This chapter covers routing protocols and concepts, a core topic for the CompTIA Network+ N10-009 exam. Understanding how routers communicate to dynamically learn and maintain network paths is essential for network configuration, troubleshooting, and design. Approximately 15-20% of the exam questions touch on routing fundamentals, including the differences between distance vector and link-state protocols, administrative distance, and routing metrics. You will learn the mechanisms, configuration, and verification of the most common routing protocols.
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Think of a routing protocol like a GPS navigation app on your phone, but for routers. Your GPS knows the entire road network (the network topology) and uses an algorithm to find the fastest or shortest path. It periodically checks for road closures or traffic jams (link failures or congestion) and recalculates the route. When you start a trip, the GPS gives you turn-by-turn instructions (the routing table). If you miss a turn or a road closes, the GPS recalculates and gives new directions. Similarly, a routing protocol like OSPF or EIGRP builds a map of the network, calculates the best paths, and installs them in the routing table. If a link fails, the protocol detects the change (via hello timers or LSA updates), recomputes the best path, and updates the routing table. Just as a GPS gives different routes for 'fastest' vs 'shortest', different routing protocols use different metrics (hops, bandwidth, delay) to determine the best path.
What Are Routing Protocols and Why Do They Exist?
Routing protocols are the languages and algorithms that routers use to share information about network reachability. Without routing protocols, network administrators would have to manually configure every route on every router—a task that does not scale beyond a handful of networks. Routing protocols automate this process, allowing routers to dynamically discover paths, adapt to topology changes, and select the best routes based on metrics.
Routers maintain a routing table that contains entries for each known destination network. Each entry includes the next-hop IP address, the outgoing interface, and the metric. Routing protocols populate this table with dynamic routes, as opposed to static routes which are manually configured.
How Routing Protocols Work Internally
Routing protocols operate at the network layer (Layer 3) of the OSI model. They exchange messages using protocol-specific packets. The fundamental process involves:
Neighbor Discovery: Routers identify adjacent routers running the same routing protocol. This is often done using hello packets sent to multicast or broadcast addresses.
Topology Exchange: Routers share information about known networks. The type and frequency of exchange depend on whether the protocol is distance vector or link-state.
Route Calculation: Each router runs an algorithm (e.g., Bellman-Ford for distance vector, Dijkstra for link-state) to determine the best path to each destination.
Route Installation: The best path is installed in the routing table. If multiple routing protocols provide a route to the same destination, the router uses administrative distance (AD) to decide which route to trust.
Convergence: After a topology change (link failure, new link), routers recalculate routes and update their tables. The time to reach convergence is a key performance metric.
Key Components, Values, Defaults, and Timers
#### Administrative Distance (AD)
Administrative distance is a trustworthiness rating assigned to a route source. Lower AD values are preferred. CompTIA N10-009 expects you to know these default AD values:
Connected interface: 0
Static route: 1
Enhanced Interior Gateway Routing Protocol (EIGRP) summary route: 5
External Border Gateway Protocol (eBGP): 20
Internal EIGRP: 90
Open Shortest Path First (OSPF): 110
Intermediate System-to-Intermediate System (IS-IS): 115
Routing Information Protocol (RIP): 120
External EIGRP: 170
Internal BGP (iBGP): 200
When a router learns a route from multiple protocols, it compares AD first. Only if AD is equal does it compare metrics.
#### Metrics
Each routing protocol uses its own metric to determine the best path within that protocol:
RIP: Hop count (max 15 hops). A hop is a router. 16 hops is considered unreachable.
OSPF: Cost, based on bandwidth. Default cost = 10^8 / bandwidth in bps. For Gigabit Ethernet (1 Gbps), cost = 100,000,000 / 1,000,000,000 = 0.1, but OSPF uses integer costs, so it is rounded to 1. For Fast Ethernet (100 Mbps), cost = 1.
