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Basic IP Connectivity and Troubleshooting in Cisco Express Forwarding

Chapter Description

This chapter presents the general troubleshooting used on Cisco IOS routers and switches as a first step in troubleshooting IP connectivity problems. It helps you verify whether CEF is the root cause of a particular IP connectivity problem.

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Troubleshooting Punt Adjacencies

In Example 4-22, suppose that the show ip cef command output yielded the following as the result of a configuration change, such as routing the traffic over a tunnel interface.

Example 4-22. Determining Whether the FIB Prefix Points to Punt Adjacency

Router-2#show ip cef 10.18.118.1
10.18.118.0/24, version 31, epoch 0
0 packets, 0 bytes
  via 192.168.1.1, 0 dependencies, recursive
    next hop 192.168.1.1, Tunnel1 via 192.168.1.0/24
    valid punt adjacency

The CEF table output indicates that the entry is a valid punt adjacency. The term punt in Cisco IOS refers to sending a packet to the next-level switching process. By default, Cisco IOS always uses the fastest switching method possible; however, the fastest switching method usually does not support advanced features such as Network Address Translation (NAT) and policy-based routing (PBR) early in the product life cycle. Therefore, to handle these cases effectively, Cisco IOS punts the packet to the next-level switch method that can switch the frame. The following list illustrates the typical switching method hierarchy, with the fastest, most effective switching method listed first:

  • Hardware-based dCEF (hardware-based switching on line cards)
  • Hardware-based CEF (hardware-based, centralized forwarding typically found on Cisco Catalyst switches)
  • PXF switching
  • Software-based CEF
  • Software-based fast switching
  • Software-based process switching

Generally, the fastest and most effective switching methods generally lag software-based CEF in feature support. Therefore, when using new and unique features, Cisco IOS generally supports the feature in software first in low- to mid-range routers and switches. High-end routers and switches only support advanced features in hardware because the throughput required for the high-end routers and switches exceeds the capability of software-based switching methods. Use the show cef not-cef-switched command to view packets that are not CEF switched. In later code, the show ip cef switching statistics command gives detailed information of why a pass occurs and replaces the show cef not command. Example 4-23 illustrates an example of the show cef not-cef-switched command followed by Table 4-4, which illustrates descriptions for each of fields.

Example 4-23. Sample Output from the show cef not-cef-switched Command

Router-2#show cef not-cef-switched
CEF Packets passed on to next switching layer
Slot  No_adj No_encap Unsupp'ted Redirect  Receive  Options   Access     Frag
RP         4       0           0       34     2920        0        0        0

Table 4-4. show cef not-cef-switched Command Field Descriptions

Field

Description

Slot

Refers to the slot for the respective ingress packet counts. For Cisco IOS routers that do not support dCEF, this value is always RP.

No_adj

Indicates the number of packets dropped because of incomplete adjacency.

No_encap

Indicates the number of packets sent to the processor for ARP resolution.

Unsupp'ted

Indicates the number of packets fast-dropped by CEF (drop adjacency) because of an unsupported feature.

Redirect

Indicates the number of packets requiring ICMP redirect by process switching.

Receive

Indicates the number of packets ultimately destined to the router, or packets destined to a tunnel endpoint on the router. If the decapsulated tunnel packet is IP, the packet is CEF switched. Otherwise, packets are process switched.

Options

Indicates the number of packets with options. Packets with IP options are process switched.

Access

Indicates the number of packets punted because of an access list failure.

Frag

Indicates the number of packets punted because of fragmentation failure.

MTU

Indicates the number of packets punted because of maximum transmission unit (MTU) failure. Note: This field is not supported for IPv4 packets.

As hardware-switching components evolve, more features are being supported by CEF and dCEF. Unfortunately, most legacy software features are not supported by CEF or dCEF. Nevertheless, when troubleshooting CEF, you should note whether a software feature such as NAT, PBR, or accounting supports CEF. To verify such features, check the platform-specific release notes, data sheets, and configuration guides.

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