OSPF load balance and CEF

[bapatsubodh]

Hi,

If we have two OSPF equal cost learned routes to same subnet and traffic destined for that subnet need to be load balanced between two links do we need to enable  CEF for per-packet instead of per destination that is default?

Is changing the CEF mode is of a big impact.( May be if it is clearing all cache entries.)

Please share the experience.

Thanks

Subodh

[Peter Paluch · Accepted solution]

Hello Subodh,

It is actually highly recommended to have the CEF activated because it provides a great reduction in the CPU load on the software-based routers. The CEF itself defaults to per-destination load balacing but can be instructed to perform per-packet load balacing. Please note, however, that using the per-packet load balacing may ultimately result in packets being reordered during their delivery to the destination which is an unpleasant side effect.

The per-packet load balacing must be enabled on the egress interfaces using the command

ip load-sharing per-packet

Changing the load-sharing mechanism should not have any significant impact on your router load. Neither the FIB nor the adjacency database should be affected by it. The load-sharing is performed via a hash table placed between the FIB and ADJ database so the FIB/ADJ should not be modified at all.

Best regards,

Peter

[Richard Burts · Accepted solution]

Subodh

If I understand your post correctly your requirement is that you have several equal cost paths to a subnet and want to achieve load balancing over those paths. Peter has given a very nice explanation of CEF. But I would like to approach answering your question from a slightly different perspective.

If you have a single source sending to a single destination, then the default load balancing of CEF will not produce much load balancing. But if you have multiple sources sending to multiple destinations, then the default load balancing of CEF is likely to produce pretty good load balancing without doing anything fancy.

Peter mentions the potential disadvantage of using per packet load balancing. When you configure CEF to load balance per packet you intoduce the probability that some packets will be delivered out of order. A simple example may help to illustrate this. Lets us think about a simple scenario where we have 3 packets from some source to some destination and assume that CEF is configured for per packet load balancing. Let us assume that packet 1 is of lenfth 100, packet 2 is of length 1500, and packet 3 is of length 100. The router starts sending packet 1 on a link. Then it starts sending packet 2 on the other link. Packet 1 complets transmission and the router begins transmitting packet 3 on the first link while packet 2 is still being send on the other link. so the destination receives packet 1, then packet 3, and then packet 2. Some applications are very sensitive to out of order packets. (Voice traffic is one example that comes to mind and there are quite a few others). So you should consider the applications that run in your network and whether any of them are sensitive to out of order packets before you decide to configure per packet load balancing.

HTH

Rick

[Peter Paluch]

Hello Rick,

Thank you for updating my reply - it is a most welcome and informative addition!

Regarding the sensitivity of applications to reordered packets - you are absolutely correct with the VoIP, and I have even seen some TCP implementations to react very intensively to reordered segments by aggresively slowing down the transmission rate, resulting in poor data transmission performance.

Best regards,

Peter

[bapatsubodh]

Gentlemen,

Thanks a lot.

Updated my CEF switching knowledge.

Subodh

[Dipak Singh]

Hello Rick,

Since we have this problems in "Per packet load balancing" so do we have any other method to balance the traffic in this case. by the way thanks for this information.

Regards:
DPK

[Joseph W. Doherty]

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[Richard Burts]

DPK

The original post asked a question that was about the situation where you have learned two equal cost paths via a dynamic routing protocol. And for that the main options are the per destination and per packet load sharing. It seems your question wants to explore what other options there might be. So here are a couple for you to consider.

One option is to make one path more attractive than the other so that the routing protocol would prefer one path and would dynamically switch to the other path if the first path fails. That generates a lot of traffic on the primary path. You then take something like Policy Based Routing and you direct certain types of traffic (perhaps it is Email, perhaps it is file transfers, perhaps it is data base backups, perhaps it is something else) over the other link. That gives you load sharing that is not dependent on the routing protocol.

Another option would be to configure Performance Based Routing. I have not implemented it and so can not speak to it directly. But it is my understanding that PfR has the ability to take load on links into consideration and can shift traffic patterns to achieve load sharing over several links.

HTH

Rick

[Dipak Singh]

Thanks Rick. 

[bapatsubodh]

Deepak,

As Rick explained you can tune the routing by different methods. Also CEF load-balancing per packet can be useful Rick's earlier post explains the same.

So in short, theoretically if you have many sources to many destinations per packet load balancing is the way to go in my opinion. Also assuming the packet size is fairly random and not line -  one flow is almost packet size is almost 1500 and other is very light like 100 ...

HTH

Cheers!!

Subodh