JN0-650 Enterprise Routing and Switching - Professional (JNCIP-ENT) Questions and Answers

In this scenario, router1 (AS 65001) is attempting to establish anExternal BGP (EBGP)peering with router2 (AS 65002) using theirloopback (lo0) interface addresses(192.168.1.1 and 172.16.1.1).

According to the Junos OS 24.4 documentation on BGP peering sessions:

EBGP TTL Default:By default, EBGP packets are sent with aTime-to-Live (TTL) value of 1. This assumes the peer is directly connected on a shared physical subnet.

Loopback Peering Challenge:When peering between loopback addresses, the destination IP is not on the same subnet as the physical ingress interface. Even if the routers are directly connected physically, the router treats the loopback interface as an additional "hop". With a TTL of 1, the BGP packet expires before it can be processed by the peer's routing engine.

The Multihop Solution (Option D):To allow this session to form, you must configure the multihop statement under the BGP neighbor hierarchy. This tells Junos to increase the TTL (the default becomes 64 when multihop is enabled) and allows the session to establish even though the peer's address is not directly connected.

Target Neighbor:The neighbor address specified in the configuration on router1 must be the remote loopback address of router2 (172.16.1.1).

Option A (Multipath)is incorrect because it is used for load balancing across multiple active BGP paths, not for initial session establishment.

Options B and Care incorrect because they reference physical interface addresses (10.10.20.1 or 10.10.10.1) which are not the addresses being used for the loopback-based BGP session.

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 shows the Class of Service (CoS) transmit queue information for interface ge-0/0/11. To determine the correct behavior, we must analyze the bandwidth allocations and the queue limit settings:

Bandwidth Calculation (Option A):In Junos OS, when multiple queues are assigned specific percentages of bandwidth, the "remainder" (represented by'r') is the total interface bandwidth minus the sum of the explicitly configured percentages.

Bandwidth assigned to Queue 1 (EF) = 10%.

Bandwidth assigned to Queue 2 (AF) = 10%.

Bandwidth assigned to Queue 3 (NC) = 4%.

Total explicitly assigned = $10\% + 10\% + 4\% = 24\%$.

Remainder for Queue 0 (Best Effort)= $100\% - 24\% = 76\%$.

"Limit: exact" Behavior (Option B):The exhibit shows that Queue 2 (assured-forwarding) has aLimitset toexact.

By default, a queue in Junos can consume more than its allocated bandwidth if other queues are idle.

However, when the exact keyword is applied to the transmit-rate (transmission rate), the queue is strictly rate-limited to its configured percentage.

This means that traffic in Queue 2 will be capped at 10% of the interface bandwidthregardless of whether the network is congested or not. If the traffic exceeds 10%, the excess will be dropped or buffered to match the exact rate.

Option Cis incorrect because 100% of the bandwidth is not reserved for a single queue; it is distributed across four queues.

Option Dis less accurate than B because it implies the drop behavior only occurs during congestion. The exact parameter enforces the limit even when the rest of the interface is completely idle.

In Junos OS routing policies, the order of evaluation and the impact of terminal actions are critical for predictable control.

Immediate Execution (Option D):When a route-filter contains an explicit action like accept or reject within its block, that action is consideredterminalif a match is found.

Evaluation Flow:The policy engine processes terms and conditions from top to bottom. If a prefix matches a route-filter that has an embedded accept action, the policyexecutes that action immediatelyand stops further evaluation of that specific term. Consequently, any subsequent then clause in the same term is completely ignored.

Best Practice:To ensure a then clause (which might include modifications like changing a preference or community) is executed, the route-filter should be defined without an internal action, allowing the match to fall through to the term's common then statement.

This scenario describes the standard transition of Class of Service (CoS) metadata from the network edge to the network core.

