Layer 3 Switching/ CEF confusion

Dilan77

Hi All

Question to those taking or have done the BCMSN, or who have access to a 3550 EMI switch.

I'm looking at Multilayer switching, particularly the CEF variety and am confused. The Sybex book I am using says that to CEF is the L3 switching method on 3550 switches, and to enable it you use;

conf t
ip routing
ip cef

You then have to convert the L2 interface to L3 by using the command 'No switchport'. CEF is then enabled on this switch.

But if you do not enter 'ip cef' does this mean that the 3550 will not route packets? I understand that 'ip routing' has to be enabled, but wasn't aware that the cef command had to be carried out too. Additionally, what interface are you converting to L3? The one that is connected to a L2 switch in a 'router on a stick' capacity?

Confused!

keenon

Dilan77 wrote:

Hi All

Question to those taking or have done the BCMSN, or who have access to a 3550 EMI switch.

I'm looking at Multilayer switching, particularly the CEF variety and am confused. The Sybex book I am using says that to CEF is the L3 switching method on 3550 switches, and to enable it you use;

conf t
ip routing
ip cef

You then have to convert the L2 interface to L3 by using the command 'No switchport'. CEF is then enabled on this switch.

But if you do not enter 'ip cef' does this mean that the 3550 will not route packets? I understand that 'ip routing' has to be enabled, but wasn't aware that the cef command had to be carried out too. Additionally, what interface are you converting to L3? The one that is connected to a L2 switch in a 'router on a stick' capacity?

Confused!

i'll try to best answer

when you want to make a L3 switch route packets "no switchport" command needs to be entered. additionally an ip address must be assigned to that interface ( just like a router) the routing for the vlans connected to this switch would be routed from the "Interface Vlan # , with an ip address

ip routing has to be turned on to route packets CEF just makes it more effective/faster in short.. you should do a lookup on cisco site for cef