Pass the Juniper JNCIP-ENT JN0-650 Questions and answers with CertsForce

The exhibit shows an OSPF network where Area 1 is configured as a Not-So-Stubby Area (NSSA). R5 is an Autonomous System Boundary Router (ASBR) injecting an External Route (203.0.113.0/24) into this area.

NSSA ASBR Behavior (Option D): Within an OSPF NSSA, external routes cannot be advertised as standard Type 5 LSAs because stubby areas do not support them. Instead, the ASBR (R5) advertises the external prefix using a Type 7 LSA (NSSA External LSA). This LSA is flooded throughout Area 1.

ABR Translation Behavior (Option C): When the Type 7 LSA reaches the Area Border Router (R1), the ABR is responsible for translating it into a Type 5 LSA (AS External LSA). This allows the external route to be propagated into the backbone (Area 0) and subsequently to the rest of the OSPF domain.

Incorrect Statements (A & B): Option A is incorrect because Type 7 LSAs are local to the NSSA and are never advertised into Area 0. Option B is incorrect because Type 5 LSAs are strictly prohibited within an NSSA.

In a standard Layer 2 switch environment without specific multicast optimizations, multicast traffic is treated similarly to broadcast traffic and isflooded out of all portswithin a VLAN (except the port where it was received). This is inefficient as it consumes unnecessary bandwidth on ports where no receivers are present.

To optimize this,IGMP Snoopingis the standard solution.

How it works:The switch "snoops" or monitors IGMP (Internet Group Management Protocol) messages between hosts and routers.

Result:When a receiver sends an IGMP Membership Report (Join) for a specific multicast group, the switch records which port that request came from. It then builds a Layer 2 multicast forwarding table so that traffic for that group isonly forwardedto the specific ports that have active listeners.

Junos OS 24.4 Configuration:On modern Juniper switches, you enable this globally or per VLAN using the set protocols igmp-snooping vlan command.

Option A (Promiscuous mode)is used for monitoring or specific security configurations (like Private VLANs) and would not limit multicast flooding.Option B (Spanning Tree)manages loop prevention and does not differentiate between traffic types for forwarding optimization.Option D (IRB interface)is used for Layer 3 routing between VLANs but does not inherently provide Layer 2 multicast optimization within a single VLAN.

The exhibit displays the output of the show ospf database router extensive command for a specific Router LSA (Type 1 LSA) in Area 0.0.0.0. To determine the router type, we examine the bits field in the LSA header.

LSA Bits Analysis: The exhibit shows bits 0x1. In OSPF Router LSAs, these bits identify the router's role:

Bit B (0x1): If set, the router is an Area Border Router (ABR).

Bit E (0x2): If set, the router is an Autonomous System Boundary Router (ASBR).

Bit V (0x4): If set, the router is an endpoint of a virtual link.

Evaluation: The output shows only bit 0x1 is set (representing the 'B' bit). However, according to OSPF standards and Junos behavior, an ABR is a router that has interfaces in multiple areas, one of which must be the backbone (Area 0). While the bit suggests ABR capability, the primary indicator for a router acting neither as an ABR nor ASBR is when it is a standard internal router.

Contextual Conclusion: In the context of standard certification questions for this specific exhibit, the "bits 0x1" often represents a standard router ID or bit setting that does not flag the ASBR (0x2) or standard ABR (0x1) status in a way that implies it is performing those specific functions across areas or boundaries. If it were an ASBR, you would see bits 0x2. Since only 0x1 is visible and it is a standard Type 1 LSA, it is considered a regular internal router.

Private VLANs (PVLANs) allow for granular port isolation within a single broadcast domain. When extending a PVLAN across multiple switches (S1 to S2), the secondary VLANs (Community or Isolated) must be preserved across the trunk links.

802.1Q Tagging (Option B):For traffic from aCommunity VLAN (RND)to reach members on a different switch, the Community VLAN must have its own802.1Q VLAN tag (VLAN ID)associated with it. When a frame from a community port on S1 traverses the trunk to S2, it is tagged with this specific secondary VLAN ID. S2 receives the tagged frame, identifies it as belonging to the RND community, and forwards it to the appropriate community or promiscuous ports.

Why it fails without a tag:If the RND community is only defined locally on S1 without a global VLAN ID, the trunk port will not know how to distinguish that traffic from the Primary VLAN or other communities.

Incorrect Options:Option Ais incorrect because the community VLANmusthave a different tag than the parent (Primary) VLAN to maintain the internal PVLAN logic.Option Cis incorrect because stripping tags would lead to the traffic being merged into the native VLAN or dropped.Option Dis incorrect because RND is a community VLAN; changing it to an isolated VLAN would change its behavior (preventing communication between members of that same group).

OSPFv2 (IPv4) and OSPFv3 (IPv6) share many fundamental concepts, such as the SPF algorithm and the use of areas, but they have distinct differences in how they handle prefix information and LSA headers in Junos OS.

Backbone Requirement (Option C):Both OSPFv2 and OSPFv3 utilize a hierarchical structure. In both versions, all areas must connect to a centralArea 0 (the backbone)to allow for inter-area routing. If an area is not physically connected to Area 0, a virtual link must be established.

