What command is used to enable CEF on a Cisco switch?
A. ip cef
B. ip cef distributed
C. ip route-cache cef
D. ip cef enable
Correct Answer: A
Explanation/Reference:
Answer:
The command to enable Cisco Express Forwarding (CEF) on a Cisco switch is ip cef. This enables CEF support on the entire switch. All interfaces that are configured to use CEF will be able to. The no form of this command will disable CEF support, including support on interfaces that have CEF configured on them.
Cisco Express Forwarding allows a Layer 3 switch to determine the next-hop destination MAC address of the first frame in a transmission made of many frames, and then utilizes the much faster switching process for all the remaining frames. This requires that routing be enabled on the switch, since the route to the initial frame must be determined.
The output of the show ip interface vlan id command can be used to determine whether IP routing is enabled. Partial output of the show ip interface vlan id command for two switches is shown below. The first (Switch A) has IP routing enabled and the second (Switch B) does NOT have IP routing enabled. The second switch is missing the section about CEF, since CEF cannot be enabled unless IP routing is enabled.
The command ip cef distributed is used to enable distributed CEF (dCEF), not the CEF mentioned in the scenario.
The command ip route-cache cef is a valid command to enable CEF on an individual interface, but the command is only valid in interface configuration mode.
The command ip cef enable is an invalid command due to incorrect syntax.
Objective:
Layer 2 Technologies Sub-Objective:
Configure and verify switch administration
References:
Cisco > Cisco IOS IP Switching Command Reference > ip cef
Explanation/Reference:
Answer:
The command to enable Cisco Express Forwarding (CEF) on a Cisco switch is ip cef. This enables CEF support on the entire switch. All interfaces that are configured to use CEF will be able to. The no form of this command will disable CEF support, including support on interfaces that have CEF configured on them.
Cisco Express Forwarding allows a Layer 3 switch to determine the next-hop destination MAC address of the first frame in a transmission made of many frames, and then utilizes the much faster switching process for all the remaining frames. This requires that routing be enabled on the switch, since the route to the initial frame must be determined.
The output of the show ip interface vlan id command can be used to determine whether IP routing is enabled. Partial output of the show ip interface vlan id command for two switches is shown below. The first (Switch A) has IP routing enabled and the second (Switch B) does NOT have IP routing enabled. The second switch is missing the section about CEF, since CEF cannot be enabled unless IP routing is enabled.
The command ip cef distributed is used to enable distributed CEF (dCEF), not the CEF mentioned in the scenario.
The command ip route-cache cef is a valid command to enable CEF on an individual interface, but the command is only valid in interface configuration mode.
The command ip cef enable is an invalid command due to incorrect syntax.
Objective:
Layer 2 Technologies Sub-Objective:
Configure and verify switch administration
References:
Cisco > Cisco IOS IP Switching Command Reference > ip cef
The correct answer is B, this must be a really old question.