Can't speak for the BGP aggregation but your filter command is incorrect and likely the problem. Try one of the following depending on what you wish to match.
Match /24 and /25 prefix length:
/routing filter add action=discard chain=out-filter invert-match=yes prefix=18.104.22.168 prefix-length=24-25
Match /24 or longer prefixes:
/routing filter add action=discard chain=out-filter invert-match=yes prefix=22.214.171.124 prefix-length=24-32
Unfortunatelly I don't think the filter is the problem. I also tried the following example, with no luck:
http://wiki.mikrotik.com/wiki/Manual:Si ... ultihoming
So than I decided, well why no just remove the filter and see whether the BGP aggregation is working. To me - it doesn'r work.
So I advertised the network 126.96.36.199/25 and then configure aggregate with the summary-only option and here's what I see in /routing bgp advertisements
[admin@KA-CE002] /routing bgp advertisements> print
PEER PREFIX NEXTHOP AS-PATH ORIGIN LOCAL-PREF
Main 188.8.131.52/25 184.108.40.206 igp
Backup 220.127.116.11/25 18.104.22.168 igp
Even though I have configured this:
/routing bgp aggregate
add instance=default prefix=22.214.171.124/24
So I believe that the bgp aggregate isn't taking any effect, and the filter was working properly. It's just that the BGP process was not advertising the /24 network...
Should I open a ticket at MikroTik?