packet based load balancing over unequal paths

[ggvan]

My customer currently has a 1 Mb frame PVC fully utilized. We can't add another 1 Mb PVC, only 512 KB PVC. So we planned to use EIGRP unequal load balancing (variance + traffic-share balanced). Is it possible to combine this unequal cost load balancing with CEF per packet load balancing between these PVCs. The goal is that 2/3 of the traffic uses the 1 Mb PVC and 1/3 uses the 512 Kb PVC in a round robin fashion. Thank you very much for your help.

[Harold Ritter]

It is certainlt possible to do what you're saying. I would not recommend it though. I have seen bad things happening with per-packet load balancing. The out of sequence packets can really degrade TCP based application performance.

Hope tis helps,

[asafayan]

hritter,

I totally agree with you...I manage an EIGRP based 20 site WAN. A primary 310 wireless WAN P2P link between two sites failed. Ping tests were successful with reasonable latency but traceroutes indicated per packet load balancing across two backup links and it nailed a TCP based imaging process and it nailed our VG248 based Fax machines. VoIP traffic itself was not effected, just fax traffic.

[ruwhite]

http://www.cisco.com/en/US/partner/tech/tk827/tk831/technologies_tech_note09186a0080094806.s

html

on per packet load sharing.... I wouldn't see any problem with ding per flow load sharing over unequal cost links, though.

:-)

Russ.W

[ggvan]

Thank you.

The customer is using per packet load balancing already for other sites without problems. Due to the nature of their main application (which uses a single port for all traffic) they have to use per packet load balancing when there are multiple links.

Until now multiple links of the same speed were used. In this particuler case we will have to use PVCs with different speeds and I wondered whether EIGRP unequal load balancing and CEF per packet load balancing would work together loading the 1 Mb PVC with 2/3 of the traffic and the 512 Kb PVC with 1/3 of the traffic.

[vcjones]

If the underlying circuits have identical performance (i.e., both are full T1 or E1 pipes and only the CIR differs), then you have a chance. But if the speeds are the actual data rates on the lines, you do NOT want to use per packet load balancing unless your application is extremely tolerant of out-of-order packet delivery.

The reason many people warn you against per-packet is that it has a high probability of introducing packet ordering changes which many protocol stacks and applications cannot handle efficiently.

What you can do, if EIGRP unequal path load balancing does not work for you:

1 - use multilink PPP rather than per-packet load balancing. More CPU usage, but the routers take care of keeping everything in order (and detect/recover from link failures as well). This would be my preferred approach if the same routers are at each end of both links.

2 - Use static routes tied to addresses learned via EIGRP depending upon specific links being up with two routes defined through the fast link and one through the slow link. This won't fix ordering problems, but does allow you to make your two unequal links look like three equal links. It's a common hack with BGP.

Good luck and have fun!

Vincent C Jones

www.networkingunlimited.com