Switching, Routing, VLANs & Troubleshooting
A practical CCNA networking review sheet built around concepts and commands you should recognize quickly.
Save / Print as PDFPractice CCNASwitching Essentials
Switches learn source MAC addresses and use their MAC address tables to forward Ethernet frames. A VLAN creates a logical Layer 2 broadcast domain. Access ports normally carry traffic for one VLAN; trunks can carry multiple VLANs between network devices.
show interfaces status
show vlan brief
show interfaces trunk
IPv4 & Routing
Network directly attached to a router interface.
Route manually configured by an administrator.
Used when no more-specific route matches.
Routers learn routes using a routing protocol.
show ip route
ping 192.0.2.1
traceroute 192.0.2.1
OSPF Basics
OSPF is a link-state interior gateway routing protocol. Routers form neighbor relationships and calculate routes using link-state information. When troubleshooting, check interface state, addressing, network reachability and OSPF neighbor formation.
show ip route ospf
show ip protocols
ACL Thinking
Access control lists evaluate traffic against ordered rules. More specific intended matches should be considered before broad rules, and an unmatched packet eventually encounters the implicit deny. Always verify direction, interface placement, source/destination and protocol criteria.
Troubleshooting by Layer
Cable, optics, power, interface state.
VLAN, trunk, MAC learning, STP.
IP, mask, gateway, routes, ACLs.
DNS, DHCP and application reachability.
Fast habit: start with show ip interface brief. A surprising interface state or IP address can immediately narrow the problem.
Mini Check
Two PCs are connected to the same switch but intentionally assigned to different VLANs. They need to communicate. What additional capability is required?
Answer: Layer 3 routing between the VLANs, commonly called inter-VLAN routing.
Go from recognition to troubleshooting
After reviewing commands, practice scenario questions where you must identify the most likely fault or next verification step.
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