分类：
2012-09-17 13:47:52
cisco tac：
> Ways of Sup720 does load-balancing doesn't depend on how (via which procol) routes are inserted in routing table. So you will see same behavior for OSPF routes.
>
> Basically, hash arithmetic for Sup720's load-balances in the following way by default :
>
> The sup720 load balancing algorithm works in the following way:
>
> 1: 1
> 2: 7-8
> 3: 1-1-1
> 4: 1-1-1-2
> 5: 1-1-1-1-1
> 6: 1-2-2-2-2-2
> 7: 1-1-1-1-1-1-1
> 8: 1-1-1-2-2-2-2-2
> It means that when you have 2 equal cost paths, load-sharing will be 46%-54%, not 50%-50%.
> if on 3 equal cost paths, load-sharing will 33%-33%-33% (expected). When we have 4 paths, we will have 20%-20%-20%-40% and not 25%-25%-25%-25% as we could expect.
>
> Reason for this is to avoid CEF polarization problem. Basically, CEF polarization problem occurs in a hierarchical network where there are several routers doing load-sharing in a row. In that case, since they all use same algorithm to load-share, only the first router will use all paths, all the other routers on the path will use a single path...
>
> Sup720 uses by default the load-sharing weight described earlier but since they are quite a lot of customers who are not impacted by CEF polarization, there is a knob to disable this 'anti-polarization' weight and to use equally all paths. This knob had been introduced in 12.2(17d)SXB2 and is then available in IOS release you are running. The Command
> to disable 'anti-polarization' weight is the following :
>
> (config)# mls ip cef load-sharing full simple
>
> You could then use this command if you have 2 equal cost paths and you want to use them equally.
