Routing & Switching Fundamentals (Network+ N10-009)

Routing & Switching Fundamentals

Domain 2 (Network Implementations) covers how data moves between devices on the same network and across different networks. Routing and switching concepts appear across multiple question types on N10-009.

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Routing Fundamentals

Layer 3 Path Selection

Routers forward packets between networks using routing tables. They select the best path using longest prefix match, then administrative distance, then metric.

Static routing: Manually configured; predictable Dynamic routing: Learned via protocol; adaptive Default route: 0.0.0.0/0 — gateway of last resort Selection: Longest prefix → AD → Metric

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Switching Fundamentals

Layer 2 Frame Forwarding

Switches build a MAC address table by learning source MAC addresses per port, then forward, flood, or filter frames based on the destination MAC.

Learn: Record source MAC → port mapping Flood: Unknown destination → all ports (except ingress) Forward: Known destination → specific port Filter: Same-segment traffic → drop

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Routing Protocols

Dynamic Route Learning

Routing protocols exchange network reachability information between routers so each one automatically builds and maintains an accurate routing table.

RIPv2: Distance vector · hop count · AD 120 OSPF: Link state · cost · AD 110 · areas EIGRP: Hybrid · composite · AD 90 BGP: Path vector · policy · internet routing

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VLANs & STP

Logical Segmentation & Loop Prevention

VLANs logically segment a switch into multiple broadcast domains. Spanning Tree Protocol (STP) prevents Layer 2 loops that would crash a network.

Access port: One VLAN · untagged · end devices Trunk port: Multiple VLANs · 802.1Q tagged STP: Elects root bridge · blocks redundant paths RSTP: 802.1w · faster convergence (~1–2s)

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Exam focus: Expect questions on administrative distance values (OSPF=110, RIP=120, EIGRP=90), longest-prefix match priority, the difference between access and trunk ports, STP port states, and when to use static vs dynamic routing. These are consistently tested across N10-009 sessions.

Path Selection Order — How a Router Chooses

1st

Longest Prefix Match

Most specific route wins — /26 beats /24 regardless of protocol or AD

2nd

Administrative Distance

Lower AD wins — EIGRP (90) preferred over OSPF (110) for same prefix

3rd

Metric

Lower metric wins within the same protocol (e.g., lower hop count in RIP)

How It Works

Switch MAC learning, routing table selection, routing protocol characteristics, VLAN port types, and STP port states.

How a Switch Forwards Frames — The 5 Actions

L

Learn — Record Source MAC

Every frame a switch receives has a source MAC address. The switch records that MAC address → ingress port mapping in its CAM (Content Addressable Memory) table. Entries age out after a default of 300 seconds.

F

Flood — Unknown Destination MAC

If the destination MAC is not in the CAM table, the switch sends the frame out all ports except the ingress port. This is also called an unknown unicast flood. Broadcasts (FF:FF:FF:FF:FF:FF) and multicasts are also flooded.

F

Forward — Known Destination MAC

If the destination MAC is in the CAM table, the switch sends the frame out only the port that maps to that MAC address. This is unicast forwarding — efficient and reduces unnecessary traffic.

F

Filter — Same-Segment Traffic

If the destination MAC is on the same port as the source (both devices on the same segment/hub), the switch drops the frame. No need to forward — the destination already received it on the wire.

A

Age — Remove Stale Entries

CAM table entries that haven't been refreshed by a new frame from that MAC are removed after the aging timer (default 300s). This prevents the table from filling with stale entries from moved or disconnected devices.

Administrative Distance (AD) — Route Trustworthiness

Lower AD = more trusted source. When two protocols offer a route to the same prefix, the lower AD wins. This is only used as a tiebreaker after longest prefix match.

Route Source	AD Value	Relative Trust
Directly Connected	0
Static Route	1
eBGP (external BGP)	20
EIGRP (internal)	90
IGRP	100
OSPF	110
IS-IS	115
RIP	120
EIGRP (external)	170
iBGP (internal BGP)	200
Unknown / Unreachable	255

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Key exam values to memorize: Directly connected=0, Static=1, eBGP=20, EIGRP internal=90, OSPF=110, RIP=120. These four (EIGRP, OSPF, RIP) appear most frequently in Network+ questions.