EIGRP: Composite metric using bandwidth (slowest link along path), delay (cumulative), load, and reliability. By default, only bandwidth and delay are used. Formula: metric = (K1 * bandwidth + (K2 * bandwidth) / (256 - load) + K3 * delay) * (K5 / (reliability + K4)). Default K values: K1=1, K2=0, K3=1, K4=0, K5=0. So metric = bandwidth + delay.
#### Timers
RIP: Update timer (30 seconds), invalid timer (180 seconds), holddown timer (180 seconds), flush timer (240 seconds).
OSPF: Hello timer (10 seconds on broadcast networks, 30 seconds on non-broadcast), dead timer (40 seconds, 4x hello).
EIGRP: Hello timer (5 seconds on high-bandwidth links, 60 seconds on low-bandwidth), hold time (15 seconds, 3x hello).
Configuration and Verification Commands
#### RIP Configuration (on Cisco IOS)
router rip
version 2
network 10.0.0.0
no auto-summary#### OSPF Configuration
router ospf 1
network 10.0.0.0 0.255.255.255 area 0#### EIGRP Configuration
router eigrp 100
network 10.0.0.0#### Verification Commands
show ip route: Displays the routing table.
show ip protocols: Shows routing protocol parameters, timers, and neighbors.
show ip ospf neighbor: Lists OSPF neighbors and their states.
show ip eigrp neighbors: Lists EIGRP neighbors.
debug ip rip: Shows RIP updates in real time (use with caution).
How Routing Protocols Interact with Related Technologies
Routing protocols work with VLANs, subinterfaces, and static routes. For example, OSPF can run over VLAN trunk links. Static routes can be redistributed into dynamic protocols. Route redistribution between protocols (e.g., OSPF into EIGRP) requires careful metric translation.
Distance Vector vs. Link-State
Distance vector protocols (RIP, EIGRP) know only the distance and vector (next hop) to a destination. They share their entire routing table with neighbors periodically. This can cause slow convergence and routing loops (e.g., count-to-infinity). Mitigations include split horizon, route poisoning, and hold-down timers.
Link-state protocols (OSPF, IS-IS) know the entire network topology. Each router sends link-state advertisements (LSAs) to all routers in the area. They use the Dijkstra SPF algorithm to compute the shortest path tree. Convergence is faster and loop-free by design.
Path Vector Protocol: BGP
Border Gateway Protocol (BGP) is an exterior gateway protocol used between autonomous systems. It is a path vector protocol that uses attributes like AS_PATH and NEXT_HOP to select paths. BGP is not covered in depth on N10-009, but you should know it is used for internet routing and is very different from interior protocols.
Common Exam Traps
Confusing AD values: Many candidates mix up OSPF (110) and RIP (120). Remember: OSPF is more trustworthy (lower AD) than RIP.
Metric confusion: RIP uses hop count, OSPF uses cost, EIGRP uses composite metric. On the exam, you might be asked to identify which protocol uses which metric.
Classful vs classless: RIP version 1 is classful (does not send subnet mask), while RIPv2, OSPF, and EIGRP are classless (send subnet mask). Modern networks use classless routing.
EIGRP is not a link-state protocol: It is an advanced distance vector protocol (also called hybrid). It does not use LSAs or SPF.
Summary of Key Defaults
RIP maximum hop count: 15
OSPF default reference bandwidth: 100 Mbps
EIGRP administrative distance: 90 (internal), 170 (external)
OSPF administrative distance: 110
RIP administrative distance: 120
OSPF hello timer: 10 seconds (broadcast)
EIGRP hello timer: 5 seconds (high-bandwidth)
Router sends hello packets
When a router interface comes up, the routing protocol (e.g., OSPF, EIGRP) sends hello packets out that interface to a multicast address (224.0.0.5 for OSPF, 224.0.0.10 for EIGRP). These packets contain the router ID, hello interval, and other parameters. Neighboring routers listening on the same subnet receive the hello and add the sender to their neighbor table. This establishes bidirectional communication. The hello interval is typically 10 seconds for OSPF on broadcast networks; if no hello is received within the dead interval (40 seconds), the neighbor is declared down.
