When using BGP to carry external routes, what is the ideal interaction between BGP and IGP running within a Transit AS?
A.
The external routes should be redistributed into an IGP.
B.
The IGP should carry the routes to reach the BGP next-hops and the core subnets within the AS.
C.
The IGP should carry the core subnets within the AS. Then a mutual redistribution between BGP and the IGP should be performed.
D.
The external routes should be redistributed into a Link-State IGP (like OSPF or IS-IS) with filtering to reduce the size of the Link-State database.
Explanation:
Core routers need to run BGP and IGP and:
1) BGP shall carry all external routes.
2) IGP shall only propagate BGP next-hops and other core subnets
3) All customer routes shall also be carried in BGP
4) Reduces IGP topology database
5) Removes customer-caused route flaps from IGP: IGP becomes more stable. Reference: Configuring BGP on Cisco routers p.4-34