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CCT: What Causes Serial Link to Show “Up, Line Protocol Down” Between Two Routers?

By: Author Alex Lim

Posted on

Categories Exam

Why does a Cisco serial interface display “up, line protocol down”? Learn how encapsulation mismatches cause this issue and how to resolve it—essential for the Cisco Certified Field Technician (CCT) exam.

Table of Contents

Question

A technician needs to check the status of a serial link between two routers. The interface shows “up, line protocol down.” What is the most likely cause?

A. Incorrect IP address
B. Encapsulation mismatch
C. Router ACL blocking traffic
D. Layer 1 connectivity issue
E. Routing table corruption

Answer

B. Encapsulation mismatch

Explanation

A mismatch in encapsulation types (such as PPP vs. HDLC) between routers can prevent communication even if the physical link is up.

The most likely cause of a Cisco serial link showing “up, line protocol down” is an encapsulation mismatch between the two routers.

  • The “up” status means the physical (Layer 1) connection is working, but “line protocol down” indicates a failure at Layer 2, typically due to mismatched encapsulation settings on each end of the link.
  • Serial interfaces on Cisco routers default to HDLC encapsulation, but can be manually set to PPP, Frame Relay, or other protocols. If one router is set to HDLC and the other to PPP, the link cannot establish Layer 2 communication, resulting in the observed status.
  • Other possible causes for this status include clock rate issues, keepalive mismatches, or hardware problems, but encapsulation mismatch is the most common and should be checked first.

A mismatch in encapsulation types (such as one side using PPP and the other using HDLC) prevents Layer 2 communication, causing the interface to show “up, line protocol down” even though the physical connection is intact. Matching the encapsulation type on both ends resolves the issue.

Cisco Certified Field Technician (CCT) certification exam practice question and answer (Q&A) dump with detail explanation and reference available free, helpful to pass the Cisco Certified Field Technician (CCT) exam and earn Cisco Certified Field Technician (CCT) certification.