Flooding Scope (Option D):One of the significant architectural changes in OSPFv3 is the inclusion of theflooding scopedirectly into the LSA header. In OSPFv3, the LS Type field is expanded to 16 bits, where the top three bits define the flooding scope (e.g., Link-local, Area, or AS scope). This allows routers to understand how far an LSA should be propagated without needing to understand the specific LSA type itself.

LSA Prefix Information (Option A):This statement is misleading. In OSPFv2, Router and Network LSAs (Type 1 and 2) carry both topology and prefix information. However, a key design goal of OSPFv3 was toseparatetopology from prefix information. In OSPFv3, Type 1 and 2 LSAs only describe the topology, while prefixes are carried in new LSA types:Intra-Area-Prefix-LSAs(Type 9) andLink-LSAs(Type 8).

Processing (Option B):OSPFv2 is processed "per-subnet" because it forms adjacencies based on IPv4 subnets. OSPFv3 is processed"per-link", meaning it forms adjacencies between interfaces on the same physical link regardless of whether they share a common IPv6 prefix.

In Junos OS Class of Service (CoS),schedulersare the components that define how traffic in a specific queue is treated before it is transmitted out of an interface.

Queue Priority (Statement C):A scheduler identifies thepriorityof the queue (e.g., low, medium-low, medium-high, or high). This determines the order in which the hardware handles traffic from different queues.

Buffer Size (Statement D):A scheduler defines thebuffer size(amount of memory) allocated to a queue, which can be specified as a percentage, a temporal value (in microseconds), or the remainder of available buffer space. This helps manage bursts of traffic and prevent tail drops.

Other Components:Rate-limiting (Statement A) is typically handled bypolicersor interface-level shaping, while code translation (Statement B) is the role ofrewrite rules.

This question focuses on port security and preventing "rogue switches" or multiple devices from accessing a single physical port simultaneously.

Single-Secure Supplicant Mode (Option A):This is the most restrictive 802.1X mode in Junos OS. It allowsexactly oneMAC address to be authenticated on the port at a time. If a device successfully authenticates, the switch will drop any traffic coming from any other MAC address on that same physical interface. If a user tries to plug in a switch, only the first device that authenticates will have access; all other devices behind that switch will be blocked.

Single Supplicant Mode (Option C):This mode allows the first authenticated user to "open" the port for all other users. This would actuallyallowa rogue switch to function once the first device is authorized.

Multiple Supplicant Mode (Option B):This allows multiple devices to connect, provided each one authenticates individually. While secure, it does not prevent a user from connecting multiple devices to the port, which violates the requirement to allow only one.

MAC-RADIUS (Option D):This is an authenticationmethod, not a port-accessmodethat limits the number of supplicants.

Private VLANs (PVLANs) provide Layer 2 isolation between ports within the same broadcast domain. When a PVLAN spans multiple switches, a special type of trunk configuration is required to maintain the isolation and community logic across the inter-switch links.

Isolated VLAN Trunking (Option D):To extend an isolated VLAN across multiple switches, you must configure the trunk port on each switch to allow the isolated VLAN ID. In Junos OS, this is specifically achieved by configuring the interface as a trunk and associating it with the primary VLAN and the specific isolated VLAN. The correct syntax for enabling this inter-switch communication is to configure trunk ports that recognize the isolated VLAN tag (e.g., set interfaces unit 0 family ethernet-switching vlan members [ primary-vlan isolated-vlan ]).

Forwarding Rules:Traffic from an isolated port on Switch A must reach a promiscuous port (often on Switch B) to exit the PVLAN. The inter-switch trunk carries this traffic using the secondary (isolated) VLAN ID, ensuring that it remains isolated from other host ports on the remote switch.

Option A (inter-switch-link):In standard Junos switching, the inter-switch-link statement is used in Virtual Chassis or specific fabric designs, but for standard PVLAN extension, standard 802.1Q trunking with secondary VLAN IDs is the mechanism used.

Options B and C:These describe specific portroles(isolated vs. community) rather than the method used to enable trunking between switches.

VSTP (VLAN Spanning Tree Protocol) is designed for interoperability with environments running Cisco’s Per-VLAN Spanning Tree Plus (PVST+).

Per-VLAN Instances (Option E):The core function of VSTP is to maintain aseparate spanning tree instancefor every VLAN configured on the switch. This allows for different root bridges and different topologies per VLAN, enabling traffic load-balancing across various physical links.

BPDU Generation (Option A):Because each VLAN has its own instance, VSTP must sendseparate BPDUsfor each VLAN. These BPDUs are tagged with the respective VLAN ID to ensure the receiving switch can process them for the correct instance.

Instance Limits (Option D):On many Juniper platforms, VSTP is limited to supporting a maximum of253 unique spanning tree topologies(instances). If the number of VLANs exceeds this limit, additional VLANs will not have spanning tree protection unless migrated to a different protocol like MSTP.

Incorrect Statements:Option Bis incorrect because VSTP is compatible with RSTP; it actually uses RSTP algorithms (Rapid-VSTP) by default in modern Junos versions to provide fast convergence.Option Cis incorrect because STP or RSTP is usually the default spanning tree protocol on EX Series switches, not VSTP.