which command is used to know the duplex speed of serial link?

The `show line` command typically displays information about console, auxiliary, and virtual terminal (VTY) lines, primarily focusing on connection status and user sessions, not physical interface characteristics like duplex and speed.

The `show interface <interface_type> <interface_number>` command displays comprehensive statistics and configuration for a specific interface. This output includes crucial operational details such as its link status, line protocol status, configured and negotiated speed, and duplex mode (full, half, or auto), which is applicable to serial links as well as Ethernet interfaces.

The `show protocol` command is not a standard Cisco IOS command; commands like `show ip protocols` display routing protocol information, not interface duplex/speed.

The `show running-config` (or `show run`) command displays the device's current configuration, which might show explicitly configured duplex/speed settings, but it does not show the operational duplex and speed that can be negotiated, unlike `show interface`.