Routing and Switching Fundamentals
Routing and Switching Fundamentals tests whether you can explain how a frame or packet actually reaches its destination. At RSTECH level this means OSI and TCP/IP layer mapping, Ethernet MAC addressing, switch forwarding behavior, VLANs and 802.1Q trunks, IPv4 addressing and subnet masks, ARP, and the boundary between switching and routing.
Layer roles and forwarding
Layer 1 covers cables, connectors, and repeaters; Layer 2 covers switches and MAC addresses; Layer 3 covers routers and IP addresses. A switch learns the source MAC of each incoming frame, floods frames with an unknown destination or a broadcast MAC, forwards known unicasts out the matching port, and ages entries. A router does not forward broadcasts and rewrites the Layer 2 header at every hop while leaving the IP addresses unchanged unless NAT is configured. Routers choose paths by longest prefix match, then administrative distance, then metric; connected routes beat static routes, and static routes normally beat dynamic routing protocols.
VLANs, IP, and gateways
VLANs create separate broadcast domains. Access ports carry one VLAN; trunk ports carry many with 802.1Q tags. The native VLAN is untagged and must match on both ends. IPv4 hosts compare the destination IP with their mask: if the destination is local, they ARP for the host; if remote, they ARP for the default gateway. Private ranges are 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. A /24 mask gives 256 addresses, with 254 usable host addresses.
Worked example
Host 192.168.10.25/24 with gateway 192.168.10.1 sends to 192.168.20.10/24. Because the /24 mask makes 192.168.20.10 remote, the host ARPs for 192.168.10.1 and builds a frame with the router's MAC as destination and the server's IP as destination. The switch learns the host MAC and floods the unknown router MAC. The router accepts the frame, decrements TTL, looks up 192.168.20.0/24, ARPs for the server on its other interface, and sends a new frame. The server replies through its own gateway.
Traps that cost marks
- A switch does not change IP addresses; it switches frames.
- Do not confuse the MAC address table with the ARP table.
- A /25 mask creates two subnets; 192.168.10.200/25 is not in 192.168.10.0/25.
- A default gateway must be on the host's own subnet.
- Mismatched native VLANs place untagged traffic in the wrong VLAN.
- A broadcast is flooded within its VLAN but not routed by default.