Catalyst 4000 Family Switch Cisco IOS Software Configuration Guide, 12.1(8a)EW - Configuring Cisco Express Forwarding

Table Of Contents

Configuring Cisco Express Forwarding

CEF Overview

CEF Components

Forwarding Information Base

Adjacency Tables

CEF Operation Modes

Catalyst 4006 Implementation of CEF

Hardware and Software Switching

Hardware Switching

Software Switching

Load Balancing

Software Interfaces

Restrictions

Configuring CEF

Default Configuration

Enabling CEF

Configuring Load Balancing for CEF

Configuring Per-Destination Load Balancing

Viewing CEF Information

Monitoring and Maintaining CEF

Displaying IP Statistics

Configuring Cisco Express Forwarding

This chapter describes Cisco Express Forwarding (CEF) on the Catalyst 4006 switch with Supervisor Engine III. It also provides guidelines, procedures, and examples to configure this feature.

This chapter consists of the following sections:

•CEF Overview

•Catalyst 4006 Implementation of CEF

•Configuring CEF

•Monitoring and Maintaining CEF

Note For complete syntax and usage information for the commands used in this chapter, refer to the Command Reference for the Catalyst 4006 Switch with Supervisor Engine III.

CEF Overview

CEF is advanced Layer 3 IP switching technology that optimizes performance and scalability for large networks with dynamic traffic patterns or networks with intensive Web-based applications and interactive sessions.

CEF provides the following benefits:

•Improves performance over the caching schemes of multilayer switches, which often flush the entire cache when information changes in the routing tables

•Provides load balancing that distributes packets across multiple links based on layer 3 routing information. If a network device discovers multiple paths to a destination, the routing table is updated with multiple entries for that destination. Traffic to that destination is then distributed among the various paths.

Note Policy based routing is not supported in this release.

CEF Components

CEF stores information in several data structures rather than the route cache of multilayer switches. The data structures provide optimized lookup for efficient packet forwarding. Two primary components comprise the CEF operation:

•Forwarding Information Base

•Adjacency Tables

Forwarding Information Base

The FIB is a table that contains a mirror image of the forwarding information in the IP routing table. When routing or topology changes occur in the network the route processor updates the IP routing table and CEF updates the FIB. 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 route cache maintenance that is associated with switching paths, such as fast switching and optimum switching. CEF uses the FIB to make IP destination prefix-based switching decisions and maintain next-hop address information based on the information in the IP routing table.

On the Catalyst 4006 Supervisor Engine III, CEF loads the FIB in to the Integrated Switching Engine hardware to increase the performance of forwarding. The Integrated Switching Engine has a finite number of forwarding slots for storing routing information. If this limit is exceeded, CEF is automatically disabled and all packets are forwarded in software. In this situation, you should reduce the number of routes on the switch, and then re-enable hardware switching with the ip cef command.

Adjacency Tables

In addition to the FIB, CEF uses adjacency tables to prepend Layer 2 addressing information. Nodes in the network are said to be adjacent if they are within a single hop from each other. The adjacency table maintains Layer 2 next-hop addresses for all FIB entries.

Adjacency Discovery

The adjacency table is populated as new adjacent nodes are discovered. Each time an adjacency entry is created (such as through the ARP protocol), a link-layer header for that adjacent node is stored in the adjacency table. Once a route is determined, the link-layer header points to a next hop and corresponding adjacency entry. The link-layer header is subsequently used for encapsulation during CEF switching of packets.

Adjacency Resolution

A route might have several paths to a destination prefix, such as when a router is configured for simultaneous load balancing and redundancy. For each resolved path, a pointer is added for the adjacency corresponding to the next-hop interface for that path. This mechanism is used for load balancing across several paths.

Adjacency Types That Require Special Handling

In addition to adjacencies for next-hop interfaces (host-route adjacencies), other types of adjacencies are used to expedite switching when certain exception conditions exist. When the prefix is defined, prefixes requiring exception processing are cached with one of the special adjacencies listed in Table 14-1.

Table 14-1 Adjacency Types for Exception Processing

This adjacency type...

Receives this processing...

Null adjacency

Packets destined for a Null0 interface are dropped. A Null0 interface can be used as an effective form of access filtering.

Glean adjacency

When a router is connected directly to several hosts, the FIB table on the router maintains a prefix for the subnet rather than for each individual host. The subnet prefix points to a glean adjacency. When packets need to be forwarded to a specific host, the adjacency database is gleaned for the specific prefix.

Punt adjacency

Features that require special handling or features that are not yet supported by CEF switching are sent (punted) to the next higher switching level.

Discard adjacency

Packets are discarded.

Drop adjacency

Packets are dropped.

Unresolved Adjacency

When a link-layer header is prepended to packets, FIB requires the prepend to point to an adjacency corresponding to the next hop. If an adjacency was created by FIB and was not discovered through a mechanism such as ARP, the Layer 2 addressing information is not known and the adjacency is considered incomplete. When the Layer 2 information is known, the packet is forwarded to the route processor, and the adjacency is determined through ARP.

