the mechanism that enables packet forwarding. CEF information is
examined when data forwarding is not occurring as expected. Troubleshooting CEF involves comparing
the Routing Information Base (RIB) information to the software Forwarding Information Base (FIB),
verifying that the hardware is programmed correctly, verifying that the adjacencies are built correctly,
verifying the control plane is built correctly, and gathering any necessary trace information.
The only prerequisite for CEF is a valid route in the RIB.
This chapter describes techniques that you can use to troubleshoot router forwarding. It includes the
following sections:
• Troubleshooting IPv4 CEF Information, page 3-81
• Troubleshooting Adjacency Information, page 3-85
• Troubleshooting Transient Traffic Drop in Forwarding, page 3-88
• Troubleshooting Control Plane Information, page 3-90
• Troubleshooting the Interface Manager, page 3-95
• Troubleshooting the Interface Manager Distributor, page 3-106
Troubleshooting IPv4 CEF Information
CEF is an advanced, Layer 3 IP switching technology that optimizes network performance. It also
improves the scalability for networks with large and dynamic traffic patterns, such as the Internet and
networks characterized by intensive Web-based applications.
Information conventionally stored in a route cache is stored in several data structures for CEF switching.
The data structures provide optimized lookup for efficient packet forwarding. The two main components
of CEF operation are forwarding information base (FIB) and adjacency tables:
• CEF uses a FIB to make IP destination prefix-based switching decisions. FIB maintains a mirror
image of the forwarding information contained in the IP routing table. When routing or topology
changes occur in the network, the IP routing table is updated, and those changes are reflected in the
FIB. The FIB maintains next hop address information based on the information in the IP routing
table. There is a one-to-one correlation between FIB entries and routing table entries, therefore FIB
contains all known routes and eliminates the need for route cache maintenance that is associated
with switching paths such as fast switching and optimum switching.
• Nodes in the network are said to be adjacent if they can reach each other with a single hop across a
link layer. In addition to the FIB, CEF uses adjacency tables to prepend Layer 2 addressing
information. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.
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