Hello Pui Lai
So then from my understanding, you want to put R1 and R3 into their own sub-AS, correct? Well, I wasn’t sure about this so I went in and labbed it up. The fundamental question here is, can two BGP routers be put into the same sub-AS if they are not directly connected, that is, if they have to pass through another sub-AS to reach each other.
Typically, BGP routers can become neighbors even if they’re not directly connected. In the case of iBGP, this is true without any additional configurations. For eBGP, you need to add features such as multhop.
Now, what about iBGP routers within the same sub-AS? Do they need to be directly connected or can they become neighbors if they can communicate through another sub-AS? Well, it seems that they cannot. iBGP routers that belong to the same sub-AS must connect to each other either directly, or via another iBGP router in the same sub-AS. I’ve labbed it up and confirmed it.
So it seems that for your topology, the only solution is to either connect R1 and R3, or don’t create another sub-AS.
I hope this has been helpful!
Laz