Configuring Cisco Express Forwarding Consistency Checkers for Route Processors and Line Cards

First Published: May 2, 2005
Last Updated: July 31, 2009

This module contains information about and instructions for configuring Cisco Express Forwarding (consistency checkers. Cisco Express Forwarding consistency checkers help you find any database inconsistencies, such as an IP prefix missing from a line card or a Route Processor (RP). You can investigate and resolve the inconsistency by examining the associated Cisco Express Forwarding system error messages that occur and by issuing Cisco Express Forwarding debug and show commands.

Cisco Express Forwarding is an advanced Layer 3 IP switching technology. It optimizes network performance and scalability for all kinds of networks: those that carry small amounts of traffic and those that carry large amounts of traffic in complex patterns, such as the Internet and networks characterized by intensive web-based applications or interactive sessions.

Finding Feature Information

For the latest feature information and caveats, see the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the "Feature Information for Configuring Basic Cisco Express Forwarding" section.

Use Cisco Feature Navigator to find information about platform support and Cisco IOS XE software image support. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Contents

•Prerequisites for Configuring Cisco Express Forwarding Consistency Checkers

•Restrictions for Configuring Cisco Express Forwarding Consistency Checkers

•Information About Configuring Cisco Express Forwarding Consistency Checkers

•How to Configure Cisco Express Forwarding Consistency Checkers

•Configuration Examples for Configuring Cisco Express Forwarding Consistency Checkers

•Additional References

•Feature Information for Configuring Basic Cisco Express Forwarding

•Glossary

Prerequisites for Configuring Cisco Express Forwarding Consistency Checkers

Cisco Express Forwarding must be up and running on the networking device before you can configure Cisco Express Forwarding consistency checkers.

Restrictions for Configuring Cisco Express Forwarding Consistency Checkers

The Cisco Express Forwarding consistency checkers lc-detect, scan-lc-rp, apply only to devices that have distributed Cisco Express Forwarding enabled.

Information About Configuring Cisco Express Forwarding Consistency Checkers

•Cisco Platform Support for Cisco Express Forwarding and Distributed Cisco Express Forwarding

•Cisco Express Forwarding Consistency Checker Types for Cisco Express Forwarding and Distributed Cisco Express Forwarding

For links to information about other Cisco Express Forwarding and distributed Cisco Express Forwarding features that you can configure, refer to the "Additional References" section.

Cisco Platform Support for Cisco Express Forwarding and Distributed Cisco Express Forwarding

Cisco Express Forwarding is enable by default on the Cisco ASR 1000 Series Aggregation Services Routers.

To find out if Cisco Express Forwarding is enabled on your platform, enter the show ip cef command. If Cisco Express Forwarding is enabled, you receive output that looks like this: 

Router# show ip cef 


Prefix              Next Hop            Interface

[...]

10.2.61.8/24        192.168.100.1       FastEthernet1/0/0

                    192.168.101.1       FastEthernet2/1/0

[...]

If Cisco Express Forwarding is not enabled on your platform, the output for the show ip cef command looks like this: 

Router# show ip cef


%CEF not running

If Cisco Express Forwarding is not enabled on your platform, use the ip cef command to enable Cisco Express Forwarding or the ip cef distributed command to enable distributed Cisco Express Forwarding.

Cisco Express Forwarding Consistency Checker Types for Cisco Express Forwarding and Distributed Cisco Express Forwarding

Cisco Express Forwarding uses routing information that is retrieved from the Routing Information Base (RIB), the RP, and the line card databases to perform express forwarding. As these databases are updated, inconsistencies might result, due to the asynchronous nature of the distribution mechanism for these databases. Inconsistencies caused by asynchronous database distribution are of the following types:

•Missing information, such as a particular prefix, on a line card

•Different information, such as different next hop IP addresses, on the line card

Cisco Express Forwarding supports passive and active consistency checkers that run independently to uncover these forwarding inconsistencies. Table 1 describes the consistency checkers and indicates whether the checker operates on the RP or the line card.

Table 1 Types of Cisco Express Forwarding Consistency Checkers 

Checker Type
Operates On
Description

lc-detect

Line card

(Distributed Cisco Express Forwarding only) Detects missing prefixes on the line card. The information is confirmed by the RP.

