Hello.

We have two ISPs with connection to our local IX (±1k routes).

We want to balance outgoing traffic 50/50 between them.

Our current config (XXX - we, YYY - ISP 1, ZZZ - ISP2)

[admin@mk1] > ip address print where interface=lobridge
Flags: X - disabled, I - invalid, D - dynamic
# ADDRESS NETWORK INTERFACE
0 x.x.200.95/32 x.x.200.95 lobridge

BGP Peers:

1 E name="peering-ISP-1" instance=default remote-address=yyy.yyy.225.186 remote-as=YYYY tcp-md5-key="" nexthop-choice=default multihop=yes route-reflect=no hold-time=3m ttl=default
in-filter=isp1-in out-filter=isp1-out address-families=ip update-source=lobridge default-originate=never remove-private-as=no as-override=no passive=no use-bfd=no

2 E name="peering-ISP-2" instance=default remote-address=z.z.167.205 remote-as=ZZZZ tcp-md5-key="" nexthop-choice=default multihop=yes route-reflect=no hold-time=3m ttl=default in-filter=isp2-in
out-filter=isp2-out address-families=ip update-source=lobridge default-originate=never remove-private-as=no as-override=no passive=no use-bfd=no

[admin@mk1] > ip route print detail where dst-address=aaa.bbb.240.0/24
Flags: X - disabled, A - active, D - dynamic, C - connect, S - static, r - rip, b - bgp, o - ospf, m - mme, B - blackhole, U - unreachable, P - prohibit
0 ADb dst-address=aaa.bbb.240.0/24 gateway=yyy.yyy.225.186 gateway-status=yyy.yyy.225.186 reachable via vlan651 distance=20 scope=30 target-scope=30 bgp-as-path="YYYY,35647,6697" bgp-local-pref=120
bgp-local-pref=120 bgp-origin=igp received-from=peering-ISP-1

1 Db dst-address=82.209.240.0/24 gateway=zzz.zzz.167.205 gateway-status=zzz.zzz.167.205 reachable via vlan121 distance=20 scope=30 target-scope=30 bgp-as-path="ZZZZ,35647,6697" bgp-local-pref=120
bgp-origin=igp received-from=peering-ISP-2


And the same for all 1k routes - there is only one Active route..

And all outgoing traffic is going via ISP1.

How to balance it between two ISPs?

This is a bit of a complex topic... Balancing traffic to two provider via BGP is more of an art than science...
because of how Traffic Engineering is done in the BGP world.

Having said that.. if you want to select some routes to go out via ISP1 and others go out via ISP2, you can use the Local Pref parameter and change that ..(use your inbound filters to modify the local pref for those routes that you want to use as preferred outbound routes.. the higher the local pref route is preferred).