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Cheat sheet

JNCIA-Junos Cheat Sheet

Practice this exam

Networking Fundamentals

8-10% of exam
Addressing + Subnetting Ethernet + Switching Networking Basics Longest Match Layer 2 vs Layer 3

Junos OS Fundamentals

7-10% of exam
Junos Architecture Control vs Forwarding Traffic Processing RE vs PFE Transit vs Exception

User Interfaces

18-22% of exam
CLI Modes + Navigation Config Management J-Web Help + Filtering Candidate vs Active

Configuration Basics

18-23% of exam
Initial Setup User Authentication Interfaces + System Services Logging Archival Rescue Configuration Groups

Monitoring + Maintenance

12-15% of exam
Monitoring Commands Maintenance + Upgrades Interface Stats Network Tools Password Recovery

Routing Fundamentals

15-20% of exam
Route Tables + Forwarding Route Preference Routing Instances Static Routing RIB vs FIB

Policy + Firewall Filters

8-12% of exam
Routing Policy Firewall Filters Match + Action uRPF Import vs Export

Quick Facts

Exam JN0-106
Credential JNCIA-Junos
Questions 65 multiple choice
Time 90 min
Cost $200 USD
Validity 3 years
Delivery Pearson VUE
Result Pass/fail shown

Networking Basics NE

Collision domain Per switch port
Broadcast domain Per VLAN/subnet
Switch Layer 2 forwarding
Router Layer 3 forwarding
MAC address 48-bit Layer 2
IPv4 32-bit address
IPv6 128-bit address

Addressing + Subnetting NE

/24 256 addresses
/30 Point-to-point link
Network bits Left of mask
Host bits Right of mask
Longest match Most specific wins
Connectionless UDP no handshake
Connection-oriented TCP handshake

RE vs PFE JU

RE thinks, PFE forwards
RE = control plane PFE = data plane RE runs protocols PFE moves packets

Transit vs Exception JU

Transit

Through device
PFE only
Forwarded

Exception

To/from RE
Protocol/local
Punted up
Pass vs process

Junos Architecture JU

RE Routing engine Control plane
PFE Forwarding engine Data plane
Control plane Protocols + mgmt
Forwarding plane Packet forwarding
FreeBSD kernel Junos foundation
Internal link RE to PFE
Single train One Junos image

Traffic Processing JU

Transit traffic PFE forwards through
Exception traffic Sent to RE
Local traffic Destined to device
Protocol traffic OSPF/BGP to RE
ICMP to RE Exception path
Forwarding table PFE lookup copy

Commit Flow US

Edit, compare, check, commit, confirm
set/delete show | compare commit check commit rollback if bad

Candidate vs Active US

Candidate

Being edited
Not yet live
Discardable

Active

Running config
Live on device
After commit
Edit then commit

Which CLI Mode US

Prompt ends > → Operational mode ( show/monitor )
Need to change config → configure ( Enter config )
Prompt ends # → Configuration mode ( set/delete )
Run op command in config → run command ( No exit needed )
Done editing → commit ( Activate config )
Leave config mode → exit ( Back to > )

CLI Modes + Navigation US

Operational mode Prompt ends >
Configuration mode Prompt ends #
configure Enter config mode
exit / quit Leave current level
top Go hierarchy root
up Move one level
edit Move into level

Operational vs Config US

Operational

Prompt >
show/monitor
View state

Configuration

Prompt #
set/delete
Change device
Look vs change

Which Commit Action US

Apply changes now → commit
Validate only → commit check
Risky remote change → commit confirmed ( Auto rollback )
Change worked → commit ( Confirm it )
Add commit note → commit comment
Schedule later → commit at

Help + Filtering US

? Context help list
Spacebar Complete command
Tab Complete variable
| match Show matching lines
| count Count output lines
| display set Show as set
| no-more Disable pager

commit vs commit confirmed US

commit

Permanent now
No timer
Stays active

commit confirmed

Temporary
Auto rollback
Confirm to keep
Permanent vs safety net

Recover Config State US

Discard candidate edits → rollback 0
Undo last commit → rollback 1
Go further back → rollback n ( Up to 49 )
Device broken badly → rollback rescue
Forgot to commit → commit ( After rollback )
Reset everything → load factory-default

Config Management US

set Add config statement
delete Remove config statement
show | compare Candidate vs active
commit Activate candidate
commit check Validate no apply
rollback 0 Discard candidate edits
rollback n Previous active config

J-Web US

J-Web Browser GUI
HTTPS access Secure J-Web
Dashboard Device status view
Point-and-click GUI configuration
CLI parity Same commit model

Interface Format CO

type dash FPC slash PIC slash port
ge = Gigabit xe = 10G fxp0 = mgmt lo0 = loopback

