Is per-packet load sharing only by CEF? Is this something that can blow your mind?

[sydflyer2011]

Hi guys,

Just a bit confusion here. I know from Cisco documentation, per-packet load balancing is only enabled via CEF. But while I am doing labs in GNS3, I think I have enabled per-packet load sharing and at the same time disabled CEF, which means fast switching is ON! Oh well, my screenshot could tell:

Fast swiching is ON right?  And Per-packet load-sharing is on, Right?  Tell me if I am wrong. Please.

This is the config:

Cheers,
Alex

[Richard Burts]

Alex

Without seeing some other parts of the config it is difficult to tell you precisely what is going on. From what you have given us I would offer these explanations:

- the interface config specifies ip load-sharing per-packet and that is why the show cef int output includes the indication that per packet load sharing is enabled.

- but you have disabled cef on the interface, and that is indicated in the output of show cef int.

- since the ip load-sharing per-packet is used in the cef context it may be enabled on the interface but it is not affecting how packets are forwarded.

- this leaves the interface using fast switching and process switching.

HTH

Rick

[sydflyer2011]

Rick, Thanks for your reply. In fact, the per-packet load-sharing is not only configured but already started working! I can tell from the access-list that configured on both interfaces participating in load-sharing.

If you need the full configure, let me know as I don't think the other part of configs will play a big role in this situation.

[sydflyer2011]

Any more thoughts?

[Peter Paluch · Accepted solution]

Hi Alex,

I agree with Rick. My understanding is that you have configured per-packet load balancing in CEF for that interface if that interface uses CEF. You may not have CEF activated on the interface but if it was, it would be configured for per-packet load balancing, hence the output of the show cef interface command.

Now, you indicated that the per-packet load balancing is actually happening. How are you generating the flow of packets that are being per-packet load balanced? Locally originated traffic (i.e. packet originated by the sending router) are, to my best knowledge, always process-switched and therefore subject to per-packet load balancing regardlesss of the switching method of intefaces. So if you were sending, say, pings from the same router on which you changed the CEF interface settings, you will still see the packets being load balanced - but not because of CEF but rather because of process switching.

Best regards,

Peter

[sydflyer2011]

Hi Peter,

Thanks for your input.

FIrst of all, the traffic is generally from a client which is attached to this router. It is this:

And then, from the screenshot below, you can see it is working:

And then look at the output of this command: show int fa0/0 stats:

The two consecutive screenshots indicate that there were packets processed even though there wasn't any traffic from PC. Well, you know they were packets generated from routing protocol (EIGRP and iBGP).  The key point is that by far, no packets were fast switched. OK. Let's look at what the next screenshot when I issued PING command from PC:

See there are obivious packets are fast switched. Pretty clear, isn't it?

Regards,

Alex

[Peter Paluch]

Alex,

One issue to consider here: on what interface exactly have you deactivated the CEF? CEF decisions are done at ingress interface while Fast Switching decisions are performed on the egress interface - so if a packet is not to be CEF-switched (including the load-balancing enabled on egress - yes, egress!) interface, the CEF must be disabled on the ingress interface.

Best regards,

Peter

[sydflyer2011]

Hi Peter,

I've disabled on both fa0/0 and fa0/1. The other one which is connected to PC, I didn't configure anything more than ip address (I assume it is process switched?), So in this case you reckon fa0/0 and fa0/1 are all egress interfaces and the other is an ingress interface? OK then, I found this table from cisco documentation which tells me that the switching method I got after combination would be Fast Switching in the end. So it is really weird to see per-packet load-sharing is still happening under fast-switching.

Incoming Interface	Outgoing Interface	Switching Method
CEF	Process	CEF
Process	CEF	Fast
Process	Fast Switching (IP route cache)	Fast Switching
CEF	Fast Switching	CEF

[cadet alain]

Hi,

if you left ingress interface from router to PC as default then CEF is enabled and if you disabled CEF for the 2 egress interfaces then according to this table the traffic will be CEF switched and as you configured per-packet load balancing for CEF you are observing the normal behaviour imho.

Regards.

Alain

[Peter Paluch]

Alex,

I've disabled on both fa0/0 and fa0/1

But those are egress (outgoing) intefaces, aren't they? I suspect that disabling CEF on egress interface won't make your packets become fast-switched.

The other one which is connected to PC, I didn't configure anything more than ip address (I assume it is process switched?)

Not true. In default settings, it is most probably CEF-switched and that may be the reason why your packets are still load balanced. By default, all recent IOSes have ip cef activated by default and thanks to that command, all interfaces work as CEF-switched until you explicitly deactivate CEF on them.

So it is really weird to see per-packet load-sharing is still happening under fast-switching.

But I believe that because you have not explicitly disabled CEF on your input interface, your packets are still CEF-switched.

Oh, by the way, can you include the link to the document where you found that table?

Best regards,

Peter

[cadet alain]

Hi Peter,

There is a  similar table in Routing TCP/IP vol1  from Jeff Doyle.

Regards.

Alain

[Peter Paluch]

Hi Alain,

Yes, that is very probable. However, I know I stumbled across this document on Cisco's website once and I am not sure if I bookmarked it so I wanted to secure myself

Best regards,

Peter

[cadet alain]

Hi Peter,

here it is:

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

Regards.

Alain

[Peter Paluch]

Hi Alain,

Awesome - thank you!

Best regards,

Peter

[sydflyer2011]

I know this topic has finished. But a bit curious to find out what will be the switching method for packets coming from fa0/0 or fa0/1 and going to the PC? We know the traffic from LEFT to RIGHT is load balanced by CEF, what about the traffic from RIGHTto LEFT? Cheers.

Regards,

Alex