How Routing Works
Routers forward packets between networks using a routing table
When a router receives a packet, it checks the destination IP against its routing table, finds the longest prefix match, and forwards the packet out the appropriate interface.
Longest Prefix Match: The most specific route (highest prefix length) always wins. /28 beats /24 beats /0.
Sample Routing Table (show ip route)
| Code | Network | AD/Metric | Next Hop / Interface |
|---|---|---|---|
| C | 192.168.1.0/24 | 0/0 | directly connected, Gi0/0 |
| L | 192.168.1.1/32 | 0/0 | local, Gi0/0 |
| S | 10.0.0.0/8 | 1/0 | via 192.168.1.254 |
| O | 172.16.0.0/16 | 110/2 | via 10.1.1.2, Gi0/1 |
| S* | 0.0.0.0/0 | 1/0 | via 209.165.200.2 |
C = Connected
L = Local
S = Static
O = OSPF
R = RIP
S* = Default route
Administrative Distance
Router's preference when multiple protocols know the same route
Lower AD = more trustworthy. If a route is learned by two different protocols, the one with the lower AD is installed in the routing table.
| Source | AD Value |
|---|---|
| Connected interface | 0 |
| Static route | 1 |
| EIGRP summary | 5 |
| OSPF | 110 |
| RIP | 120 |
| Unknown / Unreachable | 255 |
Floating static route: Set AD higher than the dynamic protocol (e.g. AD 115 for a static backup to an OSPF route). It only activates if OSPF fails.
Static Routing
Manually configured routes — no routing protocol needed
When to use: Stub networks (one path only), small topologies, default routes to the internet, or as backup routes.
Static Route Syntax
R1(config)# ip route network mask {next-hop-IP | exit-interface} [AD]
# Via next-hop IP (recommended)
R1(config)# ip route 10.0.0.0 255.0.0.0 192.168.1.254
# Via exit interface (point-to-point only)
R1(config)# ip route 10.0.0.0 255.0.0.0 GigabitEthernet0/1
# Default route (gateway of last resort)
R1(config)# ip route 0.0.0.0 0.0.0.0 209.165.200.2
# Floating static (AD=115, backup to OSPF)
R1(config)# ip route 10.0.0.0 255.0.0.0 192.168.1.254 115
OSPFv2 Concepts
Open Shortest Path First — link-state routing protocol
OSPF routers build a complete map of the network (LSDB) and use Dijkstra's SPF algorithm to calculate the shortest path. Metric = cost (reference BW ÷ interface BW).
OSPF Neighbor Adjacency States
1
Down
2
Init
3
2-Way
4
ExStart
5
Exchange
6
Loading
7
Full ✓
Hello interval10s (broadcast) / 30s (NBMA)
Dead interval4× Hello (40s / 120s)
Reference BW100 Mbps (default)
Cost = 100,000,000 ÷ BW (bps)
DR/BDR electionHighest priority, then highest Router ID
Router ID priorityManual → Loopback → Highest active IP
OSPFv2 Configuration
Single-area OSPF setup on a Cisco router
Basic OSPF Configuration
R1(config)# router ospf 1
R1(config-router)# router-id 1.1.1.1
# Advertise networks (wildcard mask)
R1(config-router)# network 192.168.1.0 0.0.0.255 area 0
R1(config-router)# network 10.1.1.0 0.0.0.3 area 0
# Passive interface (don't send Hellos)
R1(config-router)# passive-interface GigabitEthernet0/0
# Advertise default route into OSPF
R1(config-router)# default-information originate
# Adjust reference bandwidth (match network)
R1(config-router)# auto-cost reference-bandwidth 1000
OSPF Verification Commands
# View OSPF neighbor table
R1# show ip ospf neighbor
Neighbor ID Pri State Dead Time Interface
2.2.2.2 1 Full/DR 00:00:34 Gi0/1
3.3.3.3 1 Full/BDR 00:00:38 Gi0/1
# View OSPF-learned routes
R1# show ip route ospf
# Check OSPF process details
R1# show ip ospf
R1# show ip ospf interface brief
# View link-state database
R1# show ip ospf database
Routing & OSPF Drills
Multiple choice questions on static routing, OSPF concepts, and verification.
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QUESTION 1 · ROUTING
Packet Tracer Labs
Step-by-step routing configuration walkthroughs.
Routing Topology Diagrams
Routing Cheatsheet
Static Route Commands
# Standard static route
R1(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2
# Default route (quad-zero)
R1(config)# ip route 0.0.0.0 0.0.0.0 209.165.200.2
# Floating static (backup, higher AD)
R1(config)# ip route 192.168.2.0 255.255.255.0 10.0.0.2 115
# Verify routing table
R1# show ip route
R1# show ip route static
R1# ping 192.168.2.1
R1# traceroute 192.168.2.1
OSPF Configuration
# Enable OSPF process
R1(config)# router ospf 1
R1(config-router)# router-id 1.1.1.1
# Advertise networks
R1(config-router)# network 192.168.1.0 0.0.0.255 area 0
R1(config-router)# network 10.1.1.0 0.0.0.3 area 0
# Suppress Hellos on LAN-facing ports
R1(config-router)# passive-interface Gi0/0
# Set interface cost manually
R1(config-if)# ip ospf cost 10
# Set OSPF priority (DR election)
R1(config-if)# ip ospf priority 100
OSPF Verification
# Neighbor table (must reach Full)
R1# show ip ospf neighbor
# OSPF-learned routes only
R1# show ip route ospf
# Process info (Router ID, Area, SPF)
R1# show ip ospf
# Per-interface OSPF info
R1# show ip ospf interface brief
R1# show ip ospf interface Gi0/1
# Full link-state database
R1# show ip ospf database
# Clear OSPF process (use carefully!)
R1# clear ip ospf process
Key Concepts Quick Reference
OSPF AD110
Static AD1
OSPF metricCost (ref BW ÷ link BW)
Hello / Dead10s / 40s (broadcast)
Multicast Hello224.0.0.5 (all OSPF)
Multicast LSU224.0.0.6 (DR/BDR)
DR electionHighest priority → Router ID
Default priority1 (0 = never DR)
Backbone areaArea 0
SPF algorithmDijkstra's
Interactive Calculators
Tools to speed up exam calculations.
OSPF Cost Calculator
Cost = Reference BW ÷ Interface BW (min cost = 1)
1
OSPF Cost
Common interface costs (ref=100 Mbps)
Ethernet (10M)10
FastEthernet (100M)1
GigabitEthernet (1G)1 ⚠ same as FE!
With ref=1000M: GE1 · FE = 10 · 10M = 100
Fix: set
auto-cost reference-bandwidth 1000 so GE costs 1 and FE costs 10.
AD / Route Type Reference
Administrative Distance cheat sheet
| Route Source | AD |
|---|---|
| Connected | 0 |
| Static route | 1 |
| EIGRP summary | 5 |
| External BGP | 20 |
| EIGRP internal | 90 |
| OSPF | 110 |
| IS-IS | 115 |
| RIP | 120 |
| Unreachable | 255 |
Topic Checklist
Track your progress through routing concepts.
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