CEF vs MLS

[henrybb]

Hi.

I try to understand the L3 switching issues on the Catalyst range

switches.

As I already know MLS was the mechanism of the L3 switching on the older sort of Catalysts (5000/6500MSFC1). But the MLS has evolved to the CEF switching.

Do CEF mean L3 switching ? And what's difference between CEF on router and on L3 switch ?

What about to MLS and CEF working the same time on e.g. 6500? Are they mutually exclusive or they work together in some manner? In what manner? Why can I issue "sh mls cef ... " on 6509/sup720 ?

When I issue "sh platform forward ... " on 3750, does it mean HARDWARE CEF((I call it)? How about "sh ip cef " .

It is very confusing. I don't know whether CEF equal to L3 switching or there is HARDWARE CEF and SOFTWARE CEF ?

Could someone explain the issue?

thanks!

[amit-singh]

Hi henry,

As you now MLS was a traditional type of route caching mechanism that was used on older hardware/software. It was also called as route once switch many forwarding.

CEF cisco epress forwarding is mutually exclusinve of MLS as it has a different way to work. With the advent of CEf, the routing table ( l3 FIB table forwarding information base) and adjaceny table(l2) gets download on the hardware and does the L3 switching. Its is done in hardware so no performance issue.

CEF on router and L3 switch is the same.

Please go through the links :

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_white_paper09186a00800a62d9.shtml#express

http://www.cisco.com/en/US/tech/tk827/tk831/technologies_tech_note09186a0080094806.shtml

HTH, Please rate if it does.

-amit singh

[henrybb]

thanks for your reply.

You said "CEF is done in hardware ". I know PFC on 6500 is the hardware, how about on router,for example 2600 ? Can I check status of l3 switching on 2600 like "sh mls cef ... " on 6500 ?

thanks!

[spremkumar]

Hi

you can check the status of CEF using show ip cef command and if you are interested in finding for particular interface stats you can chcek up the same using show cef interface x x/x

also you can verify this link for various CLIs which helps to understand the stats as well as the counters..

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_tech_note09186a00801e1e46.shtml

regds

[henrybb]

thanks for your reply.

I know " sh ip cef " , but I don't understand whether CEF is hardware switching(L3 switching) or CEF is comprised of HARDWARE cef and SOFTWARE cef.

For example, I can issue "sh ip cef 10.1.1.5" on 6509 RP and "sh mls cef 10.1.1.5" on 6509 SP. What's difference between RP's cef and SP's cef ?

thanks!

[henrybb]

Can anybody explain it ?

Does it mean that router platform support hardware switching(L3 switching) because it support CEF ?

I mean concept of CEF equal to HARDWARE switching.

thanks!

[lutheran1971]

Yes CEF is wire-speed switching. The L3 switch builds the routing portion of the TCAM (FIB and adjacency) table in software, but it is then copied to hardware and then used for switching, as L2 is.

[Bobby Thekkekandam]

Hi Henry,

No, CEF is simply a route caching mechanism. So although CEF does not directly imply hardware switching, this IS the case for switch platforms under normal operation. For routers, CEF based forwarding is done under interrupt. In certain circumstances, CEF forwarding on a switch can also occur under interrupt (Such as if the TCAM FIB is full, for example)

Here's a troubleshooting document on 6500 CEF that may give you some more clarity on it.

http://www.cisco.com/warp/customer/473/128.html

HTH,

Bobby