JNCIA-Junos flashcards
JNCIA-Junos · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- OSI model network layer
- Layer 3 of the OSI model handles logical addressing and path selection between networks; IP addressing and routing operate here, while Ethernet switching and MAC addresses belong to layer 2.
- Private IPv4 address ranges
- Three blocks reserved for internal use: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. They are not routable on the public internet and require NAT for external connectivity.
- TCP three-way handshake
- A TCP session is established with three messages: the client sends SYN, the server replies SYN-ACK, and the client sends ACK. Data transfer begins only after this exchange completes.
- Address Resolution Protocol
- ARP resolves a known IPv4 address to the corresponding MAC address inside a broadcast domain by broadcasting a request and caching the reply for later use.
- Usable hosts in a /24
- A /24 mask leaves 256 addresses; the first is the network address and the last is the broadcast, leaving 254 usable host addresses. Usable hosts equal 2^h minus 2.
- Control plane versus forwarding plane
- The control plane runs on the Routing Engine and builds routing and forwarding tables; the forwarding plane runs on the Packet Forwarding Engine and forwards transit packets in hardware without involving the Routing Engine.
- Candidate configuration
- A working copy of the Junos configuration that is edited in configuration mode. Changes are inactive until a commit copies them into the active configuration.
- commit confirmed
- A commit option that activates the configuration but automatically rolls back after the specified number of minutes unless a confirming commit is issued, protecting remote sessions from lockout.
- Junos OS architecture
- Junos OS is built on a FreeBSD kernel and splits functions into separate daemons such as mgd, rpd and chassisd, keeping routing processes apart from the hardware forwarding plane.
- Operational mode prompt
- Operational mode shows a '>' at the end of the prompt and is used for show, ping and request commands; configuration mode shows '#'. Switch with configure and exit configuration-mode.
- CLI pipe match filter
- Appending '| match pattern' to a CLI command displays only the output lines containing the pattern; '| except pattern' displays only the lines that do not match.
- CLI pipe count
- Appending '| count' to a CLI command displays only the number of output lines instead of the lines themselves, useful for sizing route or interface tables.
- start shell
- The 'start shell' command drops from the Junos CLI into the underlying FreeBSD shell on the Routing Engine; typing exit returns to the CLI.
- J-Web interface
- A browser-based graphical interface for Junos devices that offers configuration wizards, monitoring dashboards and a CLI tool; it is an alternative to the text-based command line.
- commit check
- Validates the candidate configuration for syntax and semantic errors and reports any problems, without activating it or replacing the current running configuration.
- rollback command
- rollback N loads a previously committed configuration into the candidate; rollback 1 loads the prior commit and rollback 0 discards uncommitted edits by restoring the active configuration.
- show | compare
- In configuration mode, 'show | compare' lists the differences between the candidate configuration and the currently active committed configuration, marking added and removed lines.
- Configuration groups
- Named, reusable blocks of configuration statements that are inherited through apply-groups, so settings common to many interfaces or protocols are defined once and referenced elsewhere.
- show interfaces terse
- Displays a single summary line per interface showing administrative and link state, addresses and protocol family, making it the fastest way to check interface status.
- monitor interface
- Streams live per-interface traffic counters and error statistics that refresh continuously until Ctrl-C is pressed, useful for watching utilization in real time.
- request system snapshot
- Copies the currently running root and configuration partitions to the alternate partitions, providing a rollback backup of the Junos software and configuration on the device.
- show log messages
- Displays the contents of /var/log/messages, which records system events, interface transitions, protocol messages and errors, with the most recent entries at the end.
- Longest prefix match
- When several routes match the same destination, the router selects and installs the route whose prefix length is longest, meaning the most specific mask wins.
- Junos route preference values
- Junos prefers lower values: connected routes are 0, static 5, OSPF internal 10, RIP 100, OSPF external 150 and BGP 170; preference breaks ties before metrics are compared.
- inet.0 routing table
- The default Junos routing table that holds IPv4 unicast routes learned from protocols, static routes and directly connected networks, and feeds the forwarding table.
- Static route configuration
- The statement set routing-options static route 10.1.0.0/16 next-hop 192.168.1.1 adds a static route; the next hop must be reachable or qualify-next-hop is required.
- Autonomous system number
- A number identifying a BGP routing domain. External BGP peers belong to different autonomous systems, while internal BGP peers share the same one.
- Routing policy default action
- If a route matches no term in a Junos routing policy, it is accepted by default, so explicit reject terms are required to block unmatched traffic.
- Routing policy term actions
- A policy term matches routes with from conditions and acts with then actions: accept, reject, next term, next policy, or attribute changes such as metric and community.
- Firewall filter implicit action
- A firewall filter ends with an implicit discard, so any packet that matches none of its terms is dropped; a final accept term is needed to permit remaining traffic.
- Firewall filter direction
- Firewall filters are stateless: they are attached as input or output on an interface and each direction must be filtered separately, since no session state is tracked.
- Applying a firewall filter
- The command set interfaces ge-0/0/0 unit 0 family inet filter input FILTER-NAME attaches an existing stateless firewall filter to the inbound direction of that interface.