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

[Peter Paluch]

Hi Alex,

As the fa0/0 and fa0/1 have CEF deactivated, the traffic will be fast-switched, assuming the egress interface at the left side is fast-switched. With fast switching, per-destination load balancing will take place.

Best regards,

Peter

[sydflyer2011]

Hi Peter,

I found new problem: the interface that connects to the PC is a switch port. That means that port is not configurable in terms of switching method. Also means that we need to scrape the previous discussions (that's what I doubt).

Regards,
Alex

[sydflyer2011]

Thanks all, it is all clear as my input interface is still on CEF mode, overally the two egress interfaces are still load-balanced by CEF.

Best Regards,
Alex

[sydflyer2011]

Hi guys, this discussion may not be finished yet because the port connecting to the PC is a switchport, so I reckon the switching method on fa0/0 and fa0/1 are just purely fast switching. Any more thoughts are welcome!

Regards,
Alex

[Peter Paluch]

Hi Alex,

Well, if the input port is a switchport then surely, there must be a SVI interface configured in your router - and that one is running with CEF active by default.

Best regards,

Peter

[sydflyer2011]

Thanks Peter. I indeed has a vlan 1 interface configured (which was by default enabled for CEF). And it looks like it became per-destination load sharing as all traffic from the PC to the other end only went out of fa0/0  after I disabled CEF on that vlan 1 interface --- which is correct according to that switching method rule table.

Just a bit confusing to see the output of show ip cef x.x.x.x still tells me that per-packet load-sharing is ON (actullay not).

Regards,
Alex

[Peter Paluch · Accepted solution]

Hi Alex,

It's good to know that your experiments start aligning with the documentation

Just a bit confusing to see the output of show ip cef x.x.x.x still tells me that per-packet load-sharing is ON (actullay not).

Well, consider the fact that in the show ip cef x.x.x.x command, you are asking about a particular destination but you are not saying through which interface did the packets to this destination came into your router - and for CEF, the ingress interface is important. So the output indeed tells you what would happen to the packets if they came via a CEF-enabled interface. After all, if you know that the ingress interface is not CEF-enabled then it doesn't make sense to ask CEF what it would do with those packets

Best regards,

Peter

[sydflyer2011]

Thanks again, Peter.

[paolo bevilacqua]

Have you ever noticed that when working with Cisco CLI, is not needed to take screenshot ? Just copy and paste text.