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Cisco IOS IP Switching Configuration Guide, Release 12.4
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Cisco IOS Switching Paths Overview
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Part 1: Cisco Express Forwarding
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Cisco Express Forwarding Features Roadmap
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Cisco Express Forwarding Overview
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Configuring Basic Cisco Express Forwarding for Improved Performance, Scalability, and Resiliency in Dynamic Networks
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Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding to Customize Switching/Forwarding for Dynamic Networks
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Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
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Configuring Epochs to Clear and Rebuild Cisco Express Forwarding and Adjacency Tables
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Configuring Cisco Express Forwarding Consistency Checkers for Route Processors and Line Cards
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Configuring Cisco Express Forwarding Network Accounting
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Customizing the Display of Recorded Cisco Express Forwarding Events
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- Part 2: Fast Switching
- Part 3: Multicast Distributed Switching
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Table Of Contents
Cisco Express Forwarding Overview
Information About Cisco Express Forwarding
Cisco Platform Support for Central Cisco Express Forwarding and Distributed Cisco Express Forwarding
Cisco Express Forwarding Benefits: Improved Performance, Scalability, and Resilience
Media Supported by Cisco Express Forwarding
Main Components of Cisco Express Forwarding Operation
Cisco Express Forwarding FIB and Load Balancing
Cisco Express Forwarding Adjacency Tables Overview
Adjacency Types That Require Special Handling
Cisco Express Forwarding Operation Modes: Central and Distributed
Central Cisco Express Forwarding Mode Operation
Distributed Cisco Express Forwarding Mode Operation
Cisco Express Forwarding Features Enabled by Default
Cisco Express Forwarding Distributed Tunnel Switching
Cisco Express Forwarding-Switched Multipoint GRE Tunnels (Cisco IOS 12.2(8)T)
Links for the Cisco Express Forwarding Features
How to Configure Cisco Express Forwarding
Configuration Examples for Cisco Express Forwarding
Feature Information for Cisco Express Forwarding Overview
Cisco Express Forwarding Overview
First Published: May 2, 2005Last Updated: June 11, 2008This module contains an overview of the Cisco Express Forwarding feature. 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
Your software release may not support all the features documented in this module. 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 Cisco Express Forwarding Overview" section.
Use Cisco Feature Navigator to find information about platform support and Cisco IOS and Catalyst OS 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
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Information About Cisco Express Forwarding
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Information About Cisco Express Forwarding
Before using Cisco Express Forwarding or distributed Cisco Express Forwarding, you should understand the following:
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Cisco Platform Support for Central Cisco Express Forwarding and Distributed Cisco Express Forwarding
Cisco Express Forwarding is enabled by default on most Cisco platforms running Cisco IOS software Release12.0 or later. When Cisco Express Forwarding is enabled on a router, the Route Processor (RP) performs the express forwarding.
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 cefPrefix Next Hop Interface[...]10.2.61.8/24 192.168.100.1 FastEthernet1/0/0192.168.101.1 FastEthernet6/1[...]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 runningDistributed Cisco Express Forwarding is enabled by default on the Catalyst 6500 series switch, the Cisco 7500 series router, and the Cisco 12000 Series Internet Router. When distributed Cisco Express Forwarding is enabled on your platform, the line cards perform the express forwarding.
If Cisco Express Forwarding is not enabled on your platform, use the ip cef command to enable (central) Cisco Express Forwarding or the ip cef distributed command to enable distributed Cisco Express Forwarding.
Cisco Express Forwarding Benefits: Improved Performance, Scalability, and Resilience
Cisco Express Forwarding offers the following benefits:
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You can use Cisco Express Forwarding in any part of a network. For example, Figure 1 shows Cisco Express Forwarding being run on Cisco 12000 Series Internet routers at aggregation points at the core of a network where traffic levels are high and performance is critical.
Figure 1 Cisco Express Forwarding Example
In a typical high-capacity Internet service provider (ISP) environment, Cisco 12000 Series Internet routers function as aggregation devices at the core of the network and support links to Cisco 7500 series routers or other feeder devices. Cisco Express Forwarding in these platforms at the network core provides the performance and scalability that networks need to respond to continued growth and steadily increasing network traffic. Cisco Express Forwarding is a distributed switching mechanism that scales linearly with the number of interface cards and the bandwidth installed in the router.
