load balancing 2BGP DR and another 2BGP with 30k prefixes

RouterOS Forwarding Protocols
1

Please help me load balance incoming connections for the following BGP scenario:

2 ISP providers - 1 MT core router - LAN
on MT there are 3 eth interfaces:
wan1

  • vlan8 - BGP session 1 - default route received - 2 prefixes and 1 ASN announced - 50mbps
  • vlan16 - BGP session 2 - 18.000 national peering received - 2 prefixes and 1 ASN announced - 150mbps
    wan2
  • vlan24 - BGP session 3 - default route received - 2 prefixes and 1 ASN announced - 10mbps
  • vlan32 - BGP session 4 - 12.000 national peering prefixes received - 2 prefixes and 1 ASN announced - 100mbps
    lan
    2 prefixes /23 and /21 and one ASN

How can i set BGP session 1 to be favorite over BGP session 3 for inbound connections? Considering the two of them active all time.
Outbound is not a problem as it is controlled by MT, but inbound global connections would be a problem as not all providers and carriers obey prepend attributes (some of them are even stripping off any prepended ASN).

2

announce more specific prefixes with session 1

for example if you want to announce 192.168.0.0/21
with session 1 announce 192.168.0.0/22 and 192.168.4.0/22
with session 3 announce 192.168.0.0/21

3

That is not an option because both ISP providers require maximum aggregation per announced prefix. ISP2 even require that announced prefixes to be /23 minimum, to establish a BGP peering.
If i manage to negotiate with ISP1 to accept at least /24 then yes, this would be a good option as a more specific subnet mask is preferred versus a more generic aggregated one.

4

use bgp as path prepend to prefer a path on inbound