Neighbors exchange routing information
After adjacency is formed, routers exchange routing information. For distance vector protocols like RIP, each router sends its entire routing table to neighbors every 30 seconds. For link-state protocols like OSPF, routers exchange link-state advertisements (LSAs) that describe their directly connected links. OSPF uses a reliable flooding mechanism to ensure all routers in an area receive the same LSAs. EIGRP, as an advanced distance vector protocol, sends only changes (partial updates) after the initial full exchange.
Each router calculates best paths
Using the information received, each router runs a routing algorithm. For distance vector protocols, the Bellman-Ford algorithm is used to compute the best path based on hop count (RIP) or composite metric (EIGRP). For link-state protocols, Dijkstra's Shortest Path First (SPF) algorithm computes the shortest path tree from the router to all destinations. The result is a set of best routes that are candidates for the routing table.
Best routes installed in routing table
The router compares the best routes from each routing protocol using administrative distance. If a route is learned from OSPF (AD 110) and another from RIP (AD 120), the OSPF route is installed. If multiple routes from the same protocol exist, the metric is used; the lowest metric wins. The chosen route is added to the routing table with the next-hop IP and outgoing interface. The routing table is consulted for every forwarded packet.
Routers detect topology changes and reconverge
When a link fails or a new link appears, the router detects the change via interface status or expired hello timers. It then sends updates to neighbors: RIP sends triggered updates immediately (not waiting for the 30-second timer); OSPF floods new LSAs; EIGRP sends query and reply packets. All routers recalculate routes using their algorithm. Convergence time varies: RIP can take minutes due to hold-down timers; OSPF and EIGRP converge in seconds.
Enterprise Scenario 1: OSPF in a Multi-Site Campus Network
A large university with three campuses uses OSPF as its interior gateway protocol. Each campus is configured as a separate OSPF area (area 0 for the backbone, area 1, area 2, etc.) to reduce routing table size and limit LSA flooding. Routers between areas are area border routers (ABRs). The network team configures OSPF with a reference bandwidth of 1 Gbps to accurately reflect modern link speeds. They use passive interfaces on access ports to prevent OSPF adjacencies with end devices. Troubleshooting involves checking show ip ospf neighbor to ensure all expected adjacencies are in FULL state and verifying that inter-area routes appear in the routing table. A common misconfiguration is mismatched hello/dead timers, which prevents adjacency formation. In production, the team sets all timers to default to avoid complexity.
Enterprise Scenario 2: EIGRP in a Branch Office WAN
A retail company with hundreds of branch offices uses EIGRP over MPLS VPN links to the corporate data center. EIGRP is chosen for its fast convergence and low bandwidth usage (only partial updates). The WAN links are T1 (1.544 Mbps) and the default EIGRP hello timer of 5 seconds is changed to 60 seconds on these low-speed links to reduce overhead. The corporate data center acts as the hub, and branches are spokes. Route summarization is configured on the hub to reduce routing table size. A common issue is that EIGRP stub routers are misconfigured, causing branches to query the hub unnecessarily. The network engineer uses show ip eigrp topology to verify the topology table and show ip eigrp traffic to monitor update packets.
Scenario 3: RIP in a Small Office (Legacy)
A small law office with a single router and two subnets uses RIP because the router is old and only supports RIP. The network administrator configures RIP version 2 to support VLSM. The office has a static default route to the internet, and RIP redistributes that default to other routers. However, the RIP update timer (30 seconds) causes periodic spikes in CPU usage on the router. The administrator monitors the routing table with show ip route and ensures no routes exceed 15 hops. A common mistake is forgetting to disable auto-summary, causing discontinuous subnets to be summarized to their classful boundary, breaking routing.
The N10-009 exam tests routing protocols under Objective 1.5 (Routing Protocols and Concepts). You must be able to:
Differentiate between distance vector and link-state protocols.
Identify the administrative distance of common protocols (connected, static, OSPF, EIGRP, RIP).
Recognize the metrics used by each protocol (hop count, cost, composite).