Media Supported by Cisco Express Forwarding
Cisco Express Forwarding currently supports the following media:
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Main Components of Cisco Express Forwarding Operation
Information conventionally stored in a route cache is stored in several data structures for Cisco Express Forwarding switching. The data structures provide optimized lookup for efficient packet forwarding. The two main components of Cisco Express Forwarding operation are the forwarding information base (FIB) and the adjacency tables.
The FIB is conceptually similar to a routing table or information base. A router uses this lookup table to make destination-based switching decisions during Cisco Express Forwarding operation. The FIB is updated when changes occur in the network and contains all routes known at the time. For more information, see the "FIB Overview" section.
Adjacency tables maintain Layer 2 next-hop addresses for all FIB entries. For more information, see the "Cisco Express Forwarding Adjacency Tables Overview" section.
This separation of the reachability information (in the Cisco Express Forwarding table) and the forwarding information (in the adjacency table), provides a number of benefits:
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FIB Overview
Cisco Express Forwarding uses a FIB to make IP destination prefix-based switching decisions.
The FIB contains the prefixes from the IP routing table structured in a way that is optimized for forwarding. 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.
Because there is a one-to-one correlation between FIB entries and routing table entries, the FIB contains all known routes and eliminates the need for the route cache maintenance that is associated with switching paths such as those used in fast switching and optimum switching.
Cisco Express Forwarding FIB and Load Balancing
Several paths can lead to a destination prefix. This occurs, for example, when a router is configured for simultaneous load balancing and redundancy. For each resolved path, the FIB contains a pointer for the adjacency corresponding to the next hop interface for that path.
Cisco Express Forwarding Adjacency Tables Overview
A node is said to be adjacent to another node if the node can be reached with a single hop across a link layer (Layer 2). Cisco Express Forwarding stores forwarding information (outbound interface and MAC header rewrite) for adjacent nodes in a data structure called the adjacency table. Cisco Express Forwarding uses adjacency tables to prepend Layer 2 addressing information to packets. The adjacency tables maintain Layer 2 next-hop addresses for all FIB entries.
The following sections provide additional information about adjacencies:
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Adjacency Discovery
Each adjacency table is populated as adjacencies are discovered. Adjacencies are added to the table either through indirect manual configuration or dynamically—discovered through a mechanism like Address Resolution Protocol (ARP) or added through the use of a routing protocol, such as Border Gateway Protocol (BGP) or Open Shortest Path First (OSPF), which forms neighbor relationships. Each time an adjacency entry is created, a link-layer header for that adjacent node is computed and stored in the adjacency table.
The adjacency information is subsequently used for encapsulation during Cisco Express Forwarding switching of packets.
Adjacency Types That Require Special Handling
In addition to adjacencies associated with next hop interfaces (host-route adjacencies), other types of adjacencies are used to expedite switching when certain exception conditions exist. Prefixes requiring exception processing or special handling are cached with one of the special adjacencies listed in Table 1.
Unresolved Adjacency
When a link-layer header is prepended to a packet, the FIB requires the prepended header to point to an adjacency corresponding to the next hop. If an adjacency was created by the FIB and not discovered through a mechanism such as ARP, the Layer 2 addressing information is not known and the adjacency is considered incomplete or unresolved. Once the Layer 2 information is known, the packet is forwarded to the RP, and the adjacency is determined through ARP. Thus, the adjacency is resolved.
Cisco Express Forwarding Operation Modes: Central and Distributed
Cisco Express Forwarding can be enabled in one of the two modes described in the following sections:
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Central Cisco Express Forwarding Mode Operation
You can use central Cisco Express Forwarding mode when line cards are not available for Cisco Express Forwarding switching, when you need to use features not compatible with distributed Cisco Express Forwarding switching, or when you are running on a nondistributed platform. When central Cisco Express Forwarding mode is enabled, the Cisco Express Forwarding FIB and adjacency tables reside on the RP, and the RP performs the express forwarding.
Figure 2 shows the relationship between the routing table, the FIB, and the adjacency table during central Cisco Express Forwarding mode operation. The Catalyst switches forward traffic from workgroup LANs to a Cisco 7500 series router on the enterprise backbone running central Cisco Express Forwarding. The RP performs the express forwarding.