MF vs. BA Classification:Edge devices useMultifield (MF) classifiersbecause incoming traffic often lacks markings. MF classification assigns traffic to an internalforwarding classbased on multiple fields (IPs, ports, etc.). Core devices useBehavior Aggregate (BA) classifiersbecause they are faster; they only look at the CoS bits (like DSCP) in the header to determine the forwarding class.

The Problem:Internal forwarding class assignments made by an MF classifier on the edge device do not automatically travel with the packet. If the packet leaves the edge device with its original (potentially zeroed) DSCP bits, the core device's BA classifier will not be able to identify the priority.

The Solution (Option A):You must implementrewrite ruleson theegress interfacesof the edge devices. A rewrite rule takes the internal forwarding class and loss priority assigned by the MF classifier and "writes" the corresponding DSCP value into the packet header as it exits toward the core. This ensures that the core device sees the correct DSCP marking and can apply the appropriate BA classification.

Incorrect Options:Rewrite rules are strictly an egress function to modify the packet for thenexthop. Applying them on ingress (Option B) or core ingress (Option C) does not solve the problem of ensuring the packet header reflects the classification logic used during the internal switching process of the edge device.

The exhibit illustrates an Autonomous System (AS 65512) receiving the same prefix ($203.0.113.0/24$) from two different neighbors, ISP A and ISP B. To influence outbound traffic from AS 65512 so that ISP A is preferred, you must manipulate the BGP Local Preference attribute.BGP Path Selection and Local Preference: Local preference is the first attribute evaluated by BGP after checking if the next hop is reachable. A higher local preference value is always preferred over a lower one. The default value in Junos OS is 100.Applying the Preference (Option B): By applying an import policy on router R1 to set a local preference higher than 100 (e.g., 200) for routes from ISP A, R1, R2, and R3 will all prefer the path through R1 to reach ISP A.Applying the Preference (Option C): Alternatively, you can achieve the same result by applying an import policy on R2 to set a local preference lower than 100 (e.g., 50) for routes from ISP B. Since the routes from R1 remain at the default of 100, the path through ISP A (100) will be preferred over ISP B (50).Why A and D are incorrect: Setting a lower preference for ISP A (Option A) or a higher preference for ISP B (Option D) would result in ISP B being preferred for outbound traffic, which is the opposite of the requirement.

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.

In a standard campus deployment on Juniper EX Series switches involving VoIP phones and 802.1X authentication, understanding the default behavior of Junos OS 24.4 is critical:

Voice VLAN (Option A):By default, an access port belongs to a single VLAN (the native or data VLAN). For an IP phone to operate correctly on the same physical port as a PC, aVoice VLANmust be explicitly configured and associated with the interface. This allows the switch to segregate voice traffic (usually tagged) from data traffic (untagged).

802.1X Configuration (Option D):Access ports do not have 802.1X authentication enabled by default. To enforce network access control for the client PCs, you must manually enable and configure the dot1x protocol on all relevant interfaces.

PoE Default (Option B):On EX Series switches that support PoE (like P or MP models),PoE is enabled by defaulton all PoE-capable ports. Therefore, additional configuration is generally not required unless you need to change power priorities or management modes.

LLDP Default (Option C):Standard LLDP is typically enabled by default in the factory configuration of most EX Series switches to facilitate neighbor discovery. While LLDP-MED is used for VoIP, the core requirement for "deploying" the described environment highlights the manual steps of VLAN and security setup rather than protocol discovery which often runs out-of-the-box.

The exhibit shows a multicast network runningPIM Sparse Modewhere R2 is configured as a localRendezvous Point (RP). However, the non-RP routers (like R5) show no learned RP in their show pim rps output. To resolve this usingstandard protocolsfor dynamic RP discovery, you must implement a mechanism that automates the distribution of RP information across the PIM domain.

Bootstrap Router (BSR) (Option B):BSR is the industry-standard (RFC 5059) mechanism for dynamically electing an RP and distributing that information throughout a PIM-SM domain.