CEF Operation Modes

CEF can be enabled in one of two modes:

•Central CEF Mode

When CEF mode is enabled, the CEF FIB and adjacency tables reside on the Integrated Switching Engine hardware, and the hardware performs the express forwarding.

•Distributed CEF Mode

The Supervisor Engine III does not support distributed CEF switching.

Catalyst 4006 Implementation of CEF

The Catalyst 4006 switch with Supervisor Engine III supports an ASIC-based Integrated Switching Engine that provides:

•Ethernet bridging at Layer 2

•IP routing at Layer 3

Because the ASIC is specifically designed to forward packets, the Integrated Switching Engine hardware can run this process much faster than CPU subsystem software.

Figure 14-1 shows a high-level view of the ASIC-based Layer 2 and Layer 3 switching process on the Integrated Switching Engine.

Figure 14-1 Logical L2/L3 Switch Components

The Integrated Switching Engine performs inter-VLAN routing on logical Layer 3 interfaces with the ASIC hardware. The ASIC hardware also supports a physical Layer 3 interface that can be configured to connect with a host, a switch, or a router.

Hardware and Software Switching

For the majority of packets, the Integrated Switching Engine performs the packet forwarding function in hardware. These packets are hardware-switched at very high rates. Exception packets are forwarded by the CPU subsystem software. Statistic reports should show that the Integrated Switching Engine is forwarding the vast majority of packets in hardware. Software forwarding is significantly slower than hardware forwarding, but packets forwarded by the CPU subsystem do not cause hardware forwarding speed to be reduced.

Figure 14-2 shows a logical view of the Integrated Switching Engine and the CPU subsystem switching components.

Figure 14-2 Hardware and Software Switching Components

The Integrated Switching Engine performs inter-VLAN routing in hardware. The CPU subsystem software supports Layer 3 interfaces to VLANs that use subnetwork access protocol (SNAP) encapsulation. The CPU subsystem software also supports a generic routing encapsulation (GRE) tunnel.

Hardware Switching

Hardware switching is the normal operation of the Supervisor Engine III.

Software Switching

Software switching occurs when traffic cannot be processed in hardware. The following types of exception packets are processed in software at a much slower rate:

•Packets that use IP header options

Note Packets that use TCP header options are switched in hardware because they do not affect the forwarding decision.

•Packets that have an expiring IP time-to-live (TTL) counter

•Packets that are forwarded to a tunnel interface

•Packets that arrive with non-supported encapsulation types

•Packets that are routed to an interface with non supported encapsulation types

•Packets that exceed the MTU of an output interface and must be fragmented

•Packets that require an IGMP redirect to be routed

•802.3 Ethernet packets

Load Balancing

The Catalyst 4006 with Supervisor Engine III supports load balancing for routing packets in the Integrated Switching Engine hardware. Load balancing is always enabled. It works when multiple routes for the same network with different next-hop addresses are configured. These routes can be configured either statically or through a routing protocol such as OSPF or EIGRP.

The hardware makes a forwarding decision by using a hardware hash function to compute a value, based on the source and destination IP addresses and the source and destination TCP port numbers (if available). This hash value is then used to select which route to use to forward the packet. All hardware switching within a particular flow (such as a TCP connection) will be routed to the same next hop, thereby reducing the chance of packet reordering occurring. Up to 8 different routes for a particular network are supported.

Software Interfaces

Cisco IOS for the Catalyst 4000 supports GRE and IP tunnel interfaces that are not a part of the hardware forwarding engine. All packets that flow to or from these interfaces must be processed in software and will have a significantly lower forwarding rate than that of hardware-switched interfaces. Also, no layer 2 features are supported on these interfaces.

Restrictions

The Integrated Switching Engine supports only ARPA and ISL/802.1q encapsulation types for Layer 3 switching in hardware. The CPU subsystem supports a number of encapsulations such as SNAP for Layer 2 switching that you can use for Layer 3 switching in software.

Configuring CEF

The following sections describe how to configure CEF:

•Default Configuration

•Enabling CEF

•Configuring Load Balancing for CEF

Note The ip mtu command is not supported in this release.

Default Configuration

Table 14-2 shows the default IP unicast configuration.

Table 14-2 Default CEF Configuration

Feature

Default Value

CEF

Enabled globally

Enabling CEF

By default, CEF is enabled on Supervisor Engine III. No configuration is required.

To disable CEF, enter the no IP cef command in global configuration mode. When you disable CEF, Cisco IOS software forwards packets using the CPU subsystem software. Do not disable CEF for normal operation.

Command

Purpose

Switch(config)# no ip cef

Disables CEF operation.

To reenable CEF, enter the IP cef command in global configuration mode:

Command

Purpose

Switch(config)# ip cef

Enables standard CEF operation.

