This chapter covers static and dynamic routing — two fundamental methods routers use to learn paths to remote networks. For the N10-009 exam, understanding the differences, use cases, and configuration of these routing methods is critical. Approximately 10–15% of exam questions touch routing concepts, including static versus dynamic routing, administrative distance, and routing protocol characteristics. You will need to know when to use each, how to configure them on Cisco IOS, and how routers select the best path when multiple routes exist.
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Static routing is like hiring a personal courier to deliver packages between two specific offices. You tell the courier exactly which streets to take and which turns to make. The courier never deviates, even if a road is closed or traffic builds up. The route is fixed, predictable, and requires no decision-making. If you need to change the route, you must personally update the courier's instructions. Dynamic routing, by contrast, is like the national postal service. The postal service does not predefine every package's route. Instead, each post office exchanges information with neighboring post offices about the best roads to use based on current traffic, road closures, and distance. When a package arrives, the local post office checks its routing table, which is automatically updated every few seconds via messages from other offices. If a road closes, the offices share that news and update their tables within seconds, rerouting packages automatically. The key difference: static routing requires manual configuration and is ideal for small, stable networks; dynamic routing uses protocols (like OSPF or EIGRP) to automatically adapt to changes, making it essential for large, complex networks. On the exam, expect questions that test your understanding of when each is appropriate and the administrative distance values that determine which route wins when both exist.
What is Static Routing?
Static routing is a method where a network administrator manually enters routes into a router's routing table. These routes do not change unless the administrator updates them. Static routes are simple, secure (no routing protocol traffic), and use no bandwidth for route advertisements. However, they do not adapt to network changes — if a link fails, the static route remains in the table, causing traffic to be dropped unless a floating static route is configured.
How Static Routing Works
When a router receives a packet destined for a network not directly connected, it consults its routing table. For a static route, the table entry specifies the destination network, subnet mask, and either the next-hop IP address or the exit interface. The router then forwards the packet to the next hop. No route discovery or exchange occurs. The command to add a static route on Cisco IOS is:
ip route [destination_network] [subnet_mask] [next_hop_ip | exit_interface]Example:
ip route 192.168.2.0 255.255.255.0 10.0.0.2This tells the router: "To reach 192.168.2.0/24, send packets to 10.0.0.2."
What is Dynamic Routing?
Dynamic routing uses routing protocols to automatically discover remote networks and maintain up-to-date routing tables. Protocols like RIP, EIGRP, OSPF, and BGP exchange routing information between routers. This allows the network to adapt to changes (link failures, new networks) without manual intervention. Dynamic routing scales well but consumes bandwidth, CPU, and memory.
How Dynamic Routing Works
Routers running a dynamic routing protocol send and receive routing updates. Each protocol has its own algorithm and metric: - Distance Vector (RIP, EIGRP): Routers send their entire routing table to neighbors periodically (RIP: every 30 seconds). EIGRP sends partial updates only when changes occur, making it more efficient. - Link State (OSPF): Routers build a complete map of the network (link-state database) and run the SPF algorithm to calculate the shortest path to each destination. Updates are sent only when a change occurs. - Path Vector (BGP): Used between autonomous systems, BGP exchanges reachability information with path attributes to prevent loops.
Key Components and Defaults
Administrative Distance (AD): A value from 0 to 255 that indicates the trustworthiness of a route source. Lower AD is preferred. Default ADs:
- Connected: 0 - Static: 1 - EIGRP: 90 (internal), 170 (external) - OSPF: 110 - RIP: 120 - External BGP: 20 - Metric: A value used by a routing protocol to determine the best path within that protocol. RIP uses hop count (max 15). OSPF uses cost (based on bandwidth). EIGRP uses a composite metric (bandwidth, delay, load, reliability). - Timers:
RIP: Update timer 30s, invalid timer 180s, hold-down timer 180s, flush timer 240s.
OSPF: Hello timer 10s (default on broadcast networks), dead timer 40s.
