JNCIA Troubleshooting Methods 2 — Questions and Answers

Question 1: Which Junos command captures live packet data on an interface for analysis?

monitor traffic interface ge-0/0/0 (Correct answer)
debug interface ge-0/0/0 capture
packet-capture interface ge-0/0/0
tcpdump ge-0/0/0

Correct answer: monitor traffic interface ge-0/0/0

The 'monitor traffic interface' command in Junos operational mode captures and displays live packet headers (similar to tcpdump) on the specified interface for real-time troubleshooting.

The command 'monitor traffic interface ge-0/0/0' captures packets on the specified interface and displays headers in real time. Options include: 'count N' (capture N packets), 'matching [filter]' (BPF-style filters like 'host 10.0.0.1'), 'detail' (verbose output), 'no-resolve' (skip DNS resolution), and 'write-file filename' (save to pcap file). The capture happens on the RE so it may miss some traffic at high rates. For line-rate capture, external tools or the PFE 'port-mirror' feature is used.

Question 2: What does 'show interfaces ge-0/0/0 extensive' display that 'show interfaces ge-0/0/0' does not?

Detailed error counters including input/output errors, drops, framing errors, and CoS queue statistics (Correct answer)
Only the IP address and status
OSPF neighbor information
The full running configuration of the interface

Correct answer: Detailed error counters including input/output errors, drops, framing errors, and CoS queue statistics

The 'extensive' keyword adds granular error counters (CRC errors, giants, runts, drops), CoS queue statistics, active alarms, and detailed physical layer information to the standard interface output.

'show interfaces extensive' provides: physical layer details (speed, duplex, autoneg), error counters (input errors, output errors, CRC, framing, carrier transitions), traffic rate statistics, CoS queue statistics (packets queued/dropped per queue), RMON statistics, optical diagnostics (if SFP supports DOM), and active alarms. This is the first command to run when investigating interface-level packet loss or errors. High CRC errors suggest physical layer problems (bad cable, SFP, or connector).

Question 3: In Junos, how do you trace the path a packet takes through the network?

traceroute [destination] (Correct answer)
trace route [destination]
show path [destination]
debug route [destination]

Correct answer: traceroute [destination]

The 'traceroute' command in Junos operational mode sends probes with increasing TTL values to discover each hop between the local router and the destination.

In Junos operational mode, 'traceroute [destination]' works similarly to Unix traceroute. Options include: 'source [IP]' (specify source address), 'routing-instance [name]' (use a specific VRF), 'ttl [max]' (maximum hops), 'wait [seconds]' (probe timeout), 'no-resolve' (skip DNS), 'inet' (force IPv4), 'inet6' (force IPv6). 'traceroute monitor [destination]' provides continuous traceroute. Results show hop IPs, RTT, and ICMP error types. Asterisks (*) indicate non-responsive hops.

Question 4: What is the purpose of Junos 'traceoptions' in protocol configuration?

Enables detailed debug logging for specific protocols to a file for troubleshooting (Correct answer)
Traces the route a packet takes
Captures packets on all interfaces simultaneously
Monitors CPU utilization during protocol convergence

Correct answer: Enables detailed debug logging for specific protocols to a file for troubleshooting

Traceoptions in Junos configuration enables protocol-level debug logging (e.g., OSPF packet exchange, BGP UPDATE messages) to a specified file in /var/log/ for detailed troubleshooting.

Traceoptions are configured under each protocol hierarchy, e.g., [edit protocols ospf traceoptions]. Common settings: 'file filename' (log file in /var/log/), 'flag all' (log everything), specific flags like 'flag packets', 'flag hello', 'flag spf'. The log is viewed with 'show log filename' or 'monitor start filename'. Traceoptions can generate high log volume — use specific flags rather than 'all' in production. Always disable traceoptions after troubleshooting to avoid log file growth.

Question 5: Which command in Junos shows the status of all BGP peers?

show bgp summary (Correct answer)
show bgp neighbors
show protocols bgp
display bgp peers

Correct answer: show bgp summary

'show bgp summary' provides a concise table of all BGP neighbors with their state (Established, Active, Idle), received prefixes, and uptime — the fastest way to assess BGP health.

'show bgp summary' displays: peer IP, AS number, state (Established/Active/Idle/Connect/OpenSent/OpenConfirm), uptime, received prefixes, and the number of routes. 'show bgp neighbor [IP]' provides detailed per-neighbor information including capabilities, timers, error codes, and message statistics. 'show route receive-protocol bgp [peer-IP]' shows received routes before policy. 'show route advertising-protocol bgp [peer-IP]' shows routes being advertised to a peer.

Question 6: What does a 'martian' route mean in Junos troubleshooting context?

A route that Junos refuses to install because the destination is a reserved or invalid address (e.g., 0.0.0.0/0, 127.x.x.x) (Correct answer)
A route learned from an untrusted BGP peer
A route with an infinite metric
A route pointing to a null interface

Correct answer: A route that Junos refuses to install because the destination is a reserved or invalid address (e.g., 0.0.0.0/0, 127.x.x.x)

Martian routes are addresses that should never appear as routable destinations (loopback, link-local, broadcast, multicast used as unicast destinations). Junos drops these to prevent routing anomalies.

Junos defines martian addresses as prefixes that should not be in the unicast routing table. By default, these include: 0.0.0.0/8 (anything starting with 0), 127.0.0.0/8 (loopback), 192.0.0.0/24 (IANA special purpose), 240.0.0.0/4 (reserved class E), and others. Routes with martian destinations are rejected before installation in the RIB. The martian table can be customized with [edit routing-options martians]. 'show route martians' displays the current martian filter.

JNCIA Troubleshooting Methods 2

Which Junos command captures live packet data on an interface for analysis?