Hi,
from my point of view you have two options here to solve your problem.
A- no need for aggregate and advertise every subnet individual
B- there is another solution but it may take non-optimal-path so if it doesn't make problem for you it will be fine
1- Configure IBGP on your network full mesh or RR where R1,R2 are RR
2- advertise aggregate route to your ISP normally.
lets talk about traffic flow from R1 --> R4 subnet now
3- in case of R1-->R2 & R1-->R4 links went down the BGP session between R1 & R4 still up as there is connectivity between them so R1 still receive update (more specific route) from R4
So traffic will go from R1 ---> R3 ---> R2 ---> R4 long path but will not cause a lot of problems for you just two more hops plus this case is might happened in outage and would be fixed quickly
Regards,
Mohamed Elhariry
JNCIE-M/T # 1059, CCNP & CCIP
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