BGP outbound traffic engineering in multi-homed network with 2 ISP's

Question

I have network with switches at access layer and 3 host networks, 2 core routers , 2 border routers , that have iBGP connection with each other and ospf with other 2 core routers, also each of them connected to 2 different ISP’s with eBGP. Border routers get only default routes from ISP’s. I want to split outbound traffic from access layer host networks between 2 ISP’s, for example 2 networks use 1st ISP connection and other 1 network uses 2nd ISP connection. If one connection fails , then all networks that use failed ISP will use another ISP. I want to know how to do it only with BGP , not PBR, ip sla …, only BGP.

BR3:

router ospf 1;
 router-id 10.1.255.2;
 redistribute bgp 200 subnets;
 passive-interface Ethernet0/2;
 default-information originate;
router bgp 200;
 bgp router-id 10.1.255.2;
 bgp log-neighbor-changes;
 network 10.0.0.0 mask 255.255.255.0;
 network 10.0.1.0 mask 255.255.255.0;
 network 10.0.2.0 mask 255.255.255.0;
 neighbor 10.1.255.1 remote-as 200;
 neighbor 10.1.255.1 update-source Loopback0;
 neighbor 10.1.255.1 next-hop-self;
 neighbor 192.168.0.6 remote-as 101;
 maximum-paths 4;

BR4:

router ospf 1;
 router-id 10.1.255.1;
 redistribute bgp 200 subnets;
 passive-interface Ethernet0/1;
 default-information originate;
router bgp 200;
 bgp router-id 10.1.255.1;
 bgp log-neighbor-changes;
 network 10.0.0.0 mask 255.255.255.0;
 network 10.0.1.0 mask 255.255.255.0;
 network 10.0.2.0 mask 255.255.255.0;
 neighbor 10.1.255.2 remote-as 200;
 neighbor 10.1.255.2 update-source Loopback0;
 neighbor 10.1.255.2 next-hop-self;
 neighbor 192.168.0.2 remote-as 100;
 maximum-paths 4;

Answer 1

BGP only deals with destinations and with filling routing tables, BGP doesn't forward packets and BGP doesn't care about source addresses. So requiring 'only BGP' doesn't make much sense.

You seem to want a forwarding engine to make decisions based on the source address of packets, and that's PBR.

Answer 2

You can use ACL to redirect traffic towards those ISPs by connecting those 02 ISP routers or on BR3 and BR4. The problem is you won't have the automatic switching and you will start having asymmetric incoming traffics depending the blocs of IPs you advertise to those ISPs assuming you have public IP of your own. https://community.cisco.com/t5/routing/how-bad-is-asymmetric-routing-in-dual-homed-bgp-please-advise/td-p/752097