Configuring Load Balancing for CEF

CEF load balancing is based on a combination of source and destination packet information; it allows you to optimize resources by distributing traffic over multiple paths for transferring data to a destination. You can configure load balancing on a per-destination basis. Load-balancing decisions are made on the outbound interface. When you configure load balancing, configure it on outbound interfaces.

You can configure two types of load balancing for CEF by performing the following optional tasks:

•Configuring Per-Destination Load Balancing

•Viewing CEF Information

Configuring Per-Destination Load Balancing

Per-destination load balancing is enabled by default when you enable CEF. To use per-destination load balancing, you do not perform any additional tasks once you enable CEF.

Per-destination load balancing allows the router to use multiple paths to achieve load sharing. Packets for a given source-destination host pair are guaranteed to take the same path, even if multiple paths are available. Traffic destined for different pairs tend to take different paths. Per-destination load balancing is enabled by default when you enable CEF, and is the load balancing method of choice in most situations.

Because per-destination load balancing depends on the statistical distribution of traffic, load sharing becomes more effective as the number of source-destination pairs increase.

You can use per-destination load balancing to ensure that packets for a given host pair arrive in order. All packets for a certain host pair are routed over the same link or links.

Disabling Per-Destination Load Balancing

To disable per-destination load balancing, enter the following command in interface configuration mode:

Command

Purpose

Switch# no ip load-sharing per-destination

Disables per-destination load balancing.

Viewing CEF Information

You can view the collected CEF information. To do so, enter the following command in EXEC mode:

Command

Purpose

Switch# show ip cef

Displays the collected CEF information.

Monitoring and Maintaining CEF

To display information about IP traffic, enter the following command:

Command

Purpose

Switch# show interface type slot/interface | begin L3

Displays a summary of IP unicast traffic.

This example shows how to display information about IP unicast traffic on interface Fast Ethernet 3/3:
Switch# show interface fastethernet 3/3 | begin L3 
  L3 in Switched: ucast: 0 pkt, 0 bytes - mcast: 12 pkt, 778 bytes mcast
  L3 out Switched: ucast: 0 pkt, 0 bytes - mcast: 0 pkt, 0 bytes 
     4046399 packets input, 349370039 bytes, 0 no buffer
     Received 3795255 broadcasts, 2 runts, 0 giants, 0 throttles
<...output truncated...>
Switch#
Note The IP unicast packet count is updated approximately every five seconds.

Displaying IP Statistics

IP unicast statistics are gathered on a per-interface basis. To display IP statistics, enter the following command:

Command

Purpose

Switch# show interface type number counters detail

Displays IP statistics.

This example shows how to display IP unicast statistics:
Switch# show interface fastethernet 3/1 counters detail 
Port                InBytes       InUcastPkts      InMcastPkts       InBcastPkts 
Fa3/1            7263539133           5998222          6412307               156 
Port               OutBytes      OutUcastPkts     OutMcastPkts      OutBcastPkts 
Fa3/1            7560137031           5079852         12140475                38 
Port              InPkts 64        OutPkts 64    InPkts 65-127    OutPkts 65-127 
Fa3/1                 11274            168536          7650482          12395769 
Port         InPkts 128-255   OutPkts 128-255   InPkts 256-511   OutPkts 256-511 
Fa3/1                 31191             55269            26923             65017 
Port        InPkts 512-1023  OutPkts 512-1023 
Fa3/1                133807            151582 
Port       InPkts 1024-1518 OutPkts 1024-1518 InPkts 1519-1548 OutPkts 1519-1548 
Fa3/1                   N/A               N/A              N/A               N/A 
Port       InPkts 1024-1522 OutPkts 1024-1522 InPkts 1523-1548 OutPkts 1523-1548 
Fa3/1               4557008           4384192                0                 0 
Port       Tx-Bytes-Queue-1  Tx-Bytes-Queue-2 Tx-Bytes-Queue-3  Tx-Bytes-Queue-4 
Fa3/1                    64                 0            91007        7666686162 
Port       Tx-Drops-Queue-1  Tx-Drops-Queue-2 Tx-Drops-Queue-3  Tx-Drops-Queue-4 
Fa3/1                     0                 0                0                 0 
Port         Rx-No-Pkt-Buff     RxPauseFrames    TxPauseFrames   PauseFramesDrop 
Fa3/1                     0                 0                0               N/A 
Port       UnsupOpcodePause 
Fa3/1                     0 
Switch# 
To display CEF (software switched) and hardware IP unicast adjacency table information, enter the following command:

Command

Purpose

Switch# show adjacency [interface] [detail | internal | summary]

Displays detailed adjacency information, including Layer 2 information, when the optional detail keyword.

This example shows how to display adjacency statistics:
Switch# show adjacency gigabitethernet 3/5 detail 
Protocol Interface                 Address
IP       GigabitEthernet9/5        172.20.53.206(11)
                                   504 packets, 6110 bytes
                                   00605C865B82
                                   000164F83FA50800
                                   ARP        03:49:31 
Note Adjacency statistics are updated approximately every 10 seconds.