Understand the purpose of timers (hello, dead, update).
Know the default maximum hop count for RIP (15).
Understand the concept of convergence.
Common Wrong Answers and Why Candidates Choose Them
'EIGRP is a link-state protocol.' Many candidates hear 'advanced' and assume it is link-state. In reality, EIGRP is an advanced distance vector (hybrid) protocol. It does not use LSAs or SPF. The exam expects you to classify EIGRP correctly.
'OSPF uses hop count as a metric.' This is a classic trap. OSPF uses cost based on bandwidth. RIP uses hop count. Candidates who memorize 'metric' but not 'which protocol' often pick the wrong one.
'RIP has an administrative distance of 110.' This is OSPF's AD. RIP's AD is 120. The exam will list multiple AD values and ask you to select the correct one for a given protocol.
'Static routes have an AD of 0.' Connected interfaces have AD 0. Static routes have AD 1. Many candidates confuse these.
'All routing protocols send updates every 30 seconds.' Only RIP does. OSPF sends LSAs only on change, and EIGRP sends partial updates. The exam may ask about update frequency.
Specific Numbers and Terms That Appear on the Exam
RIP max hop count: 15
OSPF default reference bandwidth: 100 Mbps
OSPF administrative distance: 110
EIGRP administrative distance: 90 (internal)
RIP administrative distance: 120
OSPF hello timer: 10 seconds (broadcast)
EIGRP hello timer: 5 seconds (high-speed link)
Edge Cases and Exceptions
When a route is learned via multiple routing protocols, the router compares AD first, not metric. Only if AD is equal does it compare metrics.
EIGRP has different AD for internal (90) and external (170) routes. External routes are those redistributed from another protocol.
OSPF cost can be manually overridden using the ip ospf cost command on an interface.
RIP version 1 does not support VLSM or CIDR; it is classful. Version 2 does.
How to Eliminate Wrong Answers
If a question asks about 'fastest convergence', eliminate RIP (slowest). OSPF and EIGRP converge faster.
If a question mentions 'hop count limit of 15', it is RIP.
If a question mentions 'bandwidth and delay', it is EIGRP.
If a question mentions 'cost based on bandwidth', it is OSPF.
If a question mentions 'sends entire routing table every 30 seconds', it is RIP.
Always read the question carefully: it may ask for the 'best path' or 'path selection criteria' — you must know which metric the protocol uses.
Routing protocols automate the exchange of network reachability information between routers.
Distance vector protocols (RIP, EIGRP) know only the next hop; link-state protocols (OSPF) know the full topology.
Administrative distance (AD) determines which route is trusted when multiple protocols provide a route. Lower AD is preferred.
Default AD values: connected=0, static=1, eBGP=20, EIGRP=90, OSPF=110, RIP=120.
RIP uses hop count (max 15), OSPF uses cost (10^8/bandwidth), EIGRP uses composite metric (bandwidth and delay by default).
RIP sends full routing table every 30 seconds; OSPF and EIGRP send triggered updates only on change.
OSPF hello timer = 10 seconds (broadcast), dead timer = 40 seconds. EIGRP hello = 5 seconds (high-speed), hold time = 15 seconds.
EIGRP is an advanced distance vector protocol, not link-state.
Convergence is the time for all routers to agree on the topology after a change.
RIP version 2 supports VLSM and CIDR; version 1 does not.
These come up on the exam all the time. Here's how to tell them apart.
Distance Vector (RIP)
Knows only the distance and next hop to each destination.
Sends entire routing table to neighbors periodically (every 30 seconds for RIP).
Uses Bellman-Ford algorithm.
Converges slowly; prone to routing loops (count-to-infinity).
Metric: hop count (max 15).
Link-State (OSPF)
Knows the entire network topology (link-state database).
Sends link-state advertisements (LSAs) only on change, flooded to all routers.
Uses Dijkstra's SPF algorithm.
Converges quickly; loop-free by design.
Metric: cost (based on bandwidth).
Mistake
EIGRP is a link-state routing protocol.
