Clarification on CEF Per Packet Load Balancing

[Adam Frederick]

Hi Guys

Just wanted to clear something up that I couldn't find readily availabe on CCO or forums. A colleague recently told me that in order to acheive CEF per packet load balancing, both ends had to be configured for it. Let me give you my example.

I will have 2 T1s going to the same provider, however, possibly going into different routers. I want to setup CEF per packet load sharing to distribute my outbound traffic. Will this have to be setup on the provider equipment as well?

Clarification will be greatly appreciated!

[mheusinger]

Hi,

load sharing in the SP network is achieved by IP routing in the first place. And if you are talking about internet SP then iBGP will be the protocoll of choice. Unfortunately BGP has not been built for load sharing. So it might not be possible to get load sharing for the return traffic at all unless your two T1s are terminating on the same SP router.

In this case the SP router has to perform a local decision through CEF where to send IP packets once the routing shows two equal pathes.

The default setting for CEF is per-session load sharing (which is reasonable to avoid packet reordering issues).

So, yes the SP has to do something - and maybe he can maybe not. If he can ... maybe he doesn´t want.

Hope this helps

Martin

[Adam Frederick]

Would using per-session load sharing avoid the issue of the SP having to config anything?

BGP is not an option, the SP charges way too much

Thanks,

Adam

[mheusinger · Accepted solution]

Hi Adam,

the SP is using BGP internally. So it doesn´t depend on you peering through BGP with him.

You can choose how to send your traffic - per packet or per session - from your router. But the ISP has to "decide" how to forward the return traffic to your location. His IP routing protocoll will be BGP and this might prevent him from load sharing/balancing.

One option would be to split your address range and send the traffic destined to one half over one T1 and the other half over the other T1.

Hope this helps

Martin

[Richard Burts]

Adam

To answer your original question, you control the distribution of your outbound traffic but the provider controls the distribution of your inbound traffic. So yes you are dependent on what the provider configures. And per session (or per destination) is the default. And if the T1s really do terminate on different routers at the provider then I believe that per packet would be very difficult for the provider.

I believe that there is another reason to reccommend per session balancing rather than per packet. When you do per packet you introduce the probability that some packets will be delivered out of order. I believe that terminating the T1s on different routers at the provider side would magnify the probability of out of order packets. Depending on the application (and the transport protocol) out of order packets are somewhere between a nuisance and a severe problem. You want to avoid out of order packets if possible.

For these reasons I believe that you will be better off if you use per session balancing.

HTH

Rick