Free JNCIA-Junos Exam Flashcards

A switch port (or full-duplex link) is its own collision domain. A router interface bounds a broadcast domain; a switch by default forwards broadcasts, so all ports on one VLAN are one broadcast domain.

TCP is connection-oriented and reliable: it uses a handshake, sequencing, and acknowledgments. UDP is connectionless with no delivery guarantee, used where speed matters more than reliability (DNS, VoIP).

When multiple routes match a destination, the router selects the route with the most specific (longest) subnet mask. A /32 host route beats a /24, which beats the 0.0.0.0/0 default.

IPv6 addresses are 128 bits, written as eight 16-bit hex groups. A double colon (::) compresses one run of consecutive all-zero groups, and it may appear only once in an address.

Junos cleanly separates the two: the Routing Engine (RE) runs the control plane (protocols, CLI, route calculation); the Packet Forwarding Engine (PFE) forwards traffic at high speed using the forwarding table.

The RE runs the Junos OS kernel, routing protocols, and management interface. It builds the routing table (RIB) and pushes the best routes down to the PFE as the forwarding table (FIB).

Transit traffic is forwarded by the PFE without involving the RE. Exception traffic (packets to the device itself, options, or that need control handling) is sent to the RE for processing.

A single, modular OS image with one consistent CLI runs across Juniper platforms. Daemon-based modularity means a process failure (e.g., a routing daemon) does not crash the whole system.

Operational mode (prompt ends with >) runs show, ping, and monitoring commands. Configuration mode (prompt ends with #, entered via 'configure') edits the candidate configuration.

Edits go into the candidate configuration and are not live until 'commit'. The active configuration is what the device is actually running. 'commit' makes the candidate the new active config.

Activates the candidate config but auto-rolls back after a timer (default 10 minutes) unless a second 'commit' confirms it. Protects against lockout from a bad remote change.

'rollback n' loads a previous committed configuration into the candidate (0 = last committed, up to 49). You must still 'commit' to make it active. 'rollback' alone discards uncommitted edits.

In configuration mode, 'show | compare' displays the difference between the candidate and the active configuration, with + for additions and - for deletions before you commit.

'set' adds/modifies a statement. 'delete' removes it entirely. 'deactivate' keeps the statement but marks it inactive (shown as 'inactive:'), so it can be re-enabled with 'activate'.

Append '| match', '| except', '| count', '| display set', or '| no-more' to operational/config output. 'display set' converts hierarchical config into flat 'set' commands.

'edit <hierarchy>' moves into a subtree; 'up' moves one level up; 'top' returns to the root of the configuration; 'exit' leaves configuration mode (warns if uncommitted changes exist).

Junos' built-in HTTPS web interface for configuration and monitoring. It is an alternative to the CLI for many tasks but the CLI remains the comprehensive management interface.

Out of the box a device has a minimal factory-default config. 'load factory-default' resets the candidate to it; you must then set a root authentication password before the first commit succeeds.

A login class defines permission flags (e.g., view, configure, all) and idle timeout. Each user account references a class. Predefined classes include super-user, operator, read-only, and unauthorized.

Junos tries authentication methods in the configured 'authentication-order' (e.g., radius, tacplus, password). Local password is always checked if it is in the order or if remote servers are unreachable.

Reusable config templates applied with 'apply-groups'. Use them to define common settings once (e.g., interface or system parameters) and inherit them, with explicit config overriding the group.

A known-good config saved with 'request system configuration rescue save'. 'rollback rescue' restores it. Unlike rollback 0-49, it is not overwritten by routine commits.

Junos interfaces use type-fpc/pic/port (e.g., ge-0/0/1). A logical unit is added as '.unit' (ge-0/0/1.0). lo0 is the loopback; the unit holds the address and family.

NTP synchronizes the clock (critical for accurate logs and certificates). SNMP exposes monitoring data via communities/v3 users. Syslog records system events to local files or remote servers.

