IP Connectivity
IP connectivity is the largest exam domain and centers on how routers determine the best path to forward packets. You will study the components of the routing table, administrative distance, static routing, and dynamic routing with OSPFv2. Understanding longest prefix match and next-hop resolution ties these concepts together.
The Routing Table and Longest Prefix Match
A router consults its routing table to forward each packet toward its destination network. When multiple routes match, the router selects the one with the longest prefix (most specific mask), regardless of source. Routes are learned as directly connected, static, or dynamic entries. The gateway of last resort (default route) is used only when no more specific match exists.
Administrative Distance and Metrics
Administrative distance ranks the trustworthiness of routing sources, so a lower AD wins when the same prefix is learned multiple ways. Connected routes have AD 0, static routes AD 1, and OSPF AD 110. Within a single protocol, the metric breaks ties between equal-prefix routes. This layered decision process ensures the most reliable and efficient path is installed.
Static and Default Routing
Static routes are manually configured and are useful for stub networks and predictable paths. The 'ip route' command specifies the destination network, mask, and either a next-hop address or exit interface. A default route (0.0.0.0 0.0.0.0) forwards all unmatched traffic toward an upstream gateway. Floating static routes with a higher AD provide backup when a dynamic route fails.
OSPFv2 Operation
OSPF is a link-state interior gateway protocol that floods link-state advertisements so every router builds an identical topology database. Each router runs the SPF (Dijkstra) algorithm to compute shortest paths, using cost derived from bandwidth. Neighbors must agree on hello and dead timers, area ID, subnet, and MTU to form adjacencies. A router ID uniquely identifies each router.
Next-Hop Resolution and IPv6 Routing
To forward a packet, a router must resolve the next-hop IP to a Layer 2 address using ARP for IPv4 or Neighbor Discovery for IPv6. Recursive lookups may be required when a route points to a next hop that is itself learned rather than directly connected. IPv6 routing mirrors IPv4 concepts with static and OSPFv3 options. Verifying the routing table and neighbor state confirms end-to-end reachability.