What control-plane protocols are commonly used in the underlay of an SD-WAN?

• A. OSPF, IS-IS, and BGP.
• B. EIGRP, OSPF, and RIP.
• C. BGP only.
• D. OSPF and RIP only.

Answer: (A)

In an SD-WAN underlay, routing must provide reachability between the edge devices across diverse transport paths. This is typically done by combining an interior gateway protocol (IGP) for efficient, scalable intra-domain routing with a border gateway protocol (BGP) to handle inter-domain reachability and policy across multiple providers. OSPF and IS-IS are the standard IGPs used inside a network to quickly learn the topology and compute optimal paths, while BGP manages routes between different networks, such as across MPLS, the Internet, or multi-homed links. Using this mix supports both reliable internal routing and flexible, policy-driven external connectivity, which is why the combination of OSPF, IS-IS, and BGP is common for the underlay. More limited or vendor-specific options like EIGRP or RIP don’t match the typical scalability and interoperability needs, and BGP alone doesn’t provide the detailed intra-domain path computation that OSPF or IS-IS offer.