Command Center · Reference 04
Cisco & Networking Quick Reference
Fast lookup for interfaces, addressing, VLANs, trunks, spanning tree, routing, ACLs, port security, DHCP, SSH, discovery, logging, and troubleshooting.
Use show commands first. Understand the topology and required connectivity before changing a router or switch.
Network Rule
Before changing a network device
Connectivity problems are often layered
A failed connection can come from interface state, VLAN placement, trunk configuration, routing, ACLs, name resolution, or the endpoint itself. Inspect the path in order instead of changing several things at once.
Cisco IOS / Networking
Device Baseline
show version
IOS version, uptime, hardware, image, and system details.
Caution
Record the baseline before changing configuration.
show running-config
Active configuration currently in memory.
Caution
Running-config can differ from startup-config.
show startup-config
Saved configuration used after reload.
Caution
Compare it with running-config when unsaved changes matter.
show clock detail
Device time and time-source details.
Caution
Incorrect time can make logs misleading.
Cisco IOS / Networking
Interfaces
show ip interface brief
Fast view of interface IP addresses, status, and protocol state.
Caution
Read both status columns.
show interfaces
Detailed counters, errors, duplex, bandwidth, and operational state.
Caution
Use counters as evidence, not as automatic proof of failure.
show interfaces status
Switch-port status, VLAN, duplex, speed, and type.
Caution
Interpret it with VLAN and interface configuration.
show interfaces description
Interface status and configured descriptions.
Caution
Descriptions can be outdated.
Cisco IOS / Networking
IPv4 & Routing
show ip route
IPv4 routing table and route sources.
Caution
Check route source and next hop before changing anything.
show ip route <network>
Best route for a selected network.
Caution
Use the correct prefix/network.
show arp
IPv4-to-MAC mappings learned by the device.
Caution
ARP entries are dynamic and can age out.
show ip protocols
Active routing protocols and related settings.
Caution
Use this with running-config and neighbor information.
Cisco IOS / Networking
IPv6
show ipv6 interface brief
IPv6 addresses and interface state.
Caution
Interfaces can have multiple IPv6 addresses.
show ipv6 route
IPv6 routing table.
Caution
Interpret route source and prefix length first.
show ipv6 neighbors
IPv6 neighbor-discovery mappings.
Caution
Neighbor entries are dynamic evidence.
Cisco IOS / Networking
Switching & VLANs
show mac address-table
Learned MAC addresses and associated ports/VLANs.
Caution
Dynamic MAC entries can move.
show vlan brief
VLAN names, status, and access-port membership.
Caution
Trunk behavior is not fully shown here.
show interfaces trunk
Trunk ports, native VLANs, and allowed/active VLANs.
Caution
A trunk can exist while some VLANs are not carried.
show interfaces switchport
Access/trunk mode, access VLAN, native VLAN, and allowed VLAN details.
Caution
Compare administrative and operational mode.
Cisco IOS / Networking
Spanning Tree
show spanning-tree
Root bridge, port roles, path cost, and STP state.
Caution
Do not change STP without understanding topology.
show spanning-tree vlan <vlan-id>
STP details for one VLAN.
Caution
Blocked ports can be intentional.
show spanning-tree root
Root-bridge summary.
Caution
Unexpected root placement should be investigated first.
Cisco IOS / Networking
Access Control Lists
show access-lists
Configured ACL entries and match counters where available.
Caution
ACL order matters because first match wins.
show ip interface
Shows inbound/outbound IPv4 ACLs applied to interfaces.
Caution
An ACL in configuration is not necessarily applied.
show running-config | section access-list
ACL-related configuration blocks.
Caution
Named and numbered syntax can differ.
Cisco IOS / Networking
Port Security
show port-security
Port-security summary on supported switches.
Caution
Use detailed interface output for one port.
show port-security interface <interface>
Maximum MAC count, violations, sticky MACs, and secure addresses.
Caution
Do not clear secure addresses without justification.
show running-config interface <interface>
Configured switch-port settings.
Caution
Compare configuration with current state.
Cisco IOS / Networking
DHCP & DNS
show ip dhcp binding
DHCP leases issued by the device.
Caution
Bindings are dynamic.
show ip dhcp pool
DHCP pool use and address ranges.
Caution
Check exclusions and helper addresses too.
show hosts
Cached host mappings and name-server information where supported.
Caution
Cached entries may be stale.
show running-config | include name-server
Configured DNS server lines.
Caution
Management and services may depend on DNS.
Cisco IOS / Networking
SSH & Device Management
show running-config | section line vty
VTY remote-management configuration.
Caution
Review transport input, login method, and restrictions.
show ip ssh
SSH status and version details.
Caution
SSH may be required for management.
show users
Users currently connected to device lines.
Caution
Current presence does not prove misuse.
show line
Console, auxiliary, and VTY line state.
Caution
Use it to understand active management sessions.
Cisco IOS / Networking
CDP & LLDP
show cdp neighbors
Directly connected Cisco neighbors.
Caution
CDP may be intentionally disabled.
show cdp neighbors detail
Neighbor platform, address, ports, and capabilities.
Caution
Treat discovery data as topology evidence.
show lldp neighbors
Neighbors discovered with LLDP.
Caution
LLDP may not be enabled everywhere.
show lldp neighbors detail
Detailed LLDP neighbor information.
Caution
Neighbor data can change when links move.
Cisco IOS / Networking
Logging & Time
show logging
Logging configuration and buffered messages.
Caution
Buffer size and severity can limit evidence.
show clock detail
Current device time and source.
Caution
Incorrect time undermines correlation.
show ntp associations
NTP peers and association state.
Caution
Configured does not always mean synchronized.
show ntp status
Synchronization and stratum details.
Caution
Output varies by platform/IOS version.
Cisco IOS / Networking
Troubleshooting
ping <destination>
Tests IP reachability.
Caution
Success does not prove every application works.
traceroute <destination>
Shows the path toward a destination when replies are available.
Caution
Some devices do not answer probes.
show ip route <destination>
Checks route selection for a destination.
Caution
Use routing evidence before guessing.
show interfaces counters errors
Interface error counters on supported switches.
Caution
Interpret counters over time and with traffic volume.
Route codes
Common route-source reminders
Interface states
Read the status pair correctly
up / up
Physical and line protocol operational.
down / down
Physical or link problem likely.
up / down
Physical layer up, line protocol problem.
administratively down / down
Interface is shut down by configuration.
Ports
Common service ports worth recognizing
Port numbers are hints, not conclusions. Map the port to the actual service and device role.
Troubleshooting order
Work from the path, not from guesses
Read the scenario and identify required connectivity.
Check interface state and addressing.
Check VLAN or trunk state when switching is involved.
Check the routing table and expected next hop.
Check ACL application and direction.
Check name resolution only after basic IP reachability.
Use ping and traceroute as evidence, not as the whole diagnosis.
After a change, re-run the exact show command that proves the fix.
Common mistakes
Network mistakes that make troubleshooting harder
Changing an interface before recording its current IP, VLAN, status, and description.
Assuming a VLAN problem when the interface is physically down.
Deleting an ACL without checking direction and purpose.
Treating an ACL as active without verifying where it is applied.
Removing a route before checking whether it is static, connected, or dynamically learned.
Changing trunk allowed VLANs without understanding which VLANs must cross the link.
Changing spanning-tree settings without understanding the current root topology.
Using ping alone to decide whether an application is working.