Retrieves IP prefixes found missing from the line card FIB table. If IP prefixes are missing, the line card cannot forward packets for the corresponding addresses. This consistency checker then sends IP prefixes to the RP for confirmation. If the RP finds that it has the relevant entry, an inconsistency is detected, and an error message is displayed. Finally, the RP sends a signal back to the line card confirming that the IP prefix is an inconsistency.

scan-lc-rp

Line card

(Distributed Cisco Express Forwarding only) Looks through the FIB table for a configurable time period and sends the next x prefixes to the RP. The RP does an exact lookup in its FIB table. If the RP finds that the prefix is missing, the RP reports an inconsistency. Finally, the RP sends a signal back to the line card for confirmation.

The time period and number of prefixes sent are configured with the cef table consistency-check command.

scan-rp-lc

Route Processor

Looks through the RP FIB table for a configurable time period and sends the next x prefixes to the line card. (This action is opposite to the one that the scan-lc-rp checker performs.) The line card does an exact lookup in the FIB table. If the line card finds the prefix missing, the line card reports an inconsistency and signals the RP for confirmation.

The time period and number of prefixes sent are configured with the cef table consistency-check command.

scan-rib-ios

Route Processor

Compares the Routing Information Base (RIB) to the FIB table and provides the number of entries missing from the FIB table.

scan-ios-rib

Route Processor

Compares the FIB table to the RIB and provides the number of entries missing from the RIB.


Cisco Express Forwarding consistency checkers are disabled by default. Console errors are disabled by default.

If you find a database inconsistency, such as an IP prefix missing from a line card or an RP, you can investigate and resolve it by examining the Cisco Express Forwarding system error messages and by issuing Cisco Express Forwarding debug and show commands.

For Cisco Express Forwarding consistency checker system error messages, refer to the System Messages for Cisco IOS XE Software.

How to Configure Cisco Express Forwarding Consistency Checkers

•Enabling Cisco Express Forwarding Consistency Checkers (optional)

•Displaying and Clearing Cisco Express Forwarding Table Inconsistencies (optional)

Enabling Cisco Express Forwarding Consistency Checkers

Perform the following task to enable Cisco Express Forwarding consistency checkers.

SUMMARY STEPS

1. enable

2. configure terminal

3. cef table consistency-check {ipv4| ipv6} [auto-repair [delay seconds [holddown seconds] | holddown seconds] | data-checking | error-message | type {lc-detect | scan-lc-rp | scan-rp-lc | scan-rib-ios | scan-ios-rib} [count count-number [period seconds] | period seconds]]

4. end

DETAILED STEPS

 
Command or Action
Purpose

Step 1 

enable

Example:

Router> enable

Enables privileged EXEC mode.

•Enter your password if prompted.

Step 2 

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3  

ip cef table consistency-check {ipv4 | ipv6} 
[auto-repair [delay seconds [holddown seconds] 
| holddown seconds] | data-checking | 
error-message | type {lc-detect | scan-lc-rp | 
scan-rp-lc | scan-rib-ios |scan-ios-rib} [count 
count-number [period seconds] | period 
seconds]]]
Example:

Router(config)# ip cef table consistency-check IPv4 scan-rib-ios count 100 period 60

Enables Cisco Express Forwarding table consistency checker types and parameters.

•The ipv4 keyword checks IPv4 addresses.

•The ipv6 keyword checks Ipv6 addresses.

•The auto-repair keyword enables the auto repair function. By default, this function is enabled. You can enter the no form of the command to disable auto repair or enter the default form of the command to return the auto repair settings to a 10-second delay and 300-second holddown.

•The delay seconds keyword and argument specifies how long the consistency checker waits to fix an inconsistency. The range is 10 to 300 seconds. The default delay is 10 seconds

•The holddown seconds keyword and argument specifies how long the consistency checker waits to reenable auto repair after auto repair runs. The range is from 300 to 3000 seconds. The default delay is 300 seconds.

•The data-checking keyword enables the consistency checker data-checking utility. By default, this function is disabled.

•The error-message keyword enables the consistency checker to generate an error message when it detects an inconsistency. By default, this function is disabled.

