
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 how a switch identifies a BPDU

STP how to find the root bridge

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STP prio nbr value

STP topology change process

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