JNCIA-Junos key terms

A numeric label assigned to a network interface so devices can send and receive packets on an IP network. It includes a network portion and a host portion.

A 32-bit value used with an IPv4 address to show which bits identify the network and which bits identify the host. It helps devices decide local versus remote destinations.

A compact way to write an IP address and its prefix length, such as 10.0.0.0/24. The number after the slash gives the count of leading network bits.

The IP address of a router on the local subnet that a host sends packets to when the destination is outside its own subnet. It provides the first hop off the local network.

Address Resolution Protocol. A Layer 2 protocol used on IPv4 networks to map a known IP address to the corresponding MAC address on the same broadcast domain.

A logical Layer 2 broadcast domain created within a switched network. Devices in different VLANs need a router or Layer 3 switch to communicate with each other.

Maximum Transmission Unit. The largest payload size, in bytes, that an interface can carry in one frame or packet. Mismatched MTU values can cause fragmentation or dropped traffic.

The network operating system that runs on Juniper devices. It separates control-plane functions, such as routing and management, from data-plane packet forwarding.

The control-plane component of a Juniper device. It runs routing protocols, maintains routing tables, handles management access, and programs the forwarding table used by the Packet Forwarding Engine.

The data-plane component that forwards packets through a Juniper device. It uses the forwarding table supplied by the Routing Engine and performs high-speed lookups and filtering.

The part of a network device that runs routing protocols, builds routing tables, and manages the device. It does not carry the main user data traffic.

The part of a network device that actually forwards user packets from ingress to egress. It relies on forwarding information built by the control plane.

Command-line interface. The text-based Junos interface used for operational commands and configuration edits. It has separate operational mode and configuration mode.

The Junos CLI mode entered after login, used to run monitoring and troubleshooting commands. The prompt ends with a greater-than sign.

The Junos CLI mode entered with the configure command, used to view and change the candidate configuration. The prompt ends with a hash sign.

A network management protocol that uses XML over SSH to configure and monitor Junos devices. It is commonly used by automation and orchestration systems.

A working copy of the Junos configuration that an administrator edits. Changes remain inactive until a commit command copies them into the active configuration.

The Junos command that validates the candidate configuration and, if valid, makes it the active configuration used by the device. It can also save a rollback copy.

A Junos feature that restores a previously committed configuration. The rollback 0 command returns to the most recent active configuration, while higher numbers select older saved configurations.

Reusable configuration statements in Junos that can be applied to multiple parts of the configuration. They reduce duplication and help enforce consistent settings across the device.

A Junos commit option that activates changes temporarily and automatically rolls them back unless confirmed within a set time. It helps prevent losing management access after a bad change.

A Junos operational command that displays interface status, speed, counters, errors, and configuration details. It is commonly used to verify link health and traffic statistics.

A Junos operational command that displays entries in the routing table, including destinations, next hops, preferences, and protocols. It helps verify routing behavior and route selection.

A Junos operational command that captures and displays packet headers on an interface. It is useful for quick troubleshooting without performing a full packet capture.

A Junos configuration feature that records detailed protocol or process activity to log files. It helps troubleshoot routing protocols and system behavior.

A network utility that sends ICMP echo requests to a destination and reports replies and round-trip times. It tests basic IP reachability.

A collection of known destinations and the next hops or interfaces used to reach them. Junos keeps separate tables for different protocols and uses preference to choose active routes.

The table the Packet Forwarding Engine uses to make fast packet-forwarding decisions. It contains the active routes selected from the routing table.

A manually configured route that tells the device where to send traffic for a destination. It does not change automatically when the network topology changes.

Open Shortest Path First. An interior gateway protocol that uses link-state advertisements and shortest-path calculations to route within a single autonomous system.

Border Gateway Protocol. An exterior gateway protocol used to exchange routing information between autonomous systems and to apply path-selection policies.

A Junos value that ranks routes from different sources when they describe the same destination. Lower preference values are preferred, and the winning route becomes active.

The IP address or interface a router uses to send a packet toward a destination. The forwarding table stores this information for each active route.

The routing rule that selects the most specific route for a destination, meaning the route with the longest matching prefix length. It is used when multiple routes match.

A Junos routing policy construct made of terms that match routes and apply actions. Policies can accept, reject, or modify routes before they are installed or advertised.

A component of a Junos policy statement or firewall filter that contains match conditions and actions. Terms are evaluated in order until a match is found.

A rule inside a policy term or firewall filter that traffic or routes must satisfy. Examples include prefix lists, protocol, source address, and destination address.

A Junos stateless packet filter applied to an interface or other attachment point. It evaluates packets against terms and can accept, discard, or count matching traffic.

A named list of IP prefixes used in routing policy match conditions. It simplifies matching multiple networks and can check prefix length ranges.

A Junos traffic-control component that limits bandwidth or burst size for matching traffic. It can mark, discard, or color packets that exceed configured rates.