Name two reasons not to use CEF per-packet load balancing.

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Jan 20th, 2009


Name two reasons not to use CEF per-packet load balancing.


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Giuseppe Larosa Tue, 01/20/2009 - 03:45

Hello Wang,

per packet load balancing increment jitter (delay variation) and inter-packet delay.

Some applications like VoIP can be negatively affected by this.

Another reason is that to implement per packet load-sharing a minimul state machine for each flow has to be held:

the device must remember when packet N+1 of a flow arrives out what interface has been sent out packet N or it needs to allocate a pointer to the list of links that is incremented at each packet.

CEF implements per-packet load-balancing efficiently but the side effects on jitter are still there

per destination load-balancing doesn't require a mobile pointer to the list of links: all packets of a flow are sent out the same interface.

This is more efficient

Hope to help


nyl20081224 Tue, 01/20/2009 - 04:41

hello ,qiuslar

i want to know if per destination load-balancing all packets of a flow sent out the same interface .why this method be called "load-balancing"?if bwteen a and b lsr has two link ,in cisco ios,default enable per destination load-balancing ,so i must forwarding all packets of a flow e1 of lsr a to lsr b ,another link between a and b is not waste?


Giuseppe Larosa Tue, 01/20/2009 - 09:19

Hello Wang,

why per destination load-balancing is effective in distributing traffic over multiple links ?

The answer is that when the number of distinct flows is high the per-destination load balancing is effective.

I did some lab tests almot ten years ago and increasing the number of flows the distribution becomes more fair (from 50 concurrent flows and up)

per destination load balancing fails only when there are just a few high or very high volume traffic flows like for example between two datacenters (DB replication flows)

in this case per-packet load balancing is more appropriate

Hope to help


24madaeve1 Mon, 12/22/2014 - 07:57

Hi larosa! 

I have one doubt pertaining to per-packet load-sharing. In order to connect my two data-centres- A & B, Site A is having two WAN links and Site B is having two WAN links - one from ISP1 (30Mbps link) and the other from ISP2 (50Mbps link). I am doing static route load balancing using same AD values for both the ISPs. I have configured "ip load-sharing per-packet" on both the outgoing interfaces.

The load is getting distributed equally across both the links but total bandwidth utilization across both the links is not going beyond 30Mbps. The combined bandwidth of both links is 80Mbps (50+30). However links are not getting fully utilized even though heavy load is there on the links. Can you please tell me how to make full use of both the wan links at both the ends? OR Can you tell me how I can distribute the traffic across both the links with full utilization. Moreover, my links can be configured statically only at both the ends.


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