IP Switching Configuration Guide, Cisco IOS XE Release 3S

Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding—To Customize Switching and Forwarding for Dynamic Networks

This module contains information about Cisco Express Forwarding and describes the required and optional tasks for enabling or disabling Cisco Express Forwarding and distributed Cisco Express Forwarding. 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 Enabling or Disabling Cisco Express Forwarding or Distributed 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 Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

•Restrictions for Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

•Information About Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

•How to Enable or Disable Central Cisco Express Forwarding or Distributed Cisco Express Forwarding

•Configuration Examples for Enabling or Disabling Central Cisco Express Forwarding or Distributed Cisco Express Forwarding

•Additional References

•Feature Information for Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

•Glossary

Prerequisites for Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

Cisco Express Forwarding requires a software image that includes Cisco Express Forwarding and IP routing enabled on the switch or router.

Restrictions for Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

•The Cisco ASR 1000 Series Aggregation Services Router operates only in distributed Cisco Express Forwarding mode.

•If you enable Cisco Express Forwarding and then create an access list that uses the log keyword, the packets that match the access list are not Cisco Express Forwarding switched. They are process switched. Logging disables Cisco Express Forwarding.

Information About Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

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

•When to Enable or Disable Central Cisco Express Forwarding Operation on a Router

•When to Enable Distributed Cisco Express Forwarding Operation on a Line Card

•When to Enable or Disable Cisco Express Forwarding Operation on an Interface

For links to information about other Cisco Express Forwarding and distributed Cisco Express Forwarding features you can configure, refer to the following section:

•Additional References

Cisco Platform Support for Central 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 central Cisco Express Forwarding or the ip cef distributed command to enable distributed Cisco Express Forwarding.

When to Enable or Disable Central Cisco Express Forwarding Operation on a Router

Enable central Cisco Express Forwarding operation when line cards are not available for Cisco Express Forwarding switching or when you need to use features not compatible with distributed Cisco Express Forwarding switching. When central Cisco Express Forwarding operation is enabled, the Cisco Express Forwarding Forwarding Information Base (FIB) and adjacency tables reside on the RP, and the RP performs express forwarding.

Disable central Cisco Express Forwarding on a router when you want to turn off central Cisco Express Forwarding on the router and on all interfaces on the router. You might want to do this if your router and router interfaces are configured with a feature that central Cisco Express Forwarding or distributed Cisco Express Forwarding does not support.

To disable central Cisco Express Forwarding on a router and on all interfaces on the router, use the no ip cef command.

When to Enable Distributed Cisco Express Forwarding Operation on a Line Card

Enable distributed Cisco Express Forwarding on a line card when you want the line card to perform express forwarding so that the RP can handle routing protocols or switch packets from legacy interface processors. When distributed Cisco Express Forwarding is enabled, line cards maintain an identical copy of the FIB and adjacency tables. The line cards perform express forwarding between port adapters, thus relieving the RP of involvement in the switching operation. 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.

The Cisco ASR 1000 Series Routers operate only in distributed Cisco Express Forwarding mode. In other routers you can mix various types of line cards in the same router, and all of the line cards you are using need not support Cisco Express Forwarding. When a line card that does not support Cisco Express Forwarding receives a packet, the line card forwards the packet to the next higher switching layer (the RP) or forwards the packet to the next hop for processing. This structure allows legacy interface processors to exist in the router with newer interface processors.

Note When you enable distributed Cisco Express Forwarding globally, all interfaces that support distributed Cisco Express Forwarding are enabled by default.

When to Enable or Disable Cisco Express Forwarding Operation on an Interface

You need to decide whether or not you want Cisco Express Forwarding operation on an interface. In some instances, you might want to disable Cisco Express Forwarding or distributed Cisco Express Forwarding on a particular interface because that interface is configured with a feature that Cisco Express Forwarding or distributed Cisco Express Forwarding does not support. Because all interfaces that support Cisco Express Forwarding or distributed Cisco Express Forwarding are enabled by default when you enable Cisco Express Forwarding operation globally, you must use the no form of the ip route-cache cef command to turn off Cisco Express Forwarding operation on a particular interface. To reenable Cisco Express Forwarding, use the ip route-cache cef command. To reenable distributed Cisco Express Forwarding, use the ip route-cache distributed command.

Disabling Cisco Express Forwarding or distributed Cisco Express Forwarding on an interface disables Cisco Express Forwarding switching for packets forwarded to the interface, but has no effect on packets forwarded out of the interface.

When you disable Cisco Express Forwarding or distributed Cisco Express Forwarding, Cisco IOS XE software switches packets received on the interface using the next fastest switching path. For Cisco Express Forwarding, the next fastest switching path is switching on the RP. For distributed Cisco Express Forwarding, the next fastest switching path is Cisco Express Forwarding on the RP.

The input interface determines the Cisco IOS XE switching path that a packet takes. Consider the following when enabling or disabling switching methods on a particular interface:

•You need Cisco Express Forwarding to be enabled on the incoming interface for packets to be Cisco Express Forwarding switched.