•The type keyword indicates the type of consistency check to enable.

•The lc-detect keyword enables the line card to detect a missing prefix, which is confirmed by the RP.

•The scan-lc-rp keyword performs a passive scan check of tables on the line card.

•The scan-rp-lc keyword enables a passive scan check of tables on the RP.

•The scan-rib-ios keyword compares the Routing Information Base (RIB) to the FIB and provides the number of entries missing from the FIB table.

•The scan-ios-rib keyword compares the FIB table to the RIB and provides the number of entries missing from the RIB.

•The count-number keyword-argument pair is the maximum number of prefixes to check per scan. The range is from 1 to 225.

•The period seconds keyword-argument pair is the time during which updates for a candidate prefix are ignored as inconsistencies. The range is from 1 to 3600 seconds.

Step 4 

end

Example:

Router(config)# end

Exits to privileged EXEC mode.

Displaying and Clearing Cisco Express Forwarding Table Inconsistencies

Perform the following task to display and clear Cisco Express Forwarding table inconsistency records found by the lc-detect, scan-rp-lc, scan-lc-rp, scan-rib-ios, and scan-ios-rib detection mechanisms.

SUMMARY STEPS

1. enable

2. test cef table consistency [detail]

3. clear ip cef inconsistency

4. clear cef linecard [slot-number] [adjacency | interface | prefix]

5. show cef table consistency-check

6. disable

DETAILED STEPS

Step 1 enable

Use this command to enable privileged EXEC mode. For example: 

Router> enable

Enter your password if prompted.

test cef table consistency [detail]

Use this command to test the Cisco Express Forwarding FIB for prefix consistency. For example: 

Router# test cef table consistency


full-scan-rib-ios: Checking IPv4 RIB to FIB consistency

full-scan-ios-rib: Checking IPv4 FIB to RIB consistency

No IPv4 inconsistencies found, check took 00:00:00.000

This output indicates that no inconsistencies were found when the command was entered.

The following is sample output from the test cef table consistency detail command: 

Router# test cef table consistency detail 


full-scan-rib-ios: Checking IPv4 RIB to FIB consistency

full-scan-rib-ios: FIB checked 12 prefixes, and found 0 missing.

full-scan-ios-rib: Checking IPv4 FIB to RIB consistency

full-scan-ios-rib: Checked 12 FIB prefixes in 1 pass, and found 0 extra.

full-scan-rp-lc: Sent 26 IPv4 prefixes to linecards in 1 pass

full-scan-rp-lc: Initiated IPv4 FIB check on linecards..4..1..0..

full-scan-rp-lc: FIB IPv4 check completed on linecards..1..0..4..

full-scan-rp-lc: Linecard 4 checked 26 IPv4 prefixes (ignored 0). 0 inconsistent.

full-scan-rp-lc: Linecard 1 checked 26 IPv4 prefixes (ignored 0). 0 inconsistent.

full-scan-rp-lc: Linecard 0 checked 26 IPv4 prefixes (ignored 0). 0 inconsistent.

full-scan-rib-ios: Checking IPv6 RIB to FIB consistency

full-scan-rib-ios: FIB checked 16 prefixes, and found 5 missing.

full-scan-ios-rib: Checking IPv6 FIB to RIB consistency

full-scan-ios-rib: Checked 11 FIB prefixes in 1 pass, and found 0 extra.

full-scan-rp-lc: Sent 11 IPv6 prefixes to linecards in 1 pass

full-scan-rp-lc: Initiated IPv6 FIB check on linecards..4..1..0..

full-scan-rp-lc: FIB IPv6 check completed on linecards..1..4..0..

full-scan-rp-lc: Linecard 4 checked 11 IPv6 prefixes (ignored 0). 0 inconsistent.

full-scan-rp-lc: Linecard 1 checked 11 IPv6 prefixes (ignored 0). 0 inconsistent.

full-scan-rp-lc: Linecard 0 checked 11 IPv6 prefixes (ignored 0). 0 inconsistent.

No IPv4 inconsistencies found, check took 00:00:01.444

Warning: 5 IPv6 inconsistencies found, check took 00:00:01.240

The output of this command shows that no IPv4 inconsistencies were found and five (5) IPv6 inconsistencies were found. The output also shows how many prefixes were checked by the FIB and the linecards and how many prefixes were missing, if any.

