JN0-650 Sample Questions Answers

For maintenance scenarios where a router must remain operational but should not be used as a transit path for OSPF traffic, Junos OS uses the overload functionality.

OSPF Overload (Option B): When you enable overload under protocols ospf, the router advertises its own Link-State Advertisements (LSAs) with a maximum metric (65535) for all of its transit links.

Traffic Diversion: Because OSPF uses the Shortest Path First (SPF) algorithm to find the lowest-cost path, other routers in the network will see the overloaded router as an extremely high-cost path. They will calculate alternate routes, effectively moving transit traffic off the router while still allowing management traffic directed to the router's own interfaces.

Timeout Options: You can configure this to be permanent or set a timeout so that the router automatically resumes normal metric advertisements after a set period.

The exhibit displays the default-switch.evpn.0 routing table, which is used on Juniper leaf devices to store EVPN Type 2 (MAC/IP) routes.

Route Distinguisher (Option C): In EVPN, the Route Distinguisher (RD) is an 8-byte prefix added to a route to make it unique within the BGP control plane. The RD format in the exhibit is :.

For example, the prefix 2:192.168.100.1:1::5010::... indicates an EVPN Type 2 route (2:) where 192.168.100.1:1 is the Route Distinguisher.

This RD identifies the specific routing instance on the originating VTEP that advertised the MAC/IP address.

Option A is incorrect: The RD 192.168.100.2:1 does not necessarily mean the host device has that IP; it means the originating switch has that router ID/IP used for its RD.

Option B is incorrect: While the RD often incorporates the router ID, the RD itself is the full string (e.g., 192.168.100.1:1), which is distinct from the raw Router ID used in the BGP summary.

Option D is incorrect: Looking at the entries for 10.1.1.1 and 10.1.2.3, they are associated with different identifiers in the RD strings (5010 and 5020 respectively), which typically map to different VNIs or bridge domains.

In an EVPN-VXLAN environment, different BGP route types serve distinct purposes in building the control plane. The Ethernet Segment Identifier (ESI) is a unique 10-byte identifier used to represent a multi-homed segment.

Type 1: Ethernet Auto-Discovery (A-D) Route (Option A): This route type is essential for multi-homing scenarios. It carries the ESI to support features like fast convergence and aliasing. By advertising an A-D route per Ethernet Segment, a PE tells other routers that it is connected to that specific ESI. It does not carry individual host MAC addresses; instead, it provides a "mass withdraw" mechanism to quickly update paths if the entire segment goes down.

Type 4: Ethernet Segment Route (Option D): This route is used exclusively for Designated Forwarder (DF) election. It advertises the existence of an ESI and the ES-Import Route Target to all other PE routers. This allows PE routers connected to the same multi-homed segment to discover each other automatically and elect a DF to handle BUM traffic. Like Type 1, it focuses on the segment identity and does not include MAC addresses.

Type 2: MAC/IP Advertisement Route (Option B): This is the primary route used for host reachability. It must include a MAC address (and optionally an IP). While it also includes the ESI to indicate which segment the host is behind, its primary function is MAC advertisement.

Type 3: Inclusive Multicast Ethernet Tag Route (Option C): This route is used for VTEP discovery and building the replication list for BUM traffic. It identifies the VTEP's IP and the VNI, but it does not carry the ESI or MAC addresses.

In an end-to-end CoS (Class of Service) design, there is a clear distinction between classification at the ingress and rewrite rules at the egress.

The Problem: Your edge devices are successfully classifying traffic (assigning packets to internal forwarding classes). However, the core devices—which use BA (Behavior Aggregate) classifiers—are not classifying correctly. BA classifiers look at the bits in the packet header (like DSCP or 802.1p) to determine priority.

The Missing Link (Option C): When a packet leaves the edge device, its internal forwarding class and loss priority must be "written" back into the packet header so that the next-hop (the core device) can see it. By default, Junos may not preserve or set these bits correctly on egress. You must apply rewrite rules on the edge switches' egress interfaces facing the core. These rules ensure the DSCP values in the headers match the forwarding class determined by the edge's MF classifier, allowing the core's BA classifier to function properly.

Other Options: Option A (policer) handles rate limiting, not classification. Option B is incorrect because core devices often have different hardware or scaling needs than edge switches, and standard BA configuration is preferred over templates in the core. Option D is incorrect because MF classifiers are standard for edge devices where deep packet inspection is needed to identify traffic types.

In Junos OS, particularly in BGP-based environments like EVPN-VXLAN or MPLS, the router often needs to perform a recursive lookup to resolve the BGP next-hop of a route.

Next-Hop Resolution (Option C): When an EVPN route (found in default-switch.evpn.0) is received, the protocol next-hop is typically the loopback IP of the remote VTEP. To actually forward traffic, the switch must know how to reach that loopback IP.

The inet.3 table is the default table in Junos used to store egress interface information for BGP next-hops.

While inet.0 handles standard unicast routing, inet.3 is specifically consulted for resolving the path toward BGP peers. In an EVPN context, this ensures that the VXLAN tunnel endpoint is reachable and properly resolved through the underlay.

Other Tables: * inet.2 (Option A) is used for Multicast RPF checks.

default-switch.evpn.0 (Option B) stores the EVPN routes themselves (MAC/IP), but it is the source of the lookup, not the table where the recursive resolution occurs.

vxlan.inet (Option D) is an internal table used to track active VXLAN tunnels but is not the primary table for recursive BGP next-hop validation.

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, the MF 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 results overwrite any previous assignments made by the BA classifier. Therefore, if a packet matches both, the MF classifier determines the final forwarding class assignment.

