WAN serial interfaces: Which encapsulation types can be configured on a Cisco serial interface? (Select the correct grouping from: Ethernet, Token Ring, HDLC, Frame Relay, PPP)

Introduction / Context:
This networking question checks familiarity with data link encapsulations supported on routed serial interfaces. Knowing which technologies are valid on serial links is essential for WAN configuration and troubleshooting.

Given Data / Assumptions:

• You are configuring a Cisco serial interface for a WAN connection.
• Candidate encapsulations listed: Ethernet, Token Ring, HDLC, Frame Relay, PPP.
• Assume typical IOS on routers (not switches) and physical serial interfaces (synchronous).

Concept / Approach:
Serial WAN interfaces operate at Layer 2 with WAN-focused encapsulations. On Cisco routers, the common serial encapsulations are HDLC (default), PPP, and Frame Relay. Ethernet and Token Ring are LAN technologies that require different physical interfaces and framing and are not configured as serial encapsulations.

Step-by-Step Solution:

Verification / Alternative check:
On a Cisco router interface, the command set includes 'encapsulation hdlc', 'encapsulation ppp', and 'encapsulation frame-relay'. There are no 'encapsulation ethernet' or 'encapsulation token-ring' commands for serial interfaces.

Why Other Options Are Wrong:
1 and 4: Ethernet is not a serial encapsulation.
2 only: Token Ring is a LAN MAC, not used on serial.
All of the above: Includes LAN encapsulations, which are invalid on serial.

Common Pitfalls:
Confusing physical media types with Layer 2 encapsulations and assuming any familiar LAN protocol can run over serial links.

Final Answer:
3, 4 and 5