CEF and Load-Balancing

[pblume]

If I have a remote site with only a single WAN router and 2 WAN circuits of identical EIGRP metrics back to my data center, will enabling the WAN router with CEF destination-based load balancing evenly distribute traffic over the 2 WAN circuits?

Most of my remote sites have only 3 or 4 hosts which have to access the same server at my data center. Using destination-based CEF, will the first host use WAN circuit #1, the second host use WAN circuit #2, the third user use WAN circuit #1, etc.?

[thisisshanky]

Yes it would do a destination based CEF. The default load balancing mode of CEF is per-destination based LB. If you are having a 7500 router, with VIP then, You can enable Distributed CEF. (" ip cef " is the command to enable CEF globally, if you have the 7500, "ip cef distributed" can be used.)

You can use "show ip cef " command to verify what mode of LB is being used.

", version XX, per-destination sharing" will be displayed in the output indicating the Per destination mode of load sharing.

Hope that helps!

[a.manosca]

This link should help:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1828/products_configuration_guide_chapter09186a00800ca62c.html#3589

Goodluck.

[pblume]

Thanks for the URL!! One question, what does the following paragraph mean when using CEF destination-based load balancing:

"Because per-destination load balancing depends on the statistical distribution of traffic, load sharing becomes more effective as the number of source-destination pairs increase."

This paragraph makes it sound like if I only have two hosts at a remote site with equal cost links back to the same server at my data center, both hosts MIGHT take the same WAN circuit back to the data center. Am I not reading this correctly?

Thanks for all your help.

[ruwhite]

That is correct--the source and destination addresses are passed through a hash, so the more addresses on both ends, the better the sharing will be. You can try per packet load sharing, but that may not improve things, since it may cause out of order packets, which will reduce your host/server performance in a very bad way.

Russ

[a.manosca]

Hi Russ,

Is it also possible that the 3 or 4 hosts (or probably more)

accessing the same server use the same link?

(So the other link might not be used at all?)

So that means the choice is either performance or

WAN link usage. Thanks.