It uses two roles:Candidate-RPs (C-RPs), which announce their desire to be an RP, andCandidate-BSRs (C-BSRs), which collect these announcements and flood them to all other routers in the network via PIM bootstrap messages.

BSR is a "standard protocol" as it is part of the PIMv2 specification. Once configured on R2 (as C-RP) and potentially R1 or R2 (as C-BSR), all other routers will dynamically learn that R2 is the RP for the specified group ranges.

Auto-RP (Option D):While this also provides dynamic RP discovery, it is aproprietaryCisco protocol. Although Junos supports it for interoperability, the question specifically asks for "standard protocols only," making BSR the preferred choice.

MSDP (Option A):Multicast Source Discovery Protocol (MSDP) is used to share information about active sources betweendifferentPIM domains or for Anycast-RP. It does not handle the initial discovery of an RP within a single domain for standard PIM-SM operations.

Anycast RP (Option C):This is a technique used for RP redundancy and load balancing. While it can use BSR or MSDP for synchronization, it is a design architecture rather than a standalone "standard protocol" for the basic dynamic discovery of an RP in the manner requested here.

Configuration Example for BSR in Junos OS 24.4:To enable this, you would add the following to the RP (R2):

set protocols pim rp bootstrap-discovery

set protocols pim rp candidate-rp 10.222.1.2 group-ranges 224.0.0.0/4

set protocols pim rp candidate-bsr 10.222.1.2

The exhibit illustrates anEVPN-VXLANfabric using multi-homing. Server1 is dual-homed to Leaf1 and Leaf2 using anEthernet Segment Identifier (ESI)in an active-active or active-standby configuration. Server2 is single-homed to Leaf3.

In EVPN environments, specific mechanisms are used to prevent loops and duplicate traffic when dealing with Broadcast, Unknown Unicast, and Multicast (BUM) traffic:

Broadcast Propagation (Option A):When Server2 (connected to Leaf3) generates a broadcast frame, Leaf3 encapsulates this traffic into a VXLAN tunnel and forwards it to all other VTEPs (Virtual Tunnel End Points) that participate in that specific Virtual Network Instance (VNI). Therefore, both Leaf1 and Leaf2 will receive the broadcast frame from the core fabric.

Designated Forwarder (DF) Election (Option C):To prevent a multi-homed device (like Server1) from receiving duplicate copies of the same broadcast frame from multiple leaf switches, EVPN utilizes aDesignated Forwarder (DF). For a given ESI, only one leaf switch is elected as the DF.

The exhibit explicitly states thatLeaf2 is the Designated Forwarder.

According to the Junos EVPN implementation, only the DF is responsible for decapsulating BUM traffic received from the core and forwarding it to the local access segment.

While Leaf1 receives the frame (Option A), it will see that it is not the DF for that segment and will drop the frame rather than forwarding it to Server1. This ensures Server1 receives exactly one copy of the broadcast, delivered by Leaf2.

Option B is incorrectbecause in a standard ingress replication or multicast-based underlay, all VTEPs in the VNI receive the broadcast.

Option D is incorrectbecause Leaf1 is the non-designated forwarder (non-DF) for this segment and is prohibited from forwarding BUM traffic to the ESI.

To simplify VLAN management and automate the creation and pruning of VLANs across a network, Juniper utilizes theMultiple VLAN Registration Protocol (MVRP).

MVRP on Trunk Interfaces (Option C):MVRP is the IEEE 802.1ak standard that replaces the older, proprietary GVRP. It is designed to run ontrunk interfacesbetween switches.

Dynamic Creation:When a VLAN is added to one switch, MVRP advertises its existence to neighboring switches. If those switches need to reach that VLAN, they can dynamically create it in their database.

Pruning:If a switch no longer has any active ports or downstream neighbors interested in a specific VLAN, it stops advertising that VLAN on its trunks. This "prunes" the VLAN traffic from those links, saving bandwidth.

