Since we upgraded connections between our buildings we will use central WLC (5520) on our central location.
There is around 1500APs on all locations which will be adopted to that WLC.
My concern is that when I use local mode, I got my traffic doubled on ports? Why? Is this normal?
When I switched everything to FlexConnect, works like a charm.
Well, yes it is normal because with Flex is possible to sent only control and management control to WLC, the client traffic will be forwarded locally in each branch.
The decision of local vs flexconnect is normally an architectural decision. Good thread on the difference here: https://learningnetwork.cisco.com/thread/51502
Flexconnect all the AP management traffic is managed by the WLC and gets tunnelled as CAPWAP to the WLC where as the wireless data is dropped off locally at the AP switch port.
Local mode both the AP management traffic and wireless data is tunnelled to the WLC.
Normally the decision comes down to where you want to manage the use traffic from (along with things like WAN links and latency between WLC and APs) if more of the user resources are local to the site the APs are at then it makes scenes to use flexconnect, but if they are across the WAN it might make more scenes to use local mode.
The advantage of flexconnect is if the WAN goes down then the wireless keeps working where as in local mode it stops.
Around traffic to the WLC and its ports
As the 5520 uses SFPs which support 10G SFPs you could look at using 10G and LAG to get 20G back hall from the WLC to the network using local mode. If your getting more data through than this then you are probably looking at requiring to upgrade the WLC
For test scenario I will use only 4 of mine locations.
Building A, Building B, Building C, and Central Location.
Let's say if I have direct optical link from Building A and B to Central Location (<1ms latency)
Then better to use Local mode?
If my Building C location come from far away - 300+km, and it's not direct optical link (tunnels, MPLS and so...)
Better to use Flex for that location?