Routing Protocols at a Glance

RIPv2

Distance Vector

Metric: Hop count

Max hops: 15 (16 = unreachable)

AD: 120

Updates: Every 30 seconds (multicast 224.0.0.9)

Algorithm: Bellman-Ford

Use case: Small/legacy networks

OSPF

Link State

Metric: Cost (based on bandwidth)

Max hops: No limit

AD: 110

Updates: Triggered (event-driven)

Algorithm: Dijkstra (SPF)

Use case: Enterprise networks · uses areas (area 0 = backbone)

EIGRP

Hybrid (Cisco)

Metric: Composite (BW + delay)

Max hops: 255 (default 100)

AD: 90 internal / 170 external

Updates: Triggered (DUAL algorithm)

Algorithm: DUAL (Diffusing Update)

Use case: Cisco-only environments

BGP

Path Vector

Metric: Policy / AS path attributes

Max hops: No limit

AD: eBGP=20 / iBGP=200

Updates: Incremental (TCP port 179)

Algorithm: Best path selection

Use case: Internet (between ISPs / ASes)

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IGP vs EGP: RIP, OSPF, and EIGRP are Interior Gateway Protocols (IGP) — used inside a single autonomous system (AS). BGP is an Exterior Gateway Protocol (EGP) — used between autonomous systems (how the internet routes traffic between ISPs and organizations).

VLANs — Access vs Trunk Ports

Access Port

VLANs: One VLAN only Tagging: No 802.1Q tag — frames are untagged Connected to: End devices (PCs, printers, phones) Config: Assigned to a single VLAN ID Traffic: Strips tag on ingress; adds none on egress

Trunk Port

VLANs: Multiple VLANs simultaneously Tagging: 802.1Q — inserts 4-byte VLAN tag in frame Connected to: Switches, routers, Layer 3 switches Native VLAN: One VLAN allowed untagged (default VLAN 1) Traffic: Preserves VLAN tags across the link

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Inter-VLAN routing: Devices in different VLANs cannot communicate at Layer 2 — a Layer 3 device is required. Options: (1) Router-on-a-stick — a single router interface with subinterfaces per VLAN, connected to a trunk port; (2) Layer 3 switch — performs routing internally with SVIs (Switched Virtual Interfaces), the modern enterprise approach.

STP Port States — Spanning Tree Protocol

STP (802.1D) prevents Layer 2 loops by electing a root bridge and blocking redundant paths. Each port transitions through these states on startup:

BlockingMax Age: 20s

→

ListeningForward Delay: 15s

→

LearningForward Delay: 15s

→

ForwardingActive ✅

Blocking: Receives BPDUs; does not forward frames or learn MACs. Redundant ports stay here.

Listening: Sends and receives BPDUs to determine root bridge. Does not forward frames or learn MACs.

Learning: Populates MAC address table. Still does not forward user frames. Prevents flooding on convergence.

Forwarding: Fully active — forwards frames and learns MACs. This is the only state that passes user traffic.

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RSTP (802.1w) replaces 802.1D STP with faster convergence (~1–2 seconds vs 30–50 seconds for STP). Port roles change to: Root port, Designated port, and Alternate/Backup port. Most modern networks run RSTP or its per-VLAN variants (PVST+, RPVST+).

Compare & Reference

Filter by category to study routing, switching, protocols, or VLAN concepts side by side.

