Help with identifying issues from multiple paths

[Steph1963]

Hi to All,

I am having issues with the followig Cisco question:

Which four issues result from multiple paths:

A) Suboptimal Routing

B) Unavailable Redundant Path

C) traffic that is optimized for only one application

D) uneven load shating

E) incomplete route summarization

I just would like to understand the reasonnig behind the unavailable redundant path being an issue. I am not to sure why a redundant path would not be available and more important how could we reserved a path for certain traffic condition like congestion of a path.

The uneven load sharing is also another issue that I understand. If each paths has the same metric, I do not see how this can result in uneven load sharing.

Finally, the question is talking about incomplete route summarization. Is there any way to do route summarization on multiple path and expect load sharing on these multiple paths.

Thanks for your help

Stéphane

[Mohamed Sobair]

Stephane,

With multiple paths,

1- Suboptimal routing could Take place since you could use the longest path to the destination instead of the shortest path.

2- Unavailable redundant path would take place if you have an application that doesnt require or is recommended to use Assymetrical path , there are so many application that its not recommended to use redundant path, hence for those application you might have policy based routing that always keep sending a cross a single path.

3- Uneven Loadsharing could take place although you have a routing protocol with the same cost for loadbalancing, it still depends on CEF, since CEF loadbalncing uses two methods:

a- Per destination loadbalancing.

b- Per packet loadbalancing.

with the first loadbalncing technique, traffic destined from specific source to particular destination would always use a single link, the loadbalancing take place when you have multiple destinations to route the traffic across the multiple paths Here , we could have unequal cost loadsharing.

However, with the second loadbalancing technique, We would achieve equal cost loadbalncing, but its not the recommended approach since Packet might arrive at the destination out of Order and creats issues.

HTH

Mohamed