EIGRP: Hello timer 5s (most networks), hold timer 15s.
Configuration and Verification Commands
Static route configuration:
ip route 172.16.0.0 255.255.0.0 192.168.1.1To verify the routing table:
show ip routeLook for "S" for static routes.
Dynamic routing (OSPF example):
router ospf 1
network 10.0.0.0 0.255.255.255 area 0Verification:
show ip route
show ip ospf neighbor
show ip protocolsInteraction with Related Technologies
Static and dynamic routes can coexist. When a router learns the same destination from multiple sources, the route with the lowest administrative distance is installed in the routing table. For example, if a router has a static route to 10.0.0.0/8 (AD 1) and learns the same via OSPF (AD 110), the static route is used. If the static route fails (next hop unreachable), the router will use the OSPF-learned route if it exists. This is the basis for floating static routes — a static route with a higher AD than the dynamic route, serving as a backup.
Default Routes
A default route (0.0.0.0/0) matches any destination. It can be static or learned dynamically. On Cisco IOS:
ip route 0.0.0.0 0.0.0.0 192.168.1.1Dynamic protocols can also advertise default routes (e.g., OSPF default-information originate).
Route Summarization
Static routes can be summarized manually. Dynamic protocols like OSPF and EIGRP support automatic summarization at classful boundaries (can be disabled). Summarization reduces routing table size and update traffic.
Route Redistribution
When multiple routing protocols are used, routes can be redistributed between them. For example, redistributing static routes into OSPF:
router ospf 1
redistribute static subnetsThis requires careful planning to avoid routing loops and suboptimal routing.
Performance Considerations
Static routing: Zero CPU overhead for route computation, but manual updates are error-prone. Best for small, stable networks (e.g., stub networks).
Dynamic routing: CPU and memory overhead depend on protocol. OSPF requires more CPU for SPF calculations, especially in large networks with frequent changes. EIGRP is more efficient due to partial updates. RIP is lightweight but converges slowly.
Convergence Time
Convergence is the time it takes for all routers to have a consistent view of the network after a change. Static routing has no convergence — routes must be manually updated. Dynamic protocols converge automatically:
RIP: Slow (up to 180 seconds due to hold-down timers).
EIGRP: Fast (typically < 5 seconds).
OSPF: Fast (depends on network size, usually < 10 seconds).
Security Considerations
Static routes are more secure because no routing protocol traffic is exchanged. Dynamic protocols can be vulnerable to attacks (e.g., route injection, false updates). Authentication (MD5, SHA) can be configured on OSPF, EIGRP, and BGP to mitigate this.
Exam Relevance
On N10-009, you should be able to:
Identify whether a scenario requires static or dynamic routing.
Interpret routing table entries (flags, AD, metric).
Compare routing protocol characteristics (distance vector vs link state, timers, metrics).
Understand administrative distance and how it affects path selection.
Configure basic static routes and default routes.
Recognize the role of dynamic routing in enterprise networks.
Manual Entry of Static Route
The network administrator identifies a destination network not directly connected to the router. Using the `ip route` command, the administrator specifies the destination network, subnet mask, and the next-hop IP address or exit interface. For example: `ip route 192.168.2.0 255.255.255.0 10.0.0.2`. The router adds this entry to its routing table with an administrative distance of 1 (default). The route is immediately active if the next hop is reachable. If the next hop becomes unreachable, the route is removed from the routing table (unless configured with a permanent keyword). This step requires no protocol exchange, making it secure but prone to human error. Verification is done via `show ip route` where the route is marked with 'S'.
Router Receives a Packet
When a router receives an IP packet, it examines the destination IP address. It performs a longest-prefix match lookup in its routing table. For a static route, the entry provides the next-hop IP address or exit interface. The router then performs a recursive lookup if the next hop is not directly connected. For example, if the static route points to 10.0.0.2, the router must have a route to 10.0.0.0/30 (perhaps a directly connected interface). If the next hop is directly connected, the router ARPs for the next-hop MAC address (if Ethernet) and forwards the packet. If the static route specifies an exit interface (e.g., Serial0/0/0), the router encapsulates the packet directly without an ARP. The packet is then sent out the appropriate interface.