'system archival configuration' transfers the config to a remote SCP/FTP server on each commit (transfer-on-commit) or on a timer, providing off-box backups for recovery.

Gives a compact list of every interface with admin status, link status, protocol family, and address. The fastest way to verify which interfaces are up and addressed.

Admin status reflects configuration (up unless 'disable' is set). Link status reflects the physical layer. Admin up / Link down typically means a cabling or far-end problem.

A built-in tcpdump-style capture of control/exception traffic on an interface. It does not show transit traffic forwarded by the PFE, only packets reaching the RE.

Displays real-time, refreshing input/output packet and error counters for interfaces. Different from 'monitor traffic interface', which captures actual packets.

Reboot, interrupt the boot sequence to enter single-user mode, run the recovery option to get a root shell, set a new root password with 'set system root-authentication', then commit.

'request system halt' safely stops the OS so power can be removed without corrupting the file system. 'request system reboot' performs an orderly restart. Never just pull power on a running RE.

Installs a Junos software package. The 'validate' option checks compatibility with the active config first; a reboot is typically required to run the new version.

The routing table (RIB, inet.0) holds all learned routes from every protocol. The device selects the active route per destination and installs it into the forwarding table (FIB) used by the PFE.

Lower preference wins when routes to the same prefix come from different sources. Defaults: Direct 0, Local 0, Static 5, OSPF internal 10, BGP 170. It is the Junos equivalent of Cisco admin distance.

Configured under 'routing-options static route'. A next-hop can be an IP, an interface, or 'discard'/'reject'. 'reject' returns ICMP unreachable; 'discard' drops silently.

inet.0 is the default IPv4 unicast routing table; inet6.0 is the IPv6 unicast routing table. mpls.0 holds MPLS label routes. Routing instances create their own tables.

Logical, separate routing tables on one device (e.g., a virtual-router instance) providing traffic separation. The main (default) instance uses inet.0 unless an instance is specified.

They automatically learn and adapt to topology changes and scale better than static routes. JNCIA tests the role and concepts of OSPF/BGP rather than deep protocol configuration.

Import policy filters routes coming INTO the routing table from a protocol. Export policy filters routes leaving the routing table to a protocol (advertising). Direction is relative to the routing table.

A policy is an ordered list of terms. 'from' defines match conditions; 'then' defines the action (accept, reject, next term, or modify attributes). Terms are evaluated top-down.

If no term matches, the protocol's default policy applies (e.g., OSPF accepts and exports OSPF routes; BGP accepts all from peers and exports active BGP routes). Custom policy overrides defaults.

When multiple policies are applied as a chain, they are evaluated left to right. The first terminating action (accept or reject) stops evaluation; 'next policy' continues to the next one.

Routing policy controls which ROUTES are accepted/advertised (control plane). A firewall filter controls which PACKETS are permitted, discarded, or actioned (data plane). They operate on different things.

Filter terms are evaluated in order; the first matching term's action applies and evaluation stops. Any traffic not matching any term hits an implicit discard at the end.

Terminating actions: accept, discard (silent drop), reject (drop + send ICMP). Non-terminating actions like 'count', 'log', and 'policer' can be combined with a terminating action.

A filter is applied to an interface unit's family as 'filter input' (ingress) or 'filter output' (egress). Direction is from the device's perspective on that interface.

Checks that a packet's source address is reachable via the interface it arrived on, dropping spoofed-source packets. A loose mode only requires the source to be in the routing table at all.

Applying a firewall filter to lo0 filters traffic destined to the Routing Engine itself, protecting the control plane from DoS while not affecting transit traffic.

A prefix-list is a reusable named set of prefixes referenced in 'from'. A route-filter matches prefixes with match types like exact, longer, orlonger, upto, and through for granular control.

Displays the routing table: prefix, protocol source, preference (in brackets), and next hop. '*' marks the active route installed in the forwarding table. Add a prefix to filter the output.