6.2Components and topology (switch, router, firewall, LB, and the three planes)

A topology diagram is a "map" for automation. Unless you can read which devices (switch/router/firewall/LB) line the path to the API endpoint your script calls, and which ports it traverses, you cannot decide where to look when it fails. Moreover, viewing a Cisco device as three planes—management plane, control plane, and data plane—lets you reason that "automation (NETCONF/RESTCONF/SNMP) touches the management plane, and even if that is blocked, real traffic (the data plane) keeps flowing." This section organizes device roles, reading topology and ports, and the three planes as a diagnostic framework.

6.2.1Device roles

• A switch (L2; an L3 switch is L2+L3) handles same-segment frame forwarding and VLANs, while a router (L3) handles packet forwarding and routing between subnets/WAN. Automation management connections (SSH/NETCONF/RESTCONF) are usually made to these devices' management interfaces.
• A firewall permits/denies traffic by policy at the boundary. From automation's view, whether the firewall blocks API/management ports (443/830/22, etc.) is the most important gate. A load balancer (LB) distributes requests across servers for availability/scale, but since the actual responder behind one VIP can change each time, stateful APIs and health-check design need care.
• In a topology diagram, read each interface (e.g. GigabitEthernet0/1), the devices at both ends, the IP subnets or VLANs it carries, and the port values (L4 ports, covered later). Reconstructing a path like "host -> GW -> firewall -> LB -> API server" from the diagram lets you ask which hop failed.