JNCIA Cheat Sheet 2026

The 30 highest-yield JNCIA facts, distilled from real exam questions. Print it, save it as a PDF, or study it here — free, no sign-up.

1. What should you do when troubleshooting a implementation planning issue? → Follow a systematic approach: identify, research, test, implement, verify
2. What is the Routing Engine (RE) responsible for in Junos architecture? → Running routing protocols, managing configuration, and maintaining the routing table
3. In Junos, what command enables OSPF on an interface? → set protocols ospf area 0.0.0.0 interface ge-0/0/0.0
4. What should you do when troubleshooting a network configuration issue? → Follow a systematic approach: identify, research, test, implement, verify
5. Which Junos platform is specifically designed for service provider edge routing with carrier-grade scalability? → MX Series
6. What is the function of ARP in networking? → Resolve IP addresses to MAC addresses
7. What is the best practice for monitoring network configuration systems? → Implement automated monitoring with alerting thresholds
8. What is the primary purpose of Class of Service (CoS) in Junos OS? → To prioritize traffic based on application or user needs
9. What is the default preference value for OSPF routes in Junos? → 100
10. When planning IP addressing for a Junos network, which RFC defines private IPv4 address ranges? → RFC 1918 — 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16
11. In Junos, what command shows the MPLS label forwarding table? → show route table mpls.0
12. Which user interface is used to provide a graphical view for managing Junos OS devices? → J-Web
13. How should you document performance monitoring configurations? → Maintain up-to-date documentation in a centralized, accessible location
14. What is the primary function of the Junos OS? → Serve as a modular and scalable network operating system
15. In Junos, what command verifies whether a specific prefix is in the routing table? → show route [prefix] exact
16. Which address family must be configured when creating a Junos firewall filter that matches IPv4 packets? → family inet
17. Which Junos feature prevents IP spoofing by verifying packets arrive on the expected interface? → Unicast Reverse Path Forwarding (uRPF)
18. How should you document data management configurations? → Maintain up-to-date documentation in a centralized, accessible location
19. What security consideration is most important in implementation planning? → Implementing the principle of least privilege and defense in depth
20. What is the significance of the 'inet.0' routing table in Junos? → It is the main IPv4 unicast routing table used for unicast forwarding decisions
21. Which Junos feature allows multiple configuration changes to be grouped and committed together? → Candidate configuration
22. In Junos, which command shows the configuration difference between the active and candidate configuration? → show | compare
23. What is the best practice for monitoring performance monitoring systems? → Implement automated monitoring with alerting thresholds
24. What is the Junos command to verify an LDP-signaled MPLS LSP end-to-end? → ping mpls ldp fec [prefix/len] ttl 255
25. What is the key difference between the 'reject' and 'discard' actions in a Junos firewall filter? → 'Reject' sends an ICMP unreachable to the sender; 'discard' drops the packet silently
26. What should you do when troubleshooting a security fundamentals issue? → Follow a systematic approach: identify, research, test, implement, verify
27. Which Junos command saves the current configuration to a file for backup? → save [filename]
28. What is the default behavior of a Junos SRX security policy when no policy matches traffic between two zones? → Traffic is denied (implicit deny-all at end of policy)
29. What is the Junos default BGP hold time? → 90 seconds
30. Which firewall filter action causes Junos to evaluate the current packet against the next term in the filter rather than stopping? → next term