Forum Discussion
dennypayne
Feb 28, 2008Employee
It is possible to load-balance servers that are at different sites, however there are some caveats.
Most likely, if you are not spanning L2 across the sites, the servers will be on different networks. If the servers do not have the LTM as their default gateway, you will have to SNAT all incoming client connections so that when the servers respond, they will send the traffic back to the LTM before going back to the client. This causes all traffic in server logs to appear as though it comes from the LTM, which may or may not be a problem. You can somewhat alleviate this for HTTP traffic by inserting an X-Forwarded-For header which can be logged through various mechanisms. For non-HTTP traffic though there aren't such options.
Also you must consider that health checks to the servers will have to be performed over the link between the sites and this could add delay to detecting server failures and additional chattiness on those links.
LTM does not limit how many applications you can put on the box, the only limits are throughput, CPU, etc.
L4 and L7 lb (such as with iRules, etc) allow much more intelligence in directing traffic but it does not mitigate the fact that the L2/L3 routing has to work underneath it all.
Hope that helps,
Denny