Figure 2 Central Cisco Express Forwarding Mode Operation
Distributed Cisco Express Forwarding Mode Operation
For additional scalability, Cisco Express Forwarding runs in the distributed Cisco Express Forwarding form on certain platforms by spreading processing tasks across two or more line cards. When distributed Cisco Express Forwarding mode is enabled, line cards maintain identical copies of the FIB and adjacency tables. The line cards perform the express forwarding between port adapters, relieving the RP of involvement in the switching operation, thus also enhancing system performance.
Distributed Cisco Express Forwarding uses an interprocess communication (IPC) mechanism to ensure synchronization of FIB tables and adjacency tables on the RP and line cards.
Figure 3 shows the relationship between the RP and line cards when distributed Cisco Express Forwarding mode is active.
Figure 3 Distributed Cisco Express Forwarding Mode Operation
In the Cisco 12000 Series Internet Router, shown in Figure 3, the line cards perform the switching. In other routers where you can mix various types of cards in the same router, all cards might not support distributed Cisco Express Forwarding. When a line card that does not support distributed Cisco Express Forwarding receives a packet on one of these other routers, the line card forwards the packet to the next higher switching layer (the RP). This structure allows legacy interface processors to exist in the router with newer interface processors.
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Cisco Express Forwarding Features Enabled by Default
The following features are enabled by default when Cisco Express Forwarding is enabled:
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Cisco Express Forwarding Distributed Tunnel Switching
Cisco Express Forwarding supports distributed tunnel switching, such as that made possible by GRE tunnels. Distributed tunnel switching is enabled automatically when you enable Cisco Express Forwarding or distributed Cisco Express Forwarding. You do not perform any additional tasks to enable distributed tunnel switching once you enable Cisco Express Forwarding or distributed Cisco Express Forwarding.
Cisco Express Forwarding-Switched Multipoint GRE Tunnels (Cisco IOS 12.2(8)T)
The Cisco Express Forwarding-Switched Multipoint GRE Tunnels feature enables Cisco Express Forwarding switching of IP traffic to and from multipoint GRE tunnels. Traffic can be forwarded to a prefix through a tunnel destination when both the prefix and the tunnel destination are specified by the application. GRE creates a virtual point-to-point link to other routers at remote points over an IP internetwork. GRE can encapsulate a wide variety of protocol type packets. By connecting multiprotocol subnetworks in a single-protocol backbone environment, IP tunneling using GRE allows network expansion across a single-protocol backbone environment.
Links for the Cisco Express Forwarding Features
Table 2 contains links to information about features that you can configure for use with Cisco Express Forwarding or distributed Cisco Express Forwarding operation.
Table 2 Features to Configure for Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation
Configuring and verifying basic Cisco Express Forwarding operation
Enabling or disabling Cisco Express Forwarding or distributed Cisco Express Forwarding switching and forwarding
Changing your load-balancing scheme
Configuring a Load-Balancing Scheme for Cisco Express Forwarding Traffic
Refreshing or rebuilding adjacency or Cisco Express Forwarding tables
Configuring Epochs to Clear and Rebuild Cisco Express Forwarding and Adjacency Tables
Configuring Cisco Express Forwarding consistency checkers
Configuring Cisco Express Forwarding Consistency Checkers for Route Processors and Line Cards
Configuring network accounting for Cisco Express Forwarding
Customizing the display of recorded Cisco Express Forwarding events
Customizing the Display of Recorded Cisco Express Forwarding Events
How to Configure Cisco Express Forwarding
There are no tasks for the Cisco Express Forwarding Overview module.
See the "Related Documents" section for links to configuration information for Cisco Express Forwarding features and services.
Configuration Examples for Cisco Express Forwarding
There are no configuration examples for the Cisco Express Forwarding Overview module.
See the "Related Documents" section for links to configuration information for Cisco Express Forwarding features and services.
Where to Go Next
See the "Related Documents" section for links to configuration information for Cisco Express Forwarding features and services.
Additional References
The following sections provide references related to configuring Cisco Express Forwarding.
Related Documents
IP switching commands: complete command syntax, command modes, command history, defaults, usage guidelines, and examples.
