Routing concepts are central to CompTIA Network+ N10-009, appearing across networking concepts, implementation, and troubleshooting domains. You must understand how routers make forwarding decisions, the difference between static and dynamic routing, common routing protocols (RIP, OSPF, EIGRP, BGP), and metrics used for path selection. Network+ tests routing conceptually — you need to identify which protocol to use, understand administrative distance, and troubleshoot routing failures.
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A router maintains a routing table containing known networks, the next-hop address or exit interface for each, and a metric indicating path preference. When a packet arrives, the router performs a longest prefix match — it selects the most specific matching route. A route to 192.168.1.0/24 is preferred over a default route (0.0.0.0/0) for traffic destined to 192.168.1.100.
Default route (0.0.0.0/0 or ::/0 for IPv6): matches any destination not covered by more specific routes. Used for internet traffic — 'send everything unknown to the ISP.' A router without a default route will drop traffic to unknown destinations.
Administrative distance (AD): a value that ranks routing information sources by trustworthiness. Lower AD = more trusted. Connected interface = 0. Static route = 1. EIGRP = 90. OSPF = 110. RIP = 120. External BGP = 20. If two routing protocols both have a route to the same destination, the one with lower AD wins and enters the routing table.
Static routing: routes manually configured by an administrator. Advantages: predictable, no overhead, no routing protocol traffic. Disadvantages: does not adapt to topology changes — if a link fails, traffic is not automatically rerouted. Suitable for small networks or specific routes (default route, stub networks).
Dynamic routing: routers automatically exchange route information and update routing tables when topology changes. Three categories: Distance vector (RIP, EIGRP) — routers share routing tables with neighbors. Link state (OSPF, IS-IS) — routers share topology information and each router builds its own complete map. Path vector (BGP) — routers share path attributes, used for inter-AS internet routing.
RIP (Routing Information Protocol): distance-vector, uses hop count as metric (max 15 hops — 16 = unreachable), slow convergence (30s updates), suitable only for small networks. RIPv2 adds VLSM and MD5 authentication. RIPng for IPv6.
OSPF (Open Shortest Path First): link-state, uses cost (based on bandwidth) as metric, fast convergence, supports large networks, uses Dijkstra's algorithm, organized into areas (Area 0 = backbone). Standard protocol for enterprise routing. OSPFv3 for IPv6.
EIGRP (Enhanced Interior Gateway Routing Protocol): advanced distance-vector (Cisco proprietary, though now open), uses bandwidth and delay composite metric, very fast convergence via DUAL algorithm, supports VLSM. More efficient than OSPF in Cisco-only environments.
BGP (Border Gateway Protocol): path-vector, the routing protocol of the internet. Used between autonomous systems (ISPs, large enterprises). Uses AS path, local preference, and many attributes for path selection. Extremely scalable but complex. Not typically deployed in enterprise LANs.
| Protocol | Type | Metric | Max Hops | Use Case |
|---|---|---|---|---|
| RIP | Distance-vector | Hop count | 15 | Small/legacy networks |
| OSPF | Link-state | Cost (bandwidth) | Unlimited | Enterprise LAN/WAN |
| EIGRP | Advanced distance-vector | Bandwidth + delay | Unlimited | Cisco enterprise |
| BGP | Path-vector | AS path, attributes | Unlimited | Internet/ISP routing |
| Static | Manual | N/A (AD=1) | N/A | Small nets, default routes |
OSPF metric is hop count
OSPF uses cost, which is derived from bandwidth (reference bandwidth / link bandwidth). Hop count is RIP's metric
Static routes are always better than dynamic routes
Static routes have lower administrative distance (1 vs 90/110/120) so they are preferred when both exist — but static routes do not adapt to failures, making them unsuitable as the sole routing method in redundant networks
BGP is used inside company networks
BGP is an exterior gateway protocol (EGP) designed for routing between autonomous systems (ISPs, large enterprises with multiple ISP connections). Interior routing (within a company) uses OSPF or EIGRP
These questions are representative of what you will see on Network+ exams. The correct answer and explanation are shown immediately below each question.
A network administrator configures both OSPF and RIP on a router for the same destination network. Which protocol's route will appear in the routing table?
Explanation: When two routing protocols both have a route to the same destination, the protocol with the lower administrative distance wins. OSPF has AD 110; RIP has AD 120. OSPF's route (lower AD) is preferred and installed in the routing table. The RIP route is kept as a backup but not used unless OSPF fails.
A router's routing table contains entries for 10.0.0.0/8, 10.1.0.0/16, and 10.1.1.0/24. A packet arrives destined for 10.1.1.50. Which route does the router use?
Explanation: Routers use longest prefix match — the most specific (longest) matching route wins. 10.1.1.0/24 matches 10.1.1.50 with a 24-bit prefix (most specific). 10.1.0.0/16 also matches but with only a 16-bit prefix. 10.0.0.0/8 matches with 8 bits. The /24 is most specific and is used.
Convergence is the time it takes for all routers in a network to agree on a consistent view of the network topology after a change (link failure, new route). Fast convergence is critical for minimizing downtime. OSPF and EIGRP converge in seconds; RIP can take minutes. BGP convergence can take many minutes in large internet topologies.
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