
To display a list of open [TCP](/tcp) and [UDP](/udp) ports on a Cisco IOS router, use the following command:

`show control-plane host open-ports`

An example of the output of such a command can be seen below:

```
Router# show control-plane host open-ports

Active internet connections (servers and established)
Port        Local Address      Foreign Address                  Service    State
 tcp                 *:23                  *:0                   Telnet   LISTEN
 tcp                 *:53                  *:0               DNS Server   LISTEN
 tcp                 *:80                  *:0                HTTP CORE   LISTEN
 tcp               *:1720                  *:0                    H.225   LISTEN
 tcp               *:5060                  *:0                      SIP   LISTEN
 tcp                 *:23      192.0.2.18:58714               Telnet   ESTABLISHED
 udp                 *:53                  *:0               DNS Server   LISTEN
 udp                 *:67                  *:0            DHCPD Receive   LISTEN
 udp              *:52824                  *:0                  IP SNMP   LISTEN
 udp                *:161                  *:0                  IP SNMP   LISTEN
 udp                *:162                  *:0                  IP SNMP   LISTEN
 udp               *:5060                  *:0                      SIP   LISTEN
 udp               *:2517                  *:0                CCH323_CT   LISTEN
```


Show open TCP-UDP ports on Cisco router

STP how a switch identifies a BPDU

STP how to find the root bridge

STP interface type

STP prio nbr value

STP topology change process

Security - 802.1x with WLC and AAA server

Security - AAA for RESTCONF

Security - Best Practices for Repurposing Network Devices

Security - Cisco AnyConnect Secure Mobility Client

Security - Cisco Secure Client

Security - Cisco VPN Client

Security - Cisco VPN client software

Security - Diffie-Hellman groups

Security - Extensible Authentication Protocol over LAN (EAPOL)

Security - GETVPN

Security - IEEE 802.1X

Security - IOS Usernames and Passwords

Security - IPsec nonce

Security - Kerberos

Security - MAC Authentication Bypass

Security - Network Access Control (NAC)

Security - Next Generation Firewall

Security - Secure Unique Device Identifier (SUDI)

Security - WPA 4-way handshake

Security - WebVPN

Security - accessing VTY from another VRF

Security - assigning privilege levels, syntax and behavior

Security - bogons

Security - broadcast-multicast storm

Security - configuring TACACS from another VRF

Security - privilege levels and command output

Security - privilege levels and enable keyword

Security - public key exchange

Security - root CAs and private keys

Security - spoofing

Security - storm control algorithm

Security IKE policy hash command

Security Trusted Platform Module (TPM)

Security aaa new-model

Security authentication on VTY lines

Security keyrings

Segment Routing

Serial - Data Communication Equipment (DCE)

Serial - Data Terminal Equipment(DTE)

Serial - Link between two routers

Serial Interface - Clock Rate

Serial Interface default broadcast address

Session Border Controller

Session Initiation Protocol

Spanning Tree Portfast Configuration Options

Spanning Tree Protocol (STP)

Spine and leaf architecture - connections to leaf switches

Spine and leaf architecture - connections to spine switches

Spine and leaf architecture - reducing latency

Split-horizon

StackWise - IOS upgrade switch reboot sequence

StackWise Virtual

Stackwise - IOS install mode vs bundle mode

Stackwise election process

Stackwise user priority

Static ARP entry for own IP address

Subscriber templating

Summarization

Supernetting

Switch High Availability and the Control Plane

Switch configuring a routed port

Switch high availability options

Switch protected port limitations

Switch protected port

Switched Virtual Interface (SVI)

Switching -  CEF Adjacency Table

Switching - CEF

Switching - Media Access Control (MAC) addresses

Switching - Process switching

Switching - backplane

Switching - cut through

Switching - fast switching

Switching - show cable-diagnostics

Switching - store and forward

Switching - switch fabric

Switching - switchport autostate command

Switching

Switchport - IEEE 802.1q

Switchport - ISL

Switchport - show vlan internal usage

Switchport access

Switchport modes

Switchport trunk configuration ignores access commands

Switchport trunk encapsulation

Switchport trunk

Symmetric Digital Subscriber Line (SDSL)

Syslog - Logging buffered command

Syslog - logging discriminator

Syslog - terminal monitor

Syslog relay or proxy

Syslog severity levels

Syslog

TCP - Congestion control

TCP - Determining the MSS