Cisco Express Forwarding Wayne Lewis May11

© 2010 Cisco Systems, Inc. All rights reserved. 1

Cisco Express Forwarding

Wayne Lewis, Ph.D. Cisco Networking Academy Instructor and Cisco Press Author Pacific Center for Advanced Technology Training Honolulu Community College

Virtual Academy Conference 2010

2© 2010 Cisco Systems, Inc. All rights reserved.

Agenda

Cisco Express Forwarding (CEF) Overview

Layer 3 Switching

What Is CEF?

How Does CEF Work?

How Do You Configure CEF?


Cisco Express Forwarding Overview

CEF is state-of-the-art technology used on Cisco devices that offloads traditional CPU duties to dedicated application specific integrated circuits and data structures to increase the speed of switching packets on Cisco routers and switches.

Formally, CEF is a scalable, distributed, layer 3 switching solution designed to meet the future performance requirements of the Internet and enterprise networks. It represents the latest advance in Cisco IOS switching capabilities. CEF is also a key component of Cisco's MPLS implementation.


Layer 3 Switch Processing

In Layer 3 switches, the control path and data path are relatively independent.

– The control path code, such as routing protocols, runs on the route processor.

– Data packets are forwarded by the switching fabric.


Layer 3 Switch Processing (Cont.)

Layer 3 switching takes place using one of these two methods:

1. Route caching: A Layer 3 route cache is built in hardware as the switch sees traffic flow into the switch.

2. Topology-based switching: Information from the routing table is used to populate the route cache, regardless of traffic.


CEF-Based Multilayer Switches

CEF caches routing information in the FIB table and Layer 2 next-hop addresses in the adjacency table.


Multilayer Switch Packet Forwarding Process

Some IP packets cannot be processed in hardware.

If an IP packet cannot be processed in hardware, it is processed by the Layer 3 engine.


CEF-Based MLS Lookups

1. Layer 3 packets initiate TCAM lookup.

2. The longest match returns adjacency with rewrite information.

3. The packet is rewritten per adjacency information and forwarded.


ARP Throttling


CEF-Based MLS Operation


Configuring and Verifying CEF

Configuring CEF

ip cef (enabled by default)

ip route-cache cef (VLAN interface on switch, physical interface on router)

Verifying CEF

show ip cef fa 0/1 detail

show adjacency fa 0/1 detail


The commands required to enable CEF are platform dependent:

Enabling CEF

Swouter(config)#ip cef [distributed]

Swouter(config-if)#ip route-cache cef

On a multilayer Cisco switch or a Cisco router

On a multilayer Cisco switch SVI or Cisco router interface


Verifying CEF

Switch#show ip cef [type mod/port | vlan_interface] [detail]

Switch# show ip cef vlan 11 detail

IP CEF with switching (Table Version 11), flags=0x0

10 routes, 0 reresolve, 0 unresolved (0 old, 0 new), peak 0

13 leaves, 12 nodes, 14248 bytes, 14 inserts, 1 invalidations

0 load sharing elements, 0 bytes, 0 references

universal per-destination load sharing algorithm, id 4B936A24

2(0) CEF resets, 0 revisions of existing leaves

Resolution Timer: Exponential (currently 1s, peak 1s)

0 in-place/0 aborted modifications

refcounts: 1061 leaf, 1052 node

Table epoch: 0 (13 entries at this epoch)

172.16.11.0/24, version 6, epoch 0, attached, connected

0 packets, 0 bytes

via Vlan11, 0 dependencies

valid glean adjacency


Common CEF Problems

Is ideal switching method (CEF, DCEF) in use?

Are CEF tables complete and accurate?


Verify Layer 3 Switching

Switch#show interface {{type mod/port} | {port-channel

number}} | begin L3

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

.....

Switch#


Displaying Hardware Layer 3 Switching Statistics

Switch#show interfaces {{type mod/port} | {port-channel

number}} include switched

Switch#show interfaces gigabitethernet 9/5 | include switched

L2 Switched: ucast: 8199 pkt, 1362060 bytes - mcast: 6980 pkt, 371952 bytes

L3 in Switched: ucast: 3045 pkt, 742761 bytes - mcast: 0 pkt, 0 bytes mcast

L3 out Switched: ucast: 2975 pkt, 693411 bytes - mcast: 0 pkt, 0 bytes


Adjacency Information

Switch#show adjacency [{{type mod/port} |

{port-channel number}} | detail | internal | summary]

Switch#show adjacency gigabitethernet 9/5 detail

Protocol Interface Address

IP GigabitEthernet9/5 172.20.53.206(11)

504 packets, 6110 bytes

00605C865B82

000164F83FA50800

ARP 03:49:31


Debugging CEF Operations

Switch#debug ip cef {drops | access-list | receive |

events | prefix-ipc | table}

• Displays debug information for CEF

Switch#debug ip cef {ipc | interface-ipc}

• Displays debug information related to IPC in CEF

Switch#ping ip

• Performs an extended ping


How Do You Get CEF?

Buy a Cisco router or switch! It’s already there!


Summary

Layer 3 switching is high-performance packet switching in hardware.

MLS functionality can be implemented through CEF.

CEF uses tables in hardware to forward packets.

Specific commands are used to enable and verify

CEF operations.

Commands to enable CEF are platform dependent.

CEF problems can be matched to specific solutions.

Specific commands are used to troubleshoot and solve CEF problems.

Ordered steps assist in troubleshooting CEF-based problems.


Additional Resources

Basically, Cisco.com!

Get a guest account on Cisco.com (free)

www.cisco.com/en/US/partner/tech/tk827/tk831/tk102/tsd_technology_support_sub-protocol_homeMain page for CEF on Cisco.com

New Cisco Academy CCNP Switch course

http://www.cisco.com/en/US/partner/tech/tk827/tk831/tk102/tsd_technology_support_sub-protocol_home.html






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Cisco Express Forwarding Wayne Lewis May11