Dynamic Route Discovery Begins
Routers running a dynamic routing protocol (e.g., OSPF) send hello messages to discover neighbors. On broadcast networks, hello messages are multicast to 224.0.0.5 (OSPF). Neighbors exchange information to form adjacencies. For OSPF, routers must agree on parameters like area ID, authentication, and hello/dead intervals (default 10s/40s). Once an adjacency is established, routers exchange link-state advertisements (LSAs) to build a link-state database. Each router then runs the SPF algorithm to calculate the shortest path to each destination. The resulting routes are installed in the routing table with the protocol's administrative distance (e.g., OSPF AD 110). The metric (cost) is calculated based on the sum of outgoing interface costs (reference bandwidth / interface bandwidth).
Convergence After a Link Failure
When a link fails, dynamic protocols detect the failure through missing hello packets (e.g., OSPF dead timer expires after 40s) or through link-down notifications. The router that detects the failure sends updated LSAs (OSPF) or partial updates (EIGRP) to its neighbors. Other routers recalculate their routing tables. For OSPF, the SPF algorithm is rerun. This process is called convergence. The time to converge depends on the protocol: OSPF may converge in <10s, EIGRP in <5s, RIP in up to 180s. During convergence, traffic may be black-holed or looped if not using mechanisms like hold-down timers (RIP) or fast convergence features (OSPF LSA throttling). In contrast, static routing requires manual intervention to update or remove the failed route, leaving traffic dropped until an administrator acts.
Route Selection Using AD and Metric
If a router learns the same destination from multiple sources (e.g., static and OSPF), it uses administrative distance (AD) to choose the best source. The route with the lowest AD is installed. For example, a static route (AD 1) is preferred over an OSPF route (AD 110). If the same protocol provides multiple paths, the metric is used. For OSPF, the path with the lowest cost is chosen. If equal-cost paths exist, most protocols support load balancing (ECMP). The router then populates the forwarding table (FIB) with the best path. On Cisco IOS, `show ip route` displays the selected route. The output includes the protocol code (S for static, O for OSPF), AD, metric, next hop, and timestamp.
Enterprise Branch Office Connectivity
A common scenario is connecting a small branch office to a corporate headquarters via a single WAN link. Here, static routing is ideal. The branch router has a default route pointing to the HQ router: ip route 0.0.0.0 0.0.0.0 10.0.0.1. The HQ router has a static route back to the branch's internal network: ip route 192.168.10.0 255.255.255.0 10.0.0.2. This setup is simple, secure, and requires no routing protocol overhead. If the WAN link fails, the branch loses connectivity until the link is restored or a backup static route is configured (floating static). In production, network engineers often use a primary static route with AD 1 and a backup via a different ISP with AD 2. The backup route is only used when the primary next hop is unreachable.
Large Enterprise with Multiple Sites
In a network with hundreds of routers and multiple redundant links, dynamic routing is essential. OSPF is commonly deployed because it converges quickly and scales well. For example, a company with three data centers and dozens of branch offices runs OSPF across the WAN. Each area is designed to contain routing updates. Branch routers are in stub areas to reduce their routing table size. The network engineer configures OSPF with authentication to prevent unauthorized route injection. Performance considerations include tuning hello/dead timers for faster convergence (e.g., 1s/3s on critical links) and using route summarization at area borders to minimize LSAs. Misconfiguration, such as mismatched area IDs or subnet masks, can prevent adjacencies from forming, causing routing black holes.
