Per-Packet Load Balancing over VPN

I am wondering if it would be possible to have one router at each site and connect them to two modems each and establish IPSEC tunnel interfaces through each of the modems to the other site and do equal cost per-packet load balancing i.e. the "no ip route cache" command through the tunnel interfaces to get higher bandwidth between the two sites.

LAN---[Cisco Router]---Modem1A=======Tunnel0=========Modem1B---[Cisco Router]---LAN \\---Modem2A=======Tunnel1=========Modem2B---/


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If you decide for per-packet load-balancing, then it's still better to do it with CEF. Instead of 'no ip route-cache' use 'ip load-sharing per-packet' and be sure to enable CEF.

It should be enough to ensure per packet load-sharing is done for tunnel itself - everything what's inside tunnel will inherit load-sharing. So all you need is to specify two equal cost routes for tunnel destination at each location.

Cheers, iLya

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Charlie Root

I'm not sure that's a good idea. IPSec traffic is fairly order sensitive because of the anti-replay features, so unless the paths are the same length to within a very small tolerance, you would end up with the routers seeing the packets out of order, which would not be good for performance.

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Walter Roberson



My understanding is that the "no ip route-cache" has no influence on whether load balancing occurs or not.

My model is:-

ip route-cache

Enable fast switching which occurs as long as no other enabled features or circumstances prevant the router doing fast switching.

ip route-cache cef simililarly enables cef fast switching as long as no other enabled features or circumstances prevant the router doing cef switching

Depending I suspect very much on feature set and version for example I seem to recall that certain policy routing disabled fast switching.

For me this has always been one of the scary bits with Cisco. My idea was that it was almost impossible to be sure in a complex configuration whether fast switching would work or not without testing. Now I think that it is quite hard to end up process switching but I am still wary of this issue.

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It does - with route-cache disabled, the router will perform process switching using route lookup for every single packet. Internally Cisco routers keep track of last_route_used/next_route_to_use, so route lookup will every time return next route to use. If there is only one route, then there's nothing to choose from, but with multiple routes effectively it results in per-packet load sharing. This is however expensive route, so performance is penalized. Since cef supports both per-destination and per-packet load sharing, there is no reason to disable route cache.


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