Step 2 clear ip cef inconsistency

Use this command to clear the Cisco Express Forwarding inconsistency statistics and records found by the Cisco Express Forwarding consistency checkers. For example: 

Router# clear ip cef inconsistency

Step 3 clear cef linecard [slot-number] [adjacency | interface | prefix]

Use this command to clear Cisco Express Forwarding information from line cards. For example:

Router# clear cef linecard


Step 4 show cef table consistency-check

Use this command to verify the status of Cisco Express Forwarding consistency checkers in the FIB. For example: 

Router# show cef table consistency-check


Consistency checker master control: enabled


IPv4:

 Table consistency checker state:

  scan-rib-ios: disabled

   0/0/0/0 queries sent/ignored/checked/iterated

  scan-ios-rib: disabled

   0/0/0/0 queries sent/ignored/checked/iterated

  full-scan-rib-ios: enabled [1000 prefixes checked every 60s]

   0/0/0/0 queries sent/ignored/checked/iterated

  full-scan-ios-rib: enabled [1000 prefixes checked every 60s]

   0/0/0/0 queries sent/ignored/checked/iterated

 Checksum data checking disabled

 Inconsistency error messages are disabled

 Inconsistency auto-repair is enabled (10s delay, 300s holddown)

 Inconsistency auto-repair runs: 0

 Inconsistency statistics: 0 confirmed, 0/16 recorded


IPv6:

 Table consistency checker state:

  scan-ios-rib: disabled

   0/0/0/0 queries sent/ignored/checked/iterated

  full-scan-rib-ios: enabled [1000 prefixes checked every 60s]

   0/0/0/0 queries sent/ignored/checked/iterated

  full-scan-ios-rib: enabled [1000 prefixes checked every 60s]

   0/0/0/0 queries sent/ignored/checked/iterated

 Checksum data checking disabled

 Inconsistency error messages are disabled

 Inconsistency auto-repair is enabled (10s delay, 300s holddown)

 Inconsistency auto-repair runs: 0

 Inconsistency statistics: 0 confirmed, 0/16 recorded

This sample output shows that two full scans are enabled and that every 60 seconds, 1000 prefixes are checked. It also shows that the auto repair function is enabled with the default settings of a 10-second delay and a 300-second holddown. In this example, no inconsistencies were found.

Step 5 disable

Use this command to exit to user EXEC mode. For example: 

Router# disable

Router>

Configuration Examples for Configuring Cisco Express Forwarding Consistency Checkers

•Example: Enabling Cisco Express Forwarding Consistency Checkers

Example: Enabling Cisco Express Forwarding Consistency Checkers

The following example shows how to enable the scan-rp Cisco Express Forwarding consistency checker. 

configure terminal

!

cef table consistency-check scan-rp-lc count 225 period 3600


end

The RP is configured to send 3600 prefixes to the line cards every 225 seconds.

Additional References

Related Documents

Related Topic
Document Title

Cisco IOS commands

Cisco IOS Master Commands List, All Releases

Commands for configuring and managing Cisco Express Forwarding

Cisco IOS IP Switching Command Reference

List of the features documented in the Cisco Express Forwarding modules

Cisco Express Forwarding Features Roadmap

Overview of the Cisco Express Forwarding feature

Cisco Express Forwarding Overview

Tasks for verifying basic Cisco Express Forwarding and distributed Cisco Express Forwarding operation

Configuring Basic Cisco Express Forwarding for Improved Performance, Scalability, and Resiliency in Dynamic Networks

Tasks for enabling or disabling Cisco Express Forwarding or distributed Cisco Express Forwarding

Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding to Customize Switching and Forwarding for Dynamic Networks"

Tasks for configuring a load-balancing scheme for Cisco Express Forwarding

Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic

Tasks for configuring epochs for Cisco Express Forwarding tables

Configuring Epochs to Clear and Rebuild Cisco Express Forwarding and Adjacency Tables

Tasks for configuring and verifying Cisco Express Forwarding network accounting

Configuring Cisco Express Forwarding Network Accounting

Tasks for customizing the display of Cisco Express Forwarding event trace messagess

Customizing the Display of Cisco Express Forwarding Event Trace Messages

Cisco Express Forwarding-related error messages

System Messages for Cisco IOS XE Software


Standards

Standard
Title

No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.

