IP CEF

[Hamidsattarrana]

Hi,

I want to know on which Router interfaces I have to enable IP CEF?

Interface (IP Nat Inside>> LAN Interface) OR Interface (IP Nat Outside >> WAN Interface)???

If VOIP traffic is going through those interfaces via Policy Routing? 

Whats the best practice? 

Thanks.

[KEVIN ANDRES RODRIGUEZ OVIEDO · Accepted solution]

Cisco Express Forwarding (CEF) is advanced, Layer 3 IP switching technology. CEF  optimizes network performance and scalability for networks with large and  dynamic traffic patterns, such as the Internet, on networks characterized by  intensive Web-based applications, or interactive sessions

Enable CEF when your router has interface processors that do not support dCEF. To enable CEF, use the ip cef command in global configuration mode.

Enable dCEF when you want your line cards to perform express forwarding so that the route processor (RP) can handle routing protocols or switch packets from legacy interface processors. To enable dCEF operation, use the ip cef distributed command in global configuration mode as needed.

Example Enabling or Disabling CEF or dCEF 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
 no ip route-cache cef
 end

The following example shows how to reenable central Cisco Express Forwarding operation on an interface:

configure terminal
!
interface ethernet 1/1
 ip route-cache cef
 end

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

configure terminal
!
interface e0
 no ip route-cache distributed
 end

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

configure terminal
!
ip cef distributed 
!
interface e0
# ip route-cache distributed
 end

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

configure terminal
!
ip cef 
!
interface e0
 no 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 Ethernet interface 0:

configure terminal
!
ip cef distributed 
interface e0
 no ip route-cache cef
 end

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

configure terminal
!
ip cef distributed 
!
interface e0
 ip route-cache distributed
 end

 

[Hamidsattarrana]

Thanks for your help.

[Joseph W. Doherty]

Generally, CEF is "on" by default. Further, newer IOS features often require it to be on to do whatever they do.