Dual-Homed Internet Connectivity
A company has two ISP connections for redundancy. The network engineer uses static routing with floating static routes. The primary default route points to ISP1 with AD 1: ip route 0.0.0.0 0.0.0.0 203.0.113.1. The backup default route points to ISP2 with AD 2: ip route 0.0.0.0 0.0.0.0 198.51.100.1. Under normal conditions, all traffic goes through ISP1. If the link to ISP1 fails, the static route is removed, and the backup route with AD 2 becomes active. This is simple and reliable. However, if the ISP1 link is up but the ISP's upstream network is down, the static route remains active, causing traffic to be dropped. To detect such failures, network engineers may use IP SLA tracking with the static route, which removes the route if a probe fails. This adds complexity but improves reliability.
N10-009 Objective 1.5: Compare and contrast routing concepts
The exam specifically tests your ability to differentiate between static and dynamic routing, and to select the appropriate method for a given scenario. Key areas: - Administrative Distance (AD): You must know the default AD values for connected (0), static (1), EIGRP (90), OSPF (110), RIP (120), and external EIGRP (170). The exam may present a scenario where multiple routes exist and ask which one is installed. - Floating Static Routes: A common trick — a static route with a higher AD than a dynamic route serves as a backup. Candidates often confuse this with a regular static route. Remember: a floating static route has an AD > that of the dynamic protocol, so it is only used when the dynamic route disappears. - Default Routes: Know that a default route is 0.0.0.0/0 and can be static or learned via a protocol. The exam may ask which command creates a default static route. - Routing Protocol Characteristics: Be able to identify distance vector (RIP, EIGRP) vs link state (OSPF). RIP uses hop count (max 15), OSPF uses cost (based on bandwidth), EIGRP uses composite metric. Timers: RIP update 30s, OSPF hello 10s, EIGRP hello 5s.
Common Wrong Answers 1. "Static routes are automatically updated when a link fails." — Wrong; static routes require manual intervention. 2. "RIP uses a metric of cost based on bandwidth." — Wrong; RIP uses hop count. 3. "OSPF has an AD of 90." — Wrong; OSPF AD is 110; EIGRP is 90. 4. "A floating static route has a lower AD than the dynamic route." — Wrong; it has a higher AD.
Edge Cases
- When a static route points to an interface that goes down, the route is removed from the routing table. But if the route uses the permanent keyword, it stays even if the interface is down — this can cause traffic to be black-holed.
- OSPF external routes (type E1 and E2) have different behaviors; E2 is the default and does not add internal cost.
- EIGRP can be configured to use only bandwidth and delay by default; other metrics (load, reliability) are disabled.
Elimination Strategy When you see a question about route selection, first identify all route sources. Compare their ADs — the lowest wins. If two routes have the same AD, compare metrics (if same protocol). If they are from different protocols, AD decides. Remember that a directly connected network (AD 0) always beats any other route.
Static routes are manually entered with the `ip route` command and have a default AD of 1.
Dynamic routing protocols (RIP, EIGRP, OSPF, BGP) automatically discover and maintain routes.
Administrative distance determines which route is installed when multiple sources exist; lower AD wins.
Default AD values: Connected 0, Static 1, EIGRP 90, OSPF 110, RIP 120.
RIP is a distance-vector protocol using hop count (max 15), updates every 30 seconds.
OSPF is a link-state protocol using cost (based on bandwidth), converges quickly.
EIGRP is a hybrid protocol using composite metric (bandwidth and delay by default).
Floating static routes have a higher AD than the dynamic route and serve as backups.
Default static route: `ip route 0.0.0.0 0.0.0.0 [next-hop]`.
On the exam, always compare AD first, then metric, when determining the best path.
These come up on the exam all the time. Here's how to tell them apart.
Static Routing
Routes are manually configured by an administrator.
No routing protocol overhead (bandwidth, CPU, memory).
Does not adapt to network changes; requires manual updates.
Best suited for small, stable networks or stub networks.
Administrative distance default is 1 (very trustworthy).
Dynamic Routing
Routes are automatically learned via routing protocols.
Consumes bandwidth for updates and CPU for route calculations.
Automatically adapts to link failures and topology changes.
Essential for large, complex networks with redundancy.
Administrative distance varies by protocol (e.g., OSPF 110, EIGRP 90).