—


MIBs

MIB
MIBs Link

No new or modified MIBs are supported by this feature, and support for existing MIBs has not been modified by this feature.

To locate and download MIBs for selected platforms, Cisco IOS XE software releases, and feature sets, use Cisco MIB Locator found at the following URL:

http://www.cisco.com/go/mibs


RFCs

RFC
Title

No new or modified RFCs are supported by this feature, and support for existing RFCs has not been modified by this feature.

—


Technical Assistance

Description
Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html


Feature Information for Configuring Basic Cisco Express Forwarding

Table 2 lists the features in this module.

Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS XE software images support a specific software release, feature set, or platform. To access Cisco Feature Navigator, go to http://www.cisco.com/go/cfn. An account on Cisco.com is not required.

Note Table 2 lists only the Cisco IOS XE software release that introduced support for a given feature in a given Cisco IOS XE software release train. Unless noted otherwise, subsequent releases of that Cisco IOS XE software release train also support that feature.

Table 2 Feature Information for Configuring Basic Cisco Express Forwarding 

Feature Name
Releases
Feature Configuration Information

This table is intentionally left blank because no features were introduced or modified in Cisco IOS XE Release 2.1 or later. This table will be updated when feature information is added to this module.

—

—


Glossary

adjacency—A relationship formed between selected neighboring routers and end nodes for the purpose of exchanging routing information. Adjacency is based upon the use of a common media segment by the routers and nodes involved.

Cisco Express Forwarding—A Layer 3 switching technology. Cisco Express Forwarding can also refer to central Cisco Express Forwarding mode, one of two modes of Cisco Express Forwarding operation. Cisco Express Forwarding enables a Route Processor to perform express forwarding. Distributed Cisco Express Forwarding is the other mode of Cisco Express Forwarding operation.

distributed Cisco Express Forwarding—A mode of Cisco Express Forwarding switching in which line cards maintain identical copies of the forwarding information base (FIB) and adjacency tables. The line cards perform the express forwarding between port adapters; this relieves the Route Processor of involvement in the switching operation.

FIB—forwarding information base. A component of Cisco Express Forwarding that is conceptually similar to a routing table or information base. The router uses the FIB lookup table to make destination-based switching decisions during Cisco Express Forwarding operation. The router maintains a mirror image of the forwarding information in an IP routing table.

IPC—interprocess communication. The mechanism that enables the distribution of Cisco Express Forwarding tables from the Route Processor (RP) to the line card when the router is operating in distributed Cisco Express Forwarding mode.

LIB—label information base. A database used by a label switch router (LSR) to store labels learned from other LSRs, as well as labels assigned by the local LSR.

line card—A general term for an interface processor that can be used in various Cisco products.

MPLS—Multiprotocol Label Switching. An emerging industry standard for the forwarding of packets along the normal routing paths (sometimes called MPLS hop-by-hop forwarding).

prefix—The network address portion of an IP address. A prefix is specified by a network and mask and is generally represented in the format network/mask. The mask indicates which bits are the network bits. For example, 1.0.0.0/16 means that the first 16 bits of the IP address are masked, making them the network bits. The remaining bits are the host bits. In this example, the network number is 10.0.

RIB—Routing Information Base. A central repository of routes that contains Layer 3 reachability information and destination IP addresses or prefixes. The RIB is also known as the routing table.

RP—Route Processor. The processor module in the routers that contains the CPU, system software, and most of the memory components that are used in the router. It is sometimes called a supervisory processor.

VPN—Virtual Private Network. The result of a router configuration that enables IP traffic to use tunneling to travel securely over a public TCP/IP network.

VRF—A Virtual Private Network (VPN) routing/forwarding instance. A VRF consists of an IP routing table, a derived forwarding table, a set of interfaces that use the forwarding table, and a set of rules and routing protocols that determine what goes into the forwarding table. In general, a VRF includes the routing information that defines a customer VPN site that is attached to a PE router.

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