Cisco Catalyst 3850 User manual

IP Switching Cisco Express Forwarding
Configuration Guide, Cisco IOS XE
Release 3SE (Catalyst 3850 Switches)
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C O N T E N T S
Cisco Express Forwarding 1
Finding Feature Information 1
Information About CEF 1
Cisco Platform Support for CEF and dCEF 2
CEF Benefits 2
Media Supported by CEF 3
Main Components of CEF 4
FIB Overview 4
CEF FIB and Load Balancing 4
CEF Adjacency Tables Overview 4
Adjacency Discovery 5
Adjacency Types That Require Special Handling 5
Unresolved Adjacency 6
Central CEF Mode Operation 6
Distributed CEF Mode Operation 6
CEF Features Enabled by Default 7
CEF Distributed Tunnel Switching 8
Links for the CEF Features 8
How to Configure CEF 8
Configuration Examples for CEF 9
Additional References 9
Feature Information for Cisco Express Forwarding 10
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Contents
IP Switching Cisco Express Forwarding Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850
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Cisco Express Forwarding
This 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, page 1
•Information About CEF, page 1
•How to Configure CEF, page 8
•Configuration Examples for CEF, page 9
•Additional References, page 9
•Feature Information for Cisco Express Forwarding, page 10
Finding Feature Information
Your software release may not support all the features documented in this module. For the latest caveats
and feature information, see Bug Search Tool and 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 table at the end of this module.
Use Cisco Feature Navigator to find information about platform support and Cisco software image support.
To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Information About CEF
•Cisco Platform Support for CEF and dCEF, page 2
•CEF Benefits, page 2
•Media Supported by CEF, page 3
•Main Components of CEF, page 4
•FIB Overview, page 4
•CEF Adjacency Tables Overview, page 4
•Central CEF Mode Operation, page 6
•Distributed CEF Mode Operation, page 6
•CEF Features Enabled by Default, page 7
•Links for the CEF Features, page 8
IP Switching Cisco Express Forwarding Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
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Cisco Platform Support for CEF and dCEF
Cisco Express Forwarding is enabled by default on most Cisco devices running Cisco software. When
Cisco Express Forwarding is enabled on a device, the route processor (RP) performs the express
forwarding.
To find out if Cisco Express Forwarding is enabled on your device, enter the show ip cef command. If
Cisco Express Forwarding is enabled, you receive output that looks like this:
Device# show ip cef
Prefix Next Hop Interface
[...]
10.2.61.8/24 192.168.100.1 FastEthernet1/0/0
192.168.101.1 FastEthernet6/1
[...]
If Cisco Express Forwarding is not enabled on your device, the output for the show ip cef command looks
like this:
Device# show ip cef
%CEF not running
When distributed Cisco Express Forwarding is enabled on your device, the line cards perform the express
forwarding.
If Cisco Express Forwarding is not enabled on your device, use the ip cef command to enable Cisco
Express Forwarding or use the ip cef distributed command to enable distributed Cisco Express
Forwarding.
CEF Benefits
• Improved performance—Cisco Express Forwarding is less CPU-intensive than fast-switching route
caching. As a result, more CPU processing power can be dedicated to Layer 3 services such as quality
of service (QoS) and encryption.
• Scalability—Cisco Express Forwarding offers full switching capacity at each line card when
distributed Cisco Express Forwarding mode is active. Distributed Cisco Express Forwarding is a
distributed switching mechanism that scales linearly with the number of interface cards and the
bandwidth installed in the device.
• Resilience—Cisco Express Forwarding offers an unprecedented level of switching consistency and
stability in large dynamic networks. In dynamic networks, fast-switched cache entries are frequently
invalidated by routing changes. These changes can cause traffic to be process-switched through use of
the routing table, rather than fast-switched through use of the route cache. Because the forwarding
information base (FIB) lookup table contains all known routes that exist in the routing table, it
eliminates the need for route cache maintenance and the steps involved with fast-switch or process-
switch forwarding. Cisco Express Forwarding can switch traffic more efficiently than typical demand
caching schemes.
Cisco Platform Support for CEF and dCEF
Information About CEF
IP Switching Cisco Express Forwarding Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850
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You can use Cisco Express Forwarding in any part of a network. For example, the figure below shows
Cisco Express Forwarding being run on devices at aggregation points at the core of a network where traffic
levels are high and performance is critical.
Figure 1 Cisco Express Forwarding Example
Media Supported by CEF
Cisco Express Forwarding supports the following media:
• ATM/AAL5snap, ATM/AAL5mux, and ATM/AAL5nlpid
• Ethernet
• FDDI
• Frame Relay
• High-Level Data Link Control (HDLC)
• PPP
• Spatial Reuse Protocol (SRP)
• Token Ring
• Tunnels
Media Supported by CEF
Information About CEF
IP Switching Cisco Express Forwarding Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3850 Switches)
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Main Components of CEF
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
CEF 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:
• The adjacency table can be built separately from the Cisco Express Forwarding table, allowing both to
be built without any packets being process-switched.
• The MAC header rewrite used to forward a packet is not stored in cache entries, so changes in a MAC
header rewrite string do not require validation of cache entries.
FIB Overview
Cisco Express Forwarding uses a forwarding information base (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.
•CEF FIB and Load Balancing, page 4
CEF 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.
CEF 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:
Main Components of CEF
CEF FIB and Load Balancing
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•Adjacency Discovery, page 5
•Adjacency Types That Require Special Handling, page 5
•Unresolved Adjacency, page 6
Adjacency Discovery
Each adjacency table is populated as adjacencies are discovered. Adjacencies are added to the table through
indirect manual configuration or dynamically--discovered through a mechanism like Address Resolution
Protocol (ARP). Adjacencies can also be 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 the table
below.
Table 1 Adjacency Types That Require Special Handling
Packets of This Adjacency Type Receive This Processing
Null adjacency Packets destined for a Null0 interface are dropped.
Null adjacency can be used as an effective form of
access filtering.
Glean adjacency When a device is connected to a multiaccess
medium, the FIB table on the device maintains a
prefix for the subnet rather than for the individual
host prefixes. The subnet prefix points to a glean
adjacency. A glean adjacency entry indicates that a
particular next hop should be directly connected,
but there is no MAC header rewrite information
available. When the device needs to forward
packets to a specific host on a subnet, Cisco
Express Forwarding requests an ARP entry for the
specific prefix, ARP sends the MAC address, and
the adjacency entry for the host is built.
Punt adjacency The device forwards packets requiring special
handling or packets sent by features not yet
supported in CEF switching paths to the next higher
switching level for handling.
Discard adjacency The device discards the packets.
Drop adjacency The device drops the packets.
Cisco Express Forwarding
Adjacency Discovery
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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.
Central CEF 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 platform that is not a distributed 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.
The figure below shows the relationship between the routing table, the FIB, and the adjacency table during
central Cisco Express Forwarding mode operation. Traffic is forwarded from workgroup LANs to a device
on the enterprise backbone that is running central Cisco Express Forwarding. The RP performs the express
forwarding.
Figure 2 Central Cisco Express Forwarding Mode Operation
302315
Route Processor
Workgroup LAN Workgroup LAN
FIB table
Routing
table
Interface
card
E1 E2
Interface
card
E1 E2
Interface
card
E1 E2
Adjacency
table
Cisco device
running CEF
Workgroup LAN
Distributed CEF Mode Operation
For additional scalability, Cisco Express Forwarding runs in the distributed Cisco Express Forwarding form
on certain devices 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.
Central CEF Mode Operation
Unresolved Adjacency
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