Cef

CEF

I have a question about cef,

supposing I have a 3750, that does not have any active vlan interfaces. But does have separate VLANS set up. It is trunked to a second switch/router that is doing the routing between the vlans.

So all routing is done off the switch, does the fact that the 3750 is running cef make any difference to the network.

what I am wondering is is there any way for the switch to learn/ be told by the router. that station x on vlan y is on port z and station a on vlan b is on port C

and that if data need to pass between station x and station a, this can be achived with out (all) the packets in the conversation having to travel across the trunk to the router.

I jsut head a comment that not all multilayer switch are layer 3 switchs, but all layer 3 switch are multilayer switch. And I was wondering what do you use a non layer 3 multilayer switch for.

CEF on a single switch/router (or routers with routing between them) makes perfect seance. just I don't get how cef affects non routing devices.

Cheers

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Comments

ColbyG

CEF is L3.

DevilWAH

ColbyG wrote: »

CEF is L3.

see that what i thought, but a 2950 switch can run cef and that is not a layer 3 switch ?

And I remember reading that you can without enabling routing on a layer 3 switch use it to incress routing throughput by allowing direct forwarding of traffic between vlans with out the need for all packets in a conversation to be sent to the router.

Forsaken_GA

DevilWAH wrote: »

see that what i thought, but a 2950 switch can run cef and that is not a layer 3 switch ?

Where did you get that from? That's not what I see

cisco WS-C2950T-24 (RC32300) processor (revision P0) with 19912K bytes of memory.
Last reset from system-reset
Running Enhanced Image

switch001(config)#ip ?
Global IP configuration subcommands:
access-list Named access-list
accounting-list Select hosts for which IP accounting information is kept
accounting-threshold Sets the maximum number of accounting entries
accounting-transits Sets the maximum number of transit entries
alias Alias an IP address to a TCP port
default-gateway Specify default gateway (if not routing IP)

Forsaken_GA

DevilWAH wrote: »

And I remember reading that you can without enabling routing on a layer 3 switch use it to incress routing throughput by allowing direct forwarding of traffic between vlans with out the need for all packets in a conversation to be sent to the router.

This is also not correct, cef requires IP routing to be running.

From a 3550:

SW4#sh ip cef
%IPv4 CEF not running
SW4#config t
Enter configuration commands, one per line. End with CNTL/Z.
SW4(config)#ip cef
%Must enable ip routing first

DevilWAH

yer it was from a network engineer in a network consultancy company!!

he was going on about running a set up like router on a stick. where the routing was done on the layer 3 router, but the switch could learn the routes and send packets direct across vlans. with out them needing to travel the trunk to the router.

It did seem a bit to good to be true...

as for the 2950 I have never used one in my life the idea they can run cef was what I was told and have read on internet (since first post been looking around and notice none of it was cisco documentation).... I started with 3550's and have moved up, so been a bit spoilt as far as layer 3 switch go.