List of the features documented in the Cisco Express Forwarding modules
Tasks for verifying Cisco Express Forwarding information on your router
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 Cisco Express Forwarding consistency checkers
Configuring Cisco Express Forwarding Consistency Checkers for Route Processors and Line Cards
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
Tasks for customizing the display of recorded Cisco Express Forwarding events
Customizing the Display of Recorded Cisco Express Forwarding Events
Verification steps for Cisco Express Forwarding switching
Troubleshooting tips for incomplete adjacencies
Description and use of the Cisco Express Forwarding consistency checkers available for the Cisco 7500 and 12000 series routers
Troubleshooting Prefix Inconsistencies with Cisco Express Forwarding
Information about troubleshooting Cisco Express Forwarding routing loops and suboptimal routing
Causes of common Cisco Express Forwarding-related error messages on platforms running distributed Cisco Express Forwarding switching (Cisco 7500 series routers and Cisco 12000 Series Internet routers) and how to troubleshoot them
Troubleshooting Cisco Express Forwarding-Related Error Messages
Explanation of and troubleshooting information for the Cisco IOS software implementation of Layer 3 load balancing across multiple parallel links when Cisco Express Forwarding is used
Troubleshooting Load Balancing Over Parallel Links Using Cisco Express Forwarding
Troubleshooting guide for unicast IP routing on Catalyst 6500/6000 switches with Supervisor Engine 2, Policy Feature Card 2 (PFC2), or Multilayer Switch Feature Card 2 (MSFC2)
QoS features that require Cisco Express Forwarding
Standards
No new or modified standards are supported by this feature, and support for existing standards has not been modified by this feature.
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MIBs
RFCs
RFC 1701
Generic Route Encapsulation (GRE)
RFC 2784
Generic Routing Encapsulation (GRE)
RFC 2890
Key and Sequence Number Extensions to GRE
Technical Assistance
Feature Information for Cisco Express Forwarding Overview
Table 3 lists the release history for this feature.
For information on a feature in this technology that is not documented here, see the Cisco Express Forwarding Features Roadmap.
Use Cisco Feature Navigator to find information about platform support and software image support. Cisco Feature Navigator enables you to determine which Cisco IOS and Catalyst OS 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.
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Table 3 Feature Information for Cisco Express Forwarding Overview
Cisco Express Forwarding-Switched Multipoint GRE Tunnels
12.2(8)T
This feature enables Cisco Express Forwarding switching of IP traffic to and from multipoint GRE tunnels. Prior to the introduction of this feature, only process switching was available for multipoint GRE tunnels.
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CEF Support for IP Routing between IEEE 802.1Q vLANs
Cisco IOS XE Release 2.1
This feature was introduced on Cisco ASR 1000 Series Routers.
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 operation in which line cards (such as Versatile Interface Processor (VIP) 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 Switch 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.
GRE—generic routing encapsulation. A tunneling protocol developed by Cisco that enables encapsulation of a wide variety of protocol packet types inside IP tunnels, creating a virtual point-to-point link to Cisco routers at remote points over an IP internetwork. By connecting multiprotocol subnetworks in a single-protocol backbone environment, IP tunneling using GRE allows the expansion of a network across a single-protocol backbone environment.
IPC—interprocess communication. The mechanism that enables the distribution of Cisco Express Forwarding tables from the Route Switch Processor (RSP) to the line card when the router is operating in distributed Cisco Express Forwarding mode.
label disposition—The removal of Multiprotocol Label Switching (MPLS) headers at the edge of a network. In MPLS label disposition, packets arrive on a router as MPLS packets and, with the headers removed, are transmitted as IP packets.
label imposition—The action of putting a label on a packet.
LER—label edge router. A router that performs label imposition.
LFIB—label forwarding information base. The data structure used by switching functions to switch labeled packets.
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. For example, a Versatile Interface Processor (VIP) is a line card for the Cisco 7500 series router.
LSP—label switched path. A sequence of hops (Router 0...Router n). A packet travels from R0 to Rn by means of label switching mechanisms. An LSP can be chosen dynamically, based on normal routing mechanisms, or it can be configured manually.
LSR—label switch router. A Layer 3 router that forwards a packet based on the value of a label encapsulated in the packet.
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 Cisco 7000 series 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.
RSP—Route Switch Processor. The processor module used in the Cisco 7500 series routers that integrates the functions of the Route Processor (RP) and the Switch Processor (SP).
SP—Switch Processor. The Cisco 7000-series processor module that acts as the administrator for all CxBus activities. It is sometimes called a CiscoBus controller.
VIP—Versatile Interface Processor. An interface card used in Cisco 7000 and Cisco 7500 series routers. The VIP provides multilayer switching and runs Cisco IOS.
VPN—Virtual Private Network. 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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