Initial Setup CO

Factory default Out-of-box config
root user No default password
set root-authentication Required first commit
Console port Initial CLI access
load factory-default Reset candidate
Hostname set system host-name

User Authentication CO

Login class Permission set
Predefined classes operator/read-only/super-user
Local password On-box auth
RADIUS External auth server
TACACS+ External auth server
Authentication order Method fallback list

Interfaces + System Services CO

NTP Time synchronization
SNMP Monitoring + traps
DNS Name resolution
Config groups Reusable config blocks
apply-groups Inherit group config
unit Logical interface
family inet IPv4 on unit

Logging Archival Rescue CO

syslog System event logging
messages file Default syslog file
traceoptions Per-feature debugging
Config archival Auto save on commit
Rescue config Known-good fallback
rescue save Store rescue config
rollback rescue Load rescue config

Interface Naming CO

ge-0/0/0 Gigabit Ethernet port
xe- 10-Gigabit Ethernet
et- 40/100-Gigabit Ethernet
ae0 Aggregated Ethernet
lo0 Loopback interface
fxp0 Management interface
type-fpc/pic/port Naming format

Monitoring Commands MO

show interfaces Interface status/stats
show route Routing table
show chassis Hardware status
show system uptime Boot + commit time
show log View log file
monitor interface Live counters
monitor traffic Packet capture

Network Tools MO

ping Reachability test
traceroute Path discovery
show arp IPv4 MAC bindings
clear interfaces statistics Reset counters
input/output errors Interface fault counters

Maintenance + Upgrades MO

request system software add Install Junos image
request system reboot Safe reboot
request system halt Safe shutdown
request system snapshot Backup boot media
Password recovery Single-user boot
Recovery requires console Physical access

Preference Order RO

Direct 0, Static 5, OSPF 10, RIP 100, BGP 170
Lower wins Direct/Local 0 Static 5 OSPF int 10 BGP 170

RIB vs FIB RO

RIB

Routing table
On RE
All routes

FIB

Forwarding table
On PFE
Active only
Choose vs forward

Which Route Wins RO

Compare two routes → Lowest preference ( Tiebreak first )
Static vs OSPF → Static ( 5 beats 10 )
OSPF vs BGP → OSPF ( 10/150 beats 170 )
Same preference → Protocol metric ( Lower wins )
Need active route → show route ( Asterisk marked )
What PFE uses → Forwarding table ( FIB only )

Route Tables + Forwarding RO

RIB Routing table (RE)
FIB Forwarding table (PFE)
inet.0 IPv4 unicast RIB
inet6.0 IPv6 unicast RIB
Active route Installed into FIB
show route Display RIB
show route forwarding-table Display FIB

Static vs Dynamic RO

Static

Manual config
Preference 5
No protocol

Dynamic

Learned via protocol
Auto adapts
Higher preference
Manual vs learned

Route Preference Values RO

Direct 0 Most preferred
Local 0
Static 5
OSPF internal 10
IS-IS L1 internal 15
RIP 100
OSPF external 150
BGP 170

Static + Routing Instances RO

static route Manual next hop
next-hop IP forwarding target
discard Drop silently
reject Drop send ICMP
0.0.0.0/0 Default route
Routing instance Separate route table
Lower preference More preferred route

Filter Default PO

No match means discard
First match wins Implicit deny Order matters then accept needed

Routing Policy vs Filter PO

Routing policy

Acts on routes
Import/export
Control plane

Firewall filter

Acts on packets
Match traffic
Data plane
Routes vs packets

Routing Policy PO

Import policy Routes into RIB
Export policy Routes out advertised
Term from + then block
from Match condition
then accept Allow route
then reject Deny route
Default policy Per-protocol behavior

Firewall Filters PO

Firewall filter Packet match action
Filter term from + then
then accept Permit packet
then discard Drop packet
Implicit deny Unmatched dropped
First match Top-down evaluation
uRPF Source address check

Common Traps

Edit vs apply

set edits candidate ≠ commit makes active

rollback 0 vs rollback 1

0 discards candidate ≠ 1 prior active config

RIB vs FIB

RIB on RE ≠ FIB on PFE

Preference vs metric

Preference compared first ≠ Metric breaks ties

Lower preference better

Lower = preferred ≠ Not highest value

Policy vs filter

Policy on routes ≠ Filter on packets

commit confirmed timer

Auto rolls back ≠ Must commit again

Last Minute

1. Exam = JN0-106 now
2. 65 questions, 90 minutes
3. set edits; commit activates
4. rollback 0 = discard candidate
5. commit confirmed auto-rolls back
6. Direct 0, Static 5, OSPF 10
7. BGP preference = 170
8. Lower preference = preferred
9. RIB = RE; FIB = PFE
10. Policy = routes; filter = packets
11. Filter unmatched = discard
12. fxp0 = management interface

JNCIA-Junos

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