Mistake
Static routing is always more secure than dynamic routing.
Correct
Static routing eliminates routing protocol attacks, but it can be less secure if misconfigured (e.g., pointing to a wrong next hop). Dynamic protocols support authentication (MD5, SHA) to prevent route injection, making them equally secure when properly configured.
Mistake
RIP is a link-state protocol.
Correct
RIP is a distance-vector protocol. It sends full routing table updates every 30 seconds and uses hop count as metric. Link-state protocols (like OSPF) build a topology map and send only changes.
Mistake
A floating static route is a static route with AD 1.
Correct
A floating static route has an AD higher than the dynamic protocol's AD (e.g., AD 150 for OSPF backup). It is only used when the dynamic route is unavailable. The default AD for static is 1, which would override the dynamic route, not back it up.
Mistake
OSPF uses hop count as its metric.
Correct
OSPF uses cost, which is derived from the bandwidth of the link (cost = 100 Mbps / interface bandwidth). The default reference bandwidth is 100 Mbps, but it can be changed. Hop count is not considered.
Mistake
Dynamic routing always converges faster than static routing.
Correct
Static routing does not converge automatically — it requires manual updates, which can be slow. However, a well-designed static network with no failures has zero convergence time. Dynamic routing converges automatically but can take seconds depending on the protocol (RIP can take up to 3 minutes).
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The default administrative distance of a static route on Cisco IOS is 1. This means a static route is preferred over routes learned by most dynamic protocols (e.g., OSPF AD 110, RIP AD 120) but is less preferred than directly connected routes (AD 0). You can change the AD by appending a value to the `ip route` command, e.g., `ip route 10.0.0.0 255.0.0.0 192.168.1.1 200` creates a floating static route with AD 200.
A static route is a manually configured route to a specific network (e.g., `ip route 192.168.2.0 255.255.255.0 10.0.0.2`). A default route is a special static route that matches any destination network, defined as 0.0.0.0/0. It is used when no other route in the routing table matches the destination. The command is `ip route 0.0.0.0 0.0.0.0 [next-hop]`. Both are static unless learned dynamically.
A floating static route provides backup connectivity when the primary dynamic route fails. It is configured with an administrative distance higher than the dynamic protocol's AD. For example, if you have an OSPF-learned route (AD 110) to a network, you can add a static route with AD 120 to the same network via a backup link. Under normal conditions, the OSPF route is used. If OSPF fails, the static route becomes active. This ensures redundancy without manual intervention.
Distance-vector protocols (e.g., RIP, EIGRP) send their entire routing table to neighbors periodically and rely on the Bellman-Ford algorithm. They have limited knowledge of the network (only know the direction and distance to destinations). Link-state protocols (e.g., OSPF) send link-state advertisements to form a complete topology map, then run the SPF algorithm to find the shortest path. Link-state protocols converge faster and are less prone to loops, but require more CPU and memory.
OSPF uses cost as its metric. The cost of an interface is calculated as reference bandwidth (default 100 Mbps) divided by the interface bandwidth. For example, a FastEthernet interface (100 Mbps) has a cost of 1 (100/100). A T1 link (1.544 Mbps) has a cost of approximately 64 (100/1.544). The total cost to a destination is the sum of the costs of all outgoing interfaces along the path. The path with the lowest total cost is chosen.
RIP uses hop count as its metric, with a maximum of 15 hops. A route with a hop count of 16 is considered unreachable. This limits the size of a RIP network. Because of this limitation, RIP is suitable only for small networks.
Yes, a router can have both static and dynamic routes. The router will install the route with the lowest administrative distance. For example, if you have a static route to 10.0.0.0/8 (AD 1) and learn a more specific route via OSPF (e.g., 10.1.0.0/16 with AD 110), the static route is used for destinations within 10.0.0.0/8 except for 10.1.0.0/16, which is more specific and will match first. If the static route points to the same network as a dynamic route, the static route wins due to lower AD.
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