N10-009 network implementation explained
Network implementation is worth 20% of CompTIA Network+ N10-009 — around 18 of the 90 questions. This is the building domain: routing, switching, wireless and physical installation.
Routing
- Static versus dynamic routing, and when each is appropriate. Static suits small, stable networks; dynamic scales and adapts.
- Routing protocol families at a conceptual level: distance vector, link state, and the distinction between interior and exterior protocols.
- The default route and what it does.
- How a router selects a path when several exist, and what a routing table contains.
- Router-on-a-stick and inter-VLAN routing — how VLANs talk to each other.
Route selection: prefix length, administrative distance and metric
Objective 2.1 names three route selection criteria: administrative distance, prefix length and metric. Objective 5.3 in the troubleshooting domain comes back to the same ground with the routing table and default routes. Network+ wants you to apply them in the right order, not to memorise every protocol’s internals.
The order is what decides questions:
- Longest prefix match. For each packet the router looks for the most specific route that contains the destination address. A /26 beats a /24, which beats a /16, which beats the default route 0.0.0.0/0.
- Administrative distance (AD) only matters when two sources, such as a static route and OSPF, offer the same prefix and mask. Lower AD is more trusted and wins the place in the routing table. Cisco’s defaults: connected 0, static 1, OSPF 110 and RIP 120.
- Metric breaks ties between routes from the same protocol: hop count for RIP, cost for OSPF, a composite of bandwidth and delay for EIGRP. Metrics from different protocols are never compared with each other.
A default route is the route of last resort: it matches every destination, so it is used only when nothing more specific exists. On a router it is shown as the gateway of last resort; on a host it is the default gateway. In troubleshooting scenarios a missing default route shows up as “internal subnets work, the internet does not”, and show route or show ip route (listed in objective 5.5) is the command to check it.
A floating static route uses the AD rule on purpose: a static route configured with an AD higher than the dynamic protocol’s stays out of the table until the dynamic route disappears, which makes it a backup path.
Exam pitfall: do not pick a route because its AD is lower when the prefixes differ. A route learned from OSPF for 10.1.1.64/26 wins over a static route for 10.1.1.0/24 for a packet to 10.1.1.70, because prefix length is checked first and AD never overrides a more specific route. For the full worked routing table, see administrative distance and the routing table.
Dynamic routing protocols: OSPF, EIGRP and BGP
Objective 2.1 lists exactly three dynamic routing protocols, and at this level you need to know the role of each rather than configure them:
| Protocol | Type | Where it is used |
|---|---|---|
| OSPF | Interior, link state, open standard | Inside one organisation; picks the lowest total cost |
| EIGRP | Interior, advanced distance vector, originally Cisco | Inside one organisation, mostly Cisco networks |
| BGP | Exterior, path vector | Between autonomous systems: ISPs, multi-homed companies, cloud providers |
An autonomous system is a network under one administration. Interior protocols find the best path inside one; BGP exchanges reachability between them, and it is the routing protocol of the internet. BGP chooses routes by policy and by the path of autonomous systems a route has crossed, not by speed or a single metric. Its peers are configured manually and it runs over TCP port 179 (RFC 4271), where OSPF and EIGRP discover their neighbours automatically.
Typical clues: a company with two ISPs that wants to exchange routes with both is a BGP scenario. A campus or branch network that must adapt to a failed link is OSPF or EIGRP. A single connection to one ISP needs only a static default route.
Exam pitfall: choosing OSPF because it is the most familiar dynamic protocol when the scenario mentions an ISP, the internet or “between autonomous systems”. The reverse also costs marks: BGP is not the answer for fast convergence inside a single campus. The BGP basics guide covers autonomous system numbers, eBGP versus iBGP and the path attributes.
First hop redundancy and virtual IP
Objective 2.1 also lists First Hop Redundancy Protocol (FHRP), virtual IP (VIP) and subinterfaces under routing technologies. FHRP solves one specific problem: a host has a single default gateway address and cannot switch to another router by itself when that gateway fails.
With an FHRP, two or more routers share a virtual IP address and a virtual MAC address. Hosts use the virtual IP as their default gateway. One router at a time answers for it and forwards the traffic; the others listen and take over if it stops sending hello messages. The hosts notice nothing, because their gateway address and the MAC they resolved for it do not change.
The two protocols to recognise:
- HSRP is Cisco-proprietary. The forwarding router is active, the backup is standby.
- VRRP is the open standard (RFC 5798). The forwarding router is master, the backups are backup.
In both, the router with the highest priority takes the forwarding role. Whether a recovered router with a higher priority takes the role back depends on preemption, which is off by default in HSRP and on by default in VRRP.
A virtual IP is not only an FHRP idea. Load balancers and clustered firewalls also present a VIP that clients connect to while the real devices behind it change, so the general meaning is “an address not tied to one physical interface”.
