What is GRE tunneling and when is it used?
Why Interviewers Ask This
Interviewers ask this to evaluate whether you have the depth of knowledge needed to mentor others and lead technical decisions. The expected answer goes beyond definitions into practical implications and real-world consequences.
Answer
GRE (Generic Routing Encapsulation) is a simple tunneling protocol that encapsulates any Layer 3 protocol (IPv4, IPv6, IPX, MPLS) inside IPv4 packets. A GRE tunnel appears as a virtual point-to-point link between two routers. GRE header adds 24 bytes overhead. GRE itself provides no encryption or authentication — it is commonly combined with IPsec for secure GRE tunnels (GRE over IPsec). Use cases: transporting non-IP protocols over IP networks, carrying routing protocols over point-to-point links (OSPF, EIGRP over GRE tunnels in hub-and-spoke WAN), IPv6 over IPv4 (during IPv6 transition), MPLS over IP. Example Cisco config: interface Tunnel0; ip address 10.0.0.1 255.255.255.252; tunnel source 203.0.113.1; tunnel destination 198.51.100.1. mGRE (Multipoint GRE) is used in DMVPN (Dynamic Multipoint VPN) to create scalable hub-and-spoke VPN networks where spokes can communicate directly (spoke-to-spoke) after initial hub negotiation.
Common Mistake
Don't just define the term — demonstrate that you understand when to use it and when not to. Showing awareness of trade-offs is what separates average from strong Networking candidates.