Incorrect Options:Option A and Bare incorrect because GVRP is legacy and no longer the recommended protocol for modern Junos deployments.Option Dis incorrect because MVRP is meant for inter-switch trunk links to manage the fabric, not for edge access ports where VLAN membership is usually static.

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.

When 802.1X server fail fallback is enabled with the vlan-name feature, the switch provides a specific survival mechanism for both existing and new sessions when the RADIUS server becomes unreachable.

Existing Clients (Statement B): Clients that have already successfully authenticated are unaffected by the server's unavailability in the short term. They maintain their current network access until their specific re-authentication timer expires. Only then will the switch attempt to contact the server again and trigger the fallback action if the server remains down.

New Clients (Statement D): For any new device attempting to connect while the RADIUS server is down, the switch cannot perform a standard authentication. Under the vlan-name fallback configuration, these new clients are automatically placed into the specified fallback VLAN and granted access based on the local policy defined for that VLAN.

Why others are incorrect: Statement A is incorrect because disconnecting active users would cause unnecessary service disruption. Statement C is incorrect because new clients cannot "request" a VLAN during the 802.1X handshake; the switch assigns it based on the fallback configuration.

In Junos OS Class of Service (CoS) processing, there is a specific order of operation for packet classification.

Precedence of Multifield (MF) Classifiers:When both a Behavior Aggregate (BA) classifier and a Multifield (MF) classifier are applied to an interface, theMF classifier takes precedence.

Processing Order:

The BA classifier (e.g., DSCP or 802.1p) inspects the packet header and assigns an initial forwarding class and loss priority.

The MF classifier (which looks at multiple fields like source/destination IP and port) is processed later in the pipeline.

The "Final Word":Because the MF classifier is evaluated later, its resultsoverwriteany previous assignments made by the BA classifier. Therefore, if a packet matches both, the MF classifier determines the final forwarding class assignment.

In Junos OS, a physical or logical interface is typically associated with a single OSPF area. However, there are specific design requirements—such as migrating an interface from one area to another without downtime or creating specific transit topologies—where an interface needs to exist in two areas simultaneously.

Multi-area Adjacency (Option B):The secondary command under the OSPF interface hierarchy allows an interface to be part of asecondary areain addition to its primary area.

Protocol Behavior:When an interface is configured as secondary, the router sends and receives OSPF Hello packets for both the primary and secondary areas on that single link. This allows the router to form adjacencies with neighbors in both areas over the same physical connection.

Use Case:This is frequently used in hub-and-spoke topologies or during network restructuring to ensure reachability is maintained while changing area boundaries. It effectively allows a "logical" split of the interface at the OSPF protocol level.

In standard BGP operations, a router typically selects only one "best path" for a given destination prefix and installs that single path into the forwarding table. To utilize multiple paths for traffic forwarding (load-balancing), specific multipath features must be enabled.

BGP Multipath:This feature allows a router to install multiple equal-cost paths into the routing and forwarding tables. However, by default, BGP multipath requires that all candidate paths have the same neighboring Autonomous System (AS).

Multipath Multiple-AS (Option B):In the exhibit, R1 is receiving routes fromISP A (AS 65501)andISP B (AS 65502). Since these are two different neighboring ASs, the standard multipath feature is not sufficient. You must use the multipath multiple-as statement under the BGP group or neighbor hierarchy. This tells Junos to consider paths as equal even if they originate from different neighboring ASs, provided all other BGP path selection attributes (Local Preference, AS Path length, Origin, MED, etc.) are identical.

Why other options are incorrect:*Option A (multihop)is used for peering with routers that are not directly connected.

Option C (prefix limit)is a security and stability feature to limit the number of prefixes received from a neighbor.

Option D (damping)is used to penalize unstable, "flapping" routes to prevent constant routing table churn.

Configuration Example for Junos OS 24.4:To implement this on R1, you would use the following configuration:

set protocols bgp group multipath multiple-as