ip load-sharing per-packet

[24madaeve1]

hi there. I have one doubt pertaining to per-packet load-sharing. In order to connect my two remote sites- 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?

[Joseph W. Doherty · Accepted solution]

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Posting

As a general rule, you should avoid per-packet load balancing.  The reason being, many protocols don't well tolerate packets of the same flow being delivered out of order.  Even later versions of TCP may incorrectly assume packets have been lost when more than a couple are received out of order.

Load balancing packet-by-packet might also be a cause why you're unable to obtain fall transmission rates.

Other common reasons include insufficient buffer space on TCP receiving host (lookup the subject bandwidth delay product), or drops for any reason along the path.  (Visibility into the latter can be "difficult" when you do not manage link paths, end-to-end.)

[24madaeve1]

 thanks for the reply joseph. actually when I use my WAN links without any load-sharing in active-passive mode, then the bandwidth of active link gets fully used up. Its only when I switch to active-active mode using per-packet load sharing that the links are not fully utilized. 

Furthermore, I have only one or two server in site A & B and data is flowing from site A to site B. I think the servers are using a single port for communication. So the default per-destination load sharing mechanism is of no use to me. Can you tell me any scheme which I can use to distribute the traffic across both the links with full utilisation. Moreover, my links can be configured statically only at both the ends.

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If almost all your traffic appears to be single flow, I can see why you want to use per-packet load balancing.  Even so, packet re-ordering might be cause some of you performance issues.  Depending on you hosts, some protocols (e.g. some TCP stacks) might allow "tuning" how out-of-order packets impact flow reassembly.  If yours does, this should mitigate this consideration.

If you had multiple flows, you could also look at Cisco's OER/PfR.

As to other solutions, you might need to reach beyond features of you existing equipment. I think there are some appliances that can hardware mux multiple links.  I.e. an appliance that can do something like MLPPP or ATM IMA at multi-megabit speeds.  Also one that accounts for the bandwidth capacities differences.