Concept	Category	Key Value / Detail	Description	Exam Gotcha
Static Route	Routing	AD = 1; manually configured	Admin manually defines path to destination. Best for small networks or specific paths.	Does not adapt to topology changes — manual update required if link fails
Default Route	Routing	0.0.0.0/0 — matches any	Gateway of last resort — used when no more-specific route exists. Always the least specific prefix.	A more-specific route always overrides the default — longest prefix match
Dynamic Routing	Routing	Protocol-learned; adaptive	Routers exchange routes automatically. Adapts to topology changes without admin intervention.	Higher overhead than static; requires protocol configuration on all routers
Longest Prefix Match	Routing	Most specific route wins first	Router always selects the route with the longest (most specific) prefix, regardless of AD or metric.	A /26 route via RIP beats a /24 route via OSPF — prefix length trumps AD
CAM / MAC Table	Switching	MAC → port mapping	Switch builds this table by learning source MACs. Used to make forwarding decisions for unicast frames.	Full CAM table causes flooding — a MAC flooding attack exploits this
Unknown Unicast Flood	Switching	Out all ports except ingress	When destination MAC is not in CAM table, the frame is sent to all ports in the same VLAN.	Different from a broadcast — it's a unicast frame with an unknown destination
Collision Domain	Switching	Per port on a switch	Each switch port is its own collision domain. Switches eliminate collisions that hubs caused.	A hub has ONE collision domain; a switch has one per port
Broadcast Domain	Switching	Per VLAN / per router interface	All devices that receive a broadcast (FF:FF:FF:FF:FF:FF). Switches forward broadcasts within a VLAN; routers do not forward them between networks.	VLANs segment broadcast domains at Layer 2; routers at Layer 3
RIPv2	Protocol	AD=120 · Hop count · Max 15	Distance vector. Simple, classless. Updates every 30s. 16 hops = unreachable. Best for small or legacy networks.	Max 15 hops makes it unsuitable for large networks
OSPF	Protocol	AD=110 · Cost · No hop limit	Link state. Uses Dijkstra's SPF algorithm. Requires area 0 (backbone). Metric = cost = 100Mbps/interface bandwidth.	All OSPF areas must connect to area 0 (backbone area)
EIGRP	Protocol	AD=90/170 · Composite metric	Hybrid (Cisco proprietary). Uses bandwidth + delay for metric. DUAL algorithm provides fast convergence. Internal AD=90, External=170.	Cisco-only — cannot run on non-Cisco routers
BGP	Protocol	eBGP AD=20 · iBGP AD=200	Path vector. The routing protocol of the internet. Uses TCP port 179. Policy-based routing via AS path attributes.	eBGP (between ASes) AD=20; iBGP (within an AS) AD=200
Access Port	VLAN	1 VLAN · untagged	Carries traffic for a single VLAN. End devices connect here. No 802.1Q tag on frames.	Device connected to an access port has no awareness of VLANs
Trunk Port	VLAN	Multiple VLANs · 802.1Q tagged	Carries frames for multiple VLANs using 802.1Q tags. Used between switches and between switches and routers.	Native VLAN frames are untagged on a trunk — mismatch = security risk
Native VLAN	VLAN	Untagged on trunk · default VLAN 1	The VLAN whose traffic is sent untagged across a trunk link. Must match on both sides of the trunk.	Native VLAN mismatch = frames arrive in wrong VLAN; can be a security vulnerability
STP (802.1D)	VLAN/STP	Loop prevention · 30–50s convergence	Elects a root bridge (lowest bridge priority, then lowest MAC). Redundant paths are put in blocking state.	Slow convergence (30–50s). Replaced by RSTP in modern networks.
RSTP (802.1w)	VLAN/STP	Faster STP · ~1–2s convergence	Backward compatible with 802.1D. Adds Alternate and Backup port roles. Achieves sub-second convergence in many topologies.	Ports roles differ from STP: Alternate (backup to root port), Backup (backup designated port)
Router-on-a-Stick	VLAN	Single trunk → subinterfaces per VLAN	One physical router interface connects to a trunk port. Subinterfaces (e.g., Gi0/0.10, Gi0/0.20) handle inter-VLAN routing for each VLAN.	Single link is a bottleneck — Layer 3 switch is preferred in modern designs

Real-World Scenarios

How routing, switching, protocol, and VLAN concepts appear in exam-style situations.

"A router has two routes to 192.168.10.0 — one /24 learned via OSPF and one /26 learned via RIP. A packet arrives destined for 192.168.10.130. Which route does the router use?"

Scenario 1 — Routing · Longest Prefix Match

192.168.10.130 falls within both 192.168.10.0/24 AND 192.168.10.128/26 (if that's the /26 route).
Step 1: Check prefix length. /26 is more specific than /24 — it matches fewer addresses, so it's more precise.
Longest prefix match wins — the router selects the /26 route via RIP.
Administrative distance is NOT consulted — prefix length is always the first tiebreaker.
AD (OSPF=110 vs RIP=120) only matters when two routes have the SAME prefix length.

