Techzone/Cisco Networking

Cisco Networking

3 min readArticle

Cisco dominates enterprise networking — routers, switches, firewalls (ASA), wireless controllers (WLC), and everything in between. IOS (Internetwork Operating System) is the CLI you'll live in. Knowing Cisco basics is essential for network security work, assessments, and blue team roles.

IOS CLI Basics

shell 
! -- Comment in configs
> -- User EXEC mode (limited)
# -- Privileged EXEC mode (do everything)
(config)# -- Global configuration mode
(config-if)# -- Interface configuration mode
bash 
# Enter privileged mode
enable
# (enter enable password)

# Enter global config
configure terminal  # or conf t

# Exit config mode
exit  # or Ctrl+Z

# Show running config
show running-config
show run

# Show specific interface
show interface GigabitEthernet0/0
show ip interface brief

# Save config to NVRAM
write memory  # or copy running-config startup-config

Basic Router Config

shell 
! Set hostname
Router(config)# hostname Core-Router

! Set enable secret (encrypted)
Core-Router(config)# enable secret MyStrongPassword

! Configure interface
Core-Router(config)# interface GigabitEthernet0/0
Core-Router(config-if)# ip address 192.168.1.1 255.255.255.0
Core-Router(config-if)# description "LAN Interface"
Core-Router(config-if)# no shutdown

! Configure default route
Core-Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.254

VLANs (Switches)

shell 
! Create VLAN
Switch(config)# vlan 10
Switch(config-vlan)# name Management

! Assign port to VLAN (access port)
Switch(config)# interface FastEthernet0/1
Switch(config-if)# switchport mode access
Switch(config-if)# switchport access vlan 10

! Trunk port (carries multiple VLANs)
Switch(config)# interface GigabitEthernet0/1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30

! Show VLANs
Switch# show vlan brief
Switch# show interfaces trunk

ACLs (Access Control Lists)

shell 
! Standard ACL (source IP only)
Router(config)# access-list 10 permit 192.168.1.0 0.0.0.255
Router(config)# access-list 10 deny any

! Extended ACL (source, dest, protocol, port)
Router(config)# access-list 100 permit tcp 192.168.1.0 0.0.0.255 any eq 80
Router(config)# access-list 100 deny ip any any log

! Apply to interface
Router(config-if)# ip access-group 100 in

! Named ACL
Router(config)# ip access-list extended BLOCK-SSH
Router(config-ext-nacl)# deny tcp any any eq 22
Router(config-ext-nacl)# permit ip any any

Useful Show Commands

bash 
show version            # IOS version, uptime, hardware
show ip route           # Routing table
show arp                # ARP table
show cdp neighbors      # Cisco Discovery Protocol (reveals network topology!)
show interface status   # Quick interface overview
show mac address-table  # MAC table on switch
show spanning-tree      # STP info
show ip ospf neighbor   # OSPF peers
show crypto isakmp sa   # VPN tunnels

CDP Security Note

CDP (Cisco Discovery Protocol) is enabled by default and leaks device model, IOS version, IP addresses, and native VLAN to adjacent devices. In security assessments, CDP is a goldmine:

shell 
# Disable CDP globally (security best practice)
Router(config)# no cdp run

# Or per-interface
Router(config-if)# no cdp enable

Password Recovery / Security Issues

  • Default credentials for many Cisco devices: cisco/cisco, admin/admin
  • Telnet (clear text) vs SSH — always use SSH
  • Enable SSH on Cisco:
shell 
Router(config)# ip domain-name company.local
Router(config)# crypto key generate rsa modulus 2048
Router(config)# ip ssh version 2
Router(config)# line vty 0 4
Router(config-line)# transport input ssh

See Also

  • fortigate-firewall-guide - Fortinet competitor
  • networking/index - Networking overview
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