•Because Cisco Express Forwarding makes the forwarding decision on input, you need to use the no ip route-cache cef command on the ingress interface if you want to disable Cisco Express Forwarding.

How to Enable or Disable Central Cisco Express Forwarding or Distributed Cisco Express Forwarding

To enable or disable Cisco Express Forwarding or distributed Cisco Express Forwarding, perform either of the following tasks depending on whether you want to enable or disable Cisco Express Forwarding or distributed Cisco Express Forwarding on the router or to enable or disable Cisco Express Forwarding or distributed Cisco Express Forwarding on an interface:

•Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation on a Router (optional)

•Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation on an Interface (optional)

Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation on a Router

Perform the following task to enable or disable Cisco Express Forwarding or distributed Cisco Express Forwarding operation on a router. Cisco Express Forwarding can optimize your network performance and scalability.

SUMMARY STEPS

1. enable

2. show ip cef [vrf vrf-name] [interface-type interface-number [checksum | [detail | internal [checksum] | platform]]

3. configure terminal

4. ip cef distributed

5. exit

6. show ip cef [vrf vrf-name] [interface-type interface-number [checksum | [detail | internal [checksum] | platform]]

DETAILED STEPS

%CEF not running

Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation on an Interface

Perform the following task to enable or disable Cisco Express Forwarding or distributed Cisco Express Forwarding operation on an interface. Cisco Express Forwarding can optimize your network performance and scalability.

SUMMARY STEPS

1. enable

2. show cef interface [type slot/subslot/port[.subinterface-number]] [statistics] [detail]

3. configure terminal

4. interface type slot/subslot/port[.subinterface-number]

5. [no] ip route-cache cef

6. end

7. show cef interface [type slot/subslot/port[.subinterface-number]] [statistics] [detail]

DETAILED STEPS

Configuration Examples for Enabling or Disabling Central Cisco Express Forwarding or Distributed Cisco Express Forwarding

•Example: Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation

•Example: Enabling or Disabling Central Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation on an Interface

Example: Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation

You might want to disable distributed Cisco Express Forwarding if your router and router interfaces are configured with a feature that distributed Cisco Express Forwarding does not support. The following example shows how to disable distributed Cisco Express Forwarding on a router:

configure terminal

!

no ip cef distributed

end

Example: Enabling or Disabling Central Cisco Express Forwarding or Distributed Cisco Express Forwarding Operation on an Interface

All interfaces that support Cisco Express Forwarding operation (central Cisco Express Forwarding or distributed Cisco Express Forwarding) are enabled by default when you enable Cisco Express Forwarding operation globally. You might want to disable central Cisco Express Forwarding or distributed Cisco Express Forwarding on a particular interface if that interface is configured with a feature that central Cisco Express Forwarding or distributed Cisco Express Forwarding does not support.

The following example shows how to disable central Cisco Express Forwarding on a particular interface:

configure terminal

!

interface ethernet 1/1/0

 no ip route-cache cef

 end

The following example shows how to disable distributed Cisco Express Forwarding on FastEthernet interface 0/0/0:

configure terminal

!

interface fe0/0/0

 no ip route-cache cef

 end

The following example shows how to reenable distributed Cisco Express Forwarding operation on FastEthernet interface 0/0/0:

configure terminal

!

ip cef distributed 

!

interface fe0/0/0

# ip route-cache cef

 end

The following example shows how to enable distributed Cisco Express Forwarding operation on the router (globally) and turn off Cisco Express Forwarding operation on FastEthernet interface 0/0/0:

configure terminal

!

ip cef distributed 

interface fe0/0/0

 no ip route-cache cef

 end

The following example shows how to reenable distributed Cisco Express Forwarding operation on FastEthernet interface 0/0/0:

configure terminal

!

ip cef distributed 

!

interface fe0/0/0

 ip route-cache cef

 end

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Feature Information for Enabling or Disabling Cisco Express Forwarding or Distributed Cisco Express Forwarding

Table 1 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 1 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.

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 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.

GRE—generic routing encapsulation. A tunneling protocol developed by Cisco that enables encapsulation of a wide variety of protocol packet types inside IP tunnels. GRE creates 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 Processor (RP) 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 header 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.

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 you can configure the LSP 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 router 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.

Cisco and the Cisco Logo are trademarks of Cisco Systems, Inc. and/or its affiliates in the U.S. and other countries. A listing of Cisco's trademarks can be found at www.cisco.com/go/trademarks. Third party trademarks mentioned are the property of their respective owners. The use of the word partner does not imply a partnership relationship between Cisco and any other company. (1005R)

Any Internet Protocol (IP) addresses and phone numbers used in this document are not intended to be actual addresses and phone numbers. Any examples, command display output, network topology diagrams, and other figures included in the document are shown for illustrative purposes only. Any use of actual IP addresses or phone numbers in illustrative content is unintentional and coincidental.

© 2005-2009 Cisco Systems, Inc. All rights reserved.