✅ Answer: /26 via RIP wins — longest prefix match overrides administrative distance

"A new PC is connected to a switch port. The user can reach devices on the same switch but cannot reach devices across the network. The switch log shows the port is up/up."

Scenario 2 — Switching · VLAN Mismatch

Port is up/up physically — Layer 1 and Layer 2 are functional. Problem is logical.
PC can reach same-switch devices → switch is forwarding within its VLAN.
Cannot reach across network → traffic is not reaching the router or other segments.
Check the access port VLAN assignment — the port may be in VLAN 1 (default) while other devices are in VLAN 10.
Fix: Assign the port to the correct VLAN. Also verify the trunk port to the router includes that VLAN.

✅ Root cause: Access port assigned to wrong VLAN — not matching the rest of the segment

"An admin configures OSPF on all routers in a new network. One branch router cannot form a neighbor relationship with the core router despite the link being up."

Scenario 3 — Routing Protocols · OSPF Neighbor Failure

OSPF neighbor relationships (adjacencies) require matching parameters on both routers.
Check: Are both interfaces in the same OSPF area? A mismatch (area 0 vs area 1) prevents adjacency.
Check: Do the Hello and Dead timer intervals match? (Default: 10s Hello, 40s Dead on broadcast networks.)
Check: Is the subnet mask identical on both sides of the link? OSPF verifies this.
Check: Is authentication configured on one side but not the other?
Fix: Ensure area numbers, timers, subnet masks, and authentication settings match on both routers.

✅ Root cause: OSPF area mismatch or timer mismatch prevented neighbor adjacency formation

"After adding a redundant uplink between two switches, users on the network experience broadcast storms and cannot reach any devices."

Scenario 4 — VLANs & STP · Layer 2 Loop

Adding a second link between switches without loop prevention creates a Layer 2 loop.
A broadcast frame (ARP, DHCP) enters the loop and is forwarded infinitely — causing a broadcast storm.
Switches have no TTL mechanism (that's Layer 3) — frames loop forever at Layer 2.
STP/RSTP should detect the loop and place one port in blocking state — check if STP is enabled on both switches.
If STP is disabled or misconfigured, re-enable it. Alternatively, configure the redundant link as a trunk with RSTP to allow fast failover without the storm.

✅ Root cause: Layer 2 loop from redundant link without STP — infinite broadcast replication

"Devices in VLAN 10 and VLAN 20 on the same switch cannot communicate with each other, even though both VLANs are active."

Scenario 5 — VLANs · Inter-VLAN Routing Required

VLANs are separate broadcast domains — Layer 2 switching cannot move traffic between VLANs.
A Layer 3 device is required to route between VLANs: a router or a Layer 3 switch.
Option A — Router-on-a-stick: Configure a trunk link to the router; create subinterfaces (e.g., .10 and .20) each with an IP address as the default gateway for that VLAN.
Option B — Layer 3 switch: Create SVIs (Switched Virtual Interfaces) for VLAN 10 and VLAN 20, assign IP addresses, and enable IP routing.
Verify default gateways on end devices point to the correct SVI or subinterface IP.

✅ Root cause: No inter-VLAN routing configured — VLANs require a Layer 3 device to communicate

Practice Quiz

10 Network+ N10-009 style scenario questions on routing and switching fundamentals

Question 1 of 10

Routing

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Switching

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Protocols

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VLANs/STP

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🗺️ Route Planner

Describe your network issue to get a targeted diagnostic recommendation.

Memory Hooks

Tap each card to reveal the answer — 8 must-know facts for exam day

Tap any card to flip it

Quick-Recall Mnemonics

AD Values (low to high)

"Don't Start Every Internet Route Badly"
Direct(0) · Static(1) · EIGRP(90) · internal · RIP(120) · BGP(200 iBGP)

STP State Order

"Blocking Lions Love Forests"
Blocking → Listening → Learning → Forwarding

Route Selection Priority

"Prefix, then AD, then Metric"
Longest prefix first → lower AD → lower metric. Protocol doesn't matter if prefix length differs.

Switch Actions

"Learn, Flood, Forward, Filter, Age"
In that logical order — learn source, then decide what to do with destination.