Exam pitfall: FHRP protects the default gateway, not the links between switches. If the scenario says hosts lose connectivity when one router fails and must not be reconfigured, the answer is an FHRP with a virtual IP. Link aggregation and spanning tree address different problems. The HSRP and VRRP guide covers states, timers and the configuration.
Switching
The densest part of the domain:
- VLANs — logical segmentation on shared hardware. The answer whenever a scenario wants isolation without new switches.
- Trunking — carrying multiple VLANs over one link between switches, and tagging.
- Spanning Tree Protocol — preventing loops in a switched network with redundant paths. Know why loops are catastrophic: broadcast storms bring a segment down entirely.
- Port configuration: access versus trunk ports, speed and duplex.
- Port security, and link aggregation for bandwidth and redundancy.
Duplex mismatch is worth remembering as a distinct symptom: the link works, but performance is poor and errors accumulate. It appears in troubleshooting as well.
Wireless
- Standards and frequencies. 2.4 GHz travels further and penetrates better but is crowded and has only three non-overlapping channels (1, 6, 11). 5 GHz and 6 GHz offer more channels and speed over shorter range.
- Channel planning — overlapping access points on the same channel interfere. Non-overlapping assignment is the standard fix.
- Access point placement, coverage and overlap for roaming.
- Interference sources: other networks, cordless phones, microwaves, physical obstructions.
- Wireless security: WPA2 and WPA3, and enterprise versus pre-shared key authentication.
Wireless questions are usually answerable from channel overlap, range or interference rather than from standard version numbers.
Physical installation
- Racks, rack units, and mounting.
- Power: UPS, PDU, and redundancy.
- Cooling and airflow, including hot and cold aisles.
- Cable management, labelling, and patch panels.
- Environmental monitoring.
Practical and easy marks, but frequently skipped in revision.
Sample questions
Question 1. Two departments share a switch but their traffic must be separated, and no budget exists for additional hardware. What should be implemented?
- A. A crossover cable between the departments
- B. VLANs configured on the existing switch
- C. Spanning Tree Protocol
- D. Link aggregation on the uplink
Show answer
Answer: B
VLANs provide logical separation on existing switch hardware, which meets the isolation requirement without new equipment. A crossover cable connects two devices, spanning tree prevents loops rather than segmenting traffic, and link aggregation increases bandwidth.
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Question 2. A redundant link is added between two switches for resilience. Shortly afterwards the network becomes unusable with extremely high traffic. What was most likely missing?
- A. A DHCP reservation for each switch
- B. A longer DHCP lease duration
- C. A faster uplink between the switches
- D. Spanning Tree Protocol to prevent the switching loop
Show answer
Answer: D
A redundant path in a switched network creates a loop, and without Spanning Tree Protocol broadcast frames circulate indefinitely, producing a broadcast storm. A DHCP reservation, a longer lease and a faster uplink would not cause or prevent this.
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Question 3. A file transfer over a 1 Gbps link runs far slower than expected, and the switch port shows a rising error count although the link is up. What is the most likely cause?
- A. A duplex mismatch between the switch port and the host
- B. A completely failed patch cable
- C. The port is assigned to the wrong VLAN
- D. A weak wireless signal
Show answer
Answer: A
A working link with poor throughput and accumulating errors is the classic signature of a duplex mismatch between the two ends. A failed cable would break the link entirely, an incorrect VLAN would prevent communication rather than slow it, and a weak wireless signal is irrelevant to a wired link.
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Question 4. A router has a static route to 172.16.0.0/16 and an OSPF route to 172.16.5.0/24. A packet arrives for 172.16.5.20. Which route does the router use?
- A. The static route, because static routes have a lower administrative distance
- B. Both routes, load balanced
- C. The OSPF route, because it is the longest prefix match
- D. The default route
Show answer
Answer: C
The router first looks for the longest prefix that contains the destination. The /24 is more specific than the /16, so the OSPF route is used. Administrative distance is only compared when two sources offer the same prefix, so the static route’s lower AD does not apply here. The default route is used only when nothing more specific matches.
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Question 5. Two routers serve a user VLAN. When the router configured as the users' default gateway fails, users lose access to other networks until their settings are changed. What prevents this without reconfiguring hosts?
- A. A First Hop Redundancy Protocol with a virtual IP as the default gateway
- B. Link aggregation between the two routers
- C. Spanning Tree Protocol on the access switch
- D. A longer DHCP lease time
Show answer
Answer: A
A First Hop Redundancy Protocol such as HSRP or VRRP lets the routers share a virtual IP that hosts use as their gateway; if the forwarding router fails, the other takes over the virtual IP. Link aggregation bundles links, spanning tree prevents switching loops, and a longer DHCP lease does not change the gateway.
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What to practise
If you have access to a switch, configure a VLAN, make a port an access port and another a trunk, and watch what can and cannot reach what. If you do not, use a network simulator — several free ones cover everything in this domain.
Then draw a network with a redundant link and trace what a broadcast frame does without spanning tree. Understanding why the loop is catastrophic makes that question permanent.