Correct
EIGRP is an advanced distance vector (hybrid) protocol. It does not flood link-state information like OSPF; instead, it exchanges routing tables with neighbors and uses the Diffusing Update Algorithm (DUAL) for loop-free path selection.
Mistake
OSPF uses hop count as its metric.
Correct
OSPF uses cost, which is inversely proportional to bandwidth. The default cost = 10^8 / bandwidth. Hop count is used by RIP.
Mistake
RIP has an administrative distance of 110.
Correct
RIP's AD is 120. OSPF has AD 110. Candidates often mix these up because both are interior gateway protocols.
Mistake
Static routes have an administrative distance of 0.
Correct
Connected routes have AD 0. Static routes have AD 1. A static route is less trustworthy than a directly connected network.
Mistake
All routing protocols send periodic updates every 30 seconds.
Correct
Only RIP sends periodic updates every 30 seconds. OSPF and EIGRP send updates only when a topology change occurs (triggered updates). OSPF also sends LSAs every 30 minutes as a refresh, but not full routing tables.
Reveal each answer, then mark whether you got it right. Score 60%+ to unlock the next chapter.
Administrative distance (AD) is a numerical value that rates the trustworthiness of a routing information source. When a router learns the same route from multiple routing protocols (e.g., OSPF and RIP), it uses AD to decide which route to install in the routing table. Lower AD values are preferred. For example, OSPF has AD 110, while RIP has AD 120, so OSPF routes are chosen over RIP routes. Connected interfaces have AD 0, and static routes have AD 1. Understanding AD is crucial for troubleshooting routing issues where a route might not be used because a less trustworthy protocol learned it first.
Distance vector protocols (like RIP and EIGRP) work by having each router know only the distance (metric) and vector (next hop) to each destination. They share their entire routing table with neighbors periodically. This can lead to slow convergence and routing loops. Link-state protocols (like OSPF and IS-IS) have each router build a complete map of the network by exchanging link-state advertisements (LSAs). They then independently calculate the shortest path using Dijkstra's algorithm. Link-state protocols converge faster and are less prone to loops, but require more memory and CPU.
RIP uses hop count as its metric, with a maximum of 15 hops. A hop count of 16 is considered unreachable. This limit prevents routing loops in a distance vector environment by ensuring that a packet cannot traverse more than 15 routers. If a route exceeds 15 hops, it is discarded. This limitation makes RIP unsuitable for large networks.
OSPF cost is calculated as 10^8 / bandwidth (in bps). The reference bandwidth is 100 Mbps by default. For example, a Fast Ethernet link (100 Mbps) has a cost of 1 (100,000,000 / 100,000,000 = 1). A Gigabit Ethernet link (1 Gbps) would have cost 0.1, but OSPF uses integer costs, so it is rounded to 1. You can manually set the cost on an interface using the 'ip ospf cost' command. The reference bandwidth can be changed with the 'auto-cost reference-bandwidth' command to better reflect modern high-speed links.
The hello timer determines how frequently a router sends hello packets to discover and maintain neighbor relationships. For OSPF on broadcast networks, the default hello interval is 10 seconds. For EIGRP on high-speed links, it is 5 seconds. If a router does not receive a hello from a neighbor within the dead interval (typically 4 times the hello interval), it declares the neighbor down and triggers a topology change. Hello timers must match between neighbors for adjacency to form.
Convergence is the state when all routers in a network have consistent and up-to-date routing information after a topology change (link failure, addition, or metric change). During convergence, routers exchange updates, recalculate paths, and update their routing tables. Fast convergence is critical to minimize packet loss. OSPF and EIGRP converge in seconds, while RIP can take minutes due to its periodic update and hold-down timers.
Yes, a router can run multiple routing protocols simultaneously. Each protocol operates independently and populates its own routes. When the router installs routes into the routing table, it uses administrative distance to choose between routes from different protocols. You can also redistribute routes from one protocol into another, but this requires careful configuration of metrics to avoid suboptimal routing or loops.
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