Juniper Networks Routing Protocols and Configuration

Historically, Juniper Junos allows only one firewall filter to be applied per logical unit (subinterface) in each direction (input and output). While modern Junos supports filter lists to evaluate multiple filters sequentially, the fundamental interface configuration structure accommodates a single designated input and output filter per logical interface unit.

In OSPF DIS election, the router with the highest priority becomes the DIS. Priority values range from 0-255, with higher values preferred. A priority of 0 makes a router ineligible. Router D with priority 100 wins over C(70), A(64), and B(0-ineligible).

In Juniper Junos OS, the show interfaces extensive command provides detailed operational status for a specific interface, including its active alarms and defects (such as physical or link-layer faults). General commands like show alarms display system-wide or chassis alarms, not interface-specific active alarms.

The commit synchronize command in Junos OS ensures that the configuration on the primary Routing Engine is copied and committed on the backup Routing Engine, keeping both synchronized.

In Junos, inet.3 is reserved for BGP next-hop resolution using BGP-learned routes. When equal preference routes exist in both inet.0 and inet.3, BGP prefers the inet.3 version for next-hop selection.

When all OSPF routers on a broadcast network have equal priority, the router ID becomes the tiebreaker. The highest router ID is elected DR or BDR. Area ID, loopback address, and interface IP address are not used in this decision.

In route-filter syntax, 'longer' matches routes more specific than the prefix but excludes the exact prefix itself. 'orlonger' includes the exact match. The /17 in option B is too specific (only one prefix length).

From configuration mode, 'rollback 2' activates the rollback configuration numbered 2. Prefixing with 'run' exits config mode first. 'request system rollback' is operational mode syntax, not config mode.

An Autonomous System Boundary Router (ASBR) redistributes routes between OSPF and other routing protocols. ABR connects OSPF areas. Internal routers stay within one area. Backbone routers are in area 0.

The 'show ospf database' command displays the OSPF link-state database. 'show ospf neighbor' shows adjacencies. 'show protocols ospf' shows configuration. 'show ospf link-state' is not valid Junos syntax.

In Junos, 'show route receive-protocol bgp x.x.x.x' displays routes received from the specified peer. 'show bgp received routes' and 'show bgp advertised routes' are not valid syntax. 'advertising-protocol' shows what is sent, not received.

inet.0 is the default routing table for unicast IPv4 routes in Junos devices. The other routing tables serve specific purposes: inet.1 is used for the multicast forwarding cache, inet.2 for unicast routes used for multicast reverse-path forwarding (RPF) checks, and inet.3 for MPLS label-switched paths (LSPs).

The 'show alarms' command displays active alarms on the device, including interface-specific alarms. Option C has a typo ('ahow' instead of 'show'). Option D shows extensive interface details but is not the most direct command for alarm verification. The 'terse' option in B shows a summary without alarm details.

'show isis interface' displays IS-IS protocol status specifically at the interface level, showing which interfaces have IS-IS configured and their operational state. Option A shows neighbor adjacencies but doesn't verify interface configuration. Options C and D are incorrect command syntax in Junos.

Route preference (also called administrative distance) is primarily used to select between routes learned from different routing protocols to the same destination. When multiple routing protocols provide routes to the same prefix, the route with the lowest preference value is selected. Option D is grammatically incorrect ('protocol' vs 'protocols'). Route preference does not differentiate between internal/external BGP (A) - that's what LOCAL_PREF is for.

When an LSR receives a label of 0 (implicit null), it performs PHP (Penultimate Hop Popping) by popping the label and forwarding the IP packet to the egress router. However, this is operationally equivalent to a label swap operation where the incoming label is swapped for a null label (0), which is then removed. The correct answer is B because in MPLS forwarding operations, the label action is classified as a swap. Options A (pop), C (push), and D (error) do not correctly describe the MPLS forwarding operation for label 0.

The penultimate hop router (second-to-last router in the LSP) performs PHP (Penultimate Hop Popping) by removing the MPLS label before forwarding the packet to the egress router. This reduces processing load on the egress router since it doesn't need to perform a label lookup - it receives a plain IP packet. Option B is incorrect because the penultimate router pops, not pushes. Option C describes a swap operation with explicit null (label 3), not the normal PHP case. Option D is what the egress router does after PHP, not the penultimate router's job.

To advertise OSPF routes into BGP, you must apply an export policy under the BGP protocol hierarchy. An export policy controls what routes are advertised out of a protocol. Import policies control what routes are accepted into a protocol. Applying an export policy under OSPF (B) would advertise BGP routes into OSPF, not the reverse. Import policies under BGP (D) would filter what BGP routes are accepted, not what OSPF routes are advertised.