High availability and FHRP
Enterprises can't rely on one gateway. First-Hop Redundancy Protocols (HSRP, VRRP, GLBP) present a virtual gateway IP shared by two routers; if the active one fails, the standby takes over so hosts never change their gateway. Combined with redundant links and StackWise/SSO, this removes single points of failure.
Advanced Layer 3: VRF and tunnels
VRF (VRF-Lite) gives one router multiple independent routing tables - like VLANs for routing - so Corp and Guest traffic stay separated end to end. GRE tunnels carry routing between sites over the internet; add IPsec for encryption. These are the building blocks of site-to-site connectivity.
Redistribution, route maps and BGP policy
When two routing protocols meet (say OSPF and EIGRP), redistribution shares routes between them - but done carelessly it creates loops and suboptimal paths, so you tag and filter with route maps. BGP path selection is policy-driven (local preference, AS-path, MED), which is how enterprises and clouds steer traffic between providers.
Overlays and assurance
Modern enterprise WAN/campus uses overlays: MPLS and DMVPN for scalable site-to-site, SD-WAN for policy-driven transport choice, and VXLAN/SD-Access fabrics in the data centre and campus. Assurance/telemetry (model-driven, controller-based) turns raw device data into health and intent verification.