
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
```


Links:

https://www.cisco.com/c/en/us/td/docs/ios/qos/command/reference/qos_book/qos_s2.html#wp1064087



Show open TCP-UDP ports on Cisco router

STP contents of a BPDU

STP cost long mode

STP cost short mode

STP determining blocked port using cost

STP direct vs indirect link failure

STP fine tuning the spanning tree protocol

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 - 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 - 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 Interface default broadcast address

Session Border Controller

Session Initiation Protocol

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 Virtual

Stackwise election process

Stackwise user priority

Subscriber templating

Summarization

Supernetting

Switch configuring a routed port

Switch high availability options

Switch protected port limitations

Switch protected port

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 - switched virtual interface (SVI)

Switching - switchport autostate command

Switching

Switchport - IEEE 802.1q

Switchport - ISL

Switchport access

Switchport modes

Switchport trunk configuration ignores access commands

Switchport trunk encapsulation

Switchport trunk

Symmetric Digital Subscriber Line (SDSL)

Syslog - terminal monitor

Syslog relay or proxy

Syslog severity levels

TCP - Congestion control

TCP - Header

TCP - SEQ and ACK values

TCP - Slow start

TCP - Three-way handshake

TCP - Urgent pointer field

TCP - Window Size Scaling

TCP - factors affecting segment size

TCP - role of checksum field

TCP MSS and window size

TCP RST flag

TCP SYN timeout