The exhibit shows router1 (AS 65001) connected to router2 (AS 65002) via two parallel physical links. The show bgp summary output indicates that sessions are established with two neighbors: 10.10.10.1 and 10.10.20.1. Currently, for the second neighbor (10.10.20.1), there are 0 active routes despite having 4 accepted routes, which indicates that BGP has selected only one "best path" via the first neighbor for forwarding.

BGP Best Path Selection: By default, BGP only selects a single best path for any given destination prefix and installs that one path into the forwarding table. In a topology with parallel links to the same AS, this leads to underutilization of available bandwidth.

Multipath Solution (Option B): To enable active routes from both peers and allow for load-balancing (ECMP) across both links, you must enable the multipath feature.

When the multipath statement is configured under the protocols bgp group ext-peers hierarchy, it tells Junos OS to install multiple equal-cost BGP paths into the routing table and subsequent forwarding table.

Since both neighbors belong to the same peer group (ext-peers) and the same AS (65002), configuring multipath at the group level will apply to both sessions, allowing paths from both neighbors to be marked as "active".

Incorrect Options:

Option A: 172.16.1.1 is the loopback address of router2. The exhibit shows peering is currently done using physical interface addresses (10.10.10.1 and 10.10.20.1), so this address is irrelevant to the current active sessions.

Option C: Configuring the neighbor address alone without the multipath parameter will not change the best-path selection behavior.

Option D: 10.10.20.2 is the local interface IP of router1, not the neighbor's IP. BGP multipath must be configured to point to remote peer paths.

Configuration Example for Junos OS 24.4: To implement this, apply the following command: set protocols bgp group ext-peers multipath

In a standard Layer 2 switch environment without specific multicast optimizations, multicast traffic is treated similarly to broadcast traffic and is flooded out of all ports within 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 Snooping is 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 is only forwarded to 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.

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 an EVPN-VXLAN Edge-Routed Bridging (ERB) architecture, also known as a collapsed fabric, the Layer 3 default gateway functionality is moved from the core/spine layer down to the edge (the leaf or distribution layer).

Anycast Gateways: To support seamless host mobility and redundancy, multiple distribution/leaf switches are configured with the same anycast IP address and MAC address on their IRB interfaces for a given VLAN. This allows a host to move between different switches without needing to update its default gateway configuration or ARP cache.

Distributed Routing: Routing occurs locally at the edge (distribution layer). Traffic destined for a different subnet is routed by the first switch it hits (the ingress leaf), rather than being backhauled to a central core router.

Symmetric vs. Asymmetric IRB: Junos OS 24.4 supports both models, but the ERB design typically utilizes symmetric routing for better scalability, where each leaf only needs to know the routes for its locally connected VNIs and uses a transit VNI for inter-subnet communication.

Option A is incorrect because while unique IPs can be used (Method 1 in some docs), the defining characteristic of an efficient ERB design is the use of shared Anycast IPs for the gateway. Option C describes a Centrally-Routed Bridging (CRB) design, not ERB.

The exhibit shows the output of the show poe interface command on a Juniper switch.

VTEP Power Capabilities (Statement D): The exhibit shows the Pair/Mode as 2P/AT, which refers to the IEEE 802.3at (PoE+) standard. Under this standard, a switch can supply up to 30 W of power per port to attached devices (PDs). * Dynamic Learning (Statement C): Looking at the Max power column for port ge-0/0/0, it shows 19.5W(L). The (L) indicator signifies that the power has been dynamically learned (negotiated) via LLDP or hardware classification, rather than being statically configured by an administrator. Additionally, it has identified the device as Class 4, which is a standard dynamic classification for PoE+ devices.

Why others are incorrect: Statement A (60 W) refers to the 802.3bt (PoE++) standard, which is not indicated here. Statement B is incorrect because the "(L)" confirms the configuration is not static.

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.

The exhibit illustrates a standard end-to-end Class of Service (CoS) path consisting of an edge router (R1) and core/intermediate routers (R2 and R3).

Edge Classification (Option B): The scenario specifies that video traffic enters the network without any class-of-service markings. Because there are no DSCP or 802.1p bits to inspect, the ingress router (R1) cannot use simple Behavior Aggregate (BA) classification. Instead, it must use Multifield (MF) classification. MF classification allows the router to look deeper into the packet (Source/Destination IP, Protocol, Port numbers) to identify the video stream and assign it to a specific forwarding class. * Core Classification (Option D): Once R1 has identified the traffic, it will typically apply a rewrite rule on its egress interface to mark the packets with a specific CoS value (like a DSCP code). Consequently, the downstream routers (R2 and R3) do not need to perform expensive deep-packet inspection again. They can use Behavior Aggregate (BA) classification, which simply looks at the markings in the packet header to determine the appropriate forwarding priority.

Option A and C: These are incorrect because BA classification is impossible on unmarked ingress traffic, and performing MF classification in the core is inefficient and not a recommended Juniper design practice for high-speed transit.

The exhibit shows the output of the command show route protocol bgp on router R1 for the prefix 172.16.10.1/32. To determine the correct characteristics of this route, we analyze the specific BGP attributes and next-hop information provided in the routing table entry:

Multipath Parameter (Option B): The routing table shows two distinct paths for the prefix 172.16.10.1/32. The first path has two next hops (192.168.10.1 and 192.168.20.1) and is marked with the plus symbol (+) and the asterisk (*), indicating it is both an active and the best route. The presence of multiple next hops being used simultaneously for a single BGP path is a clear indication that the multipath parameter is enabled in the BGP configuration. In Junos OS, BGP multipath allows the installation of multiple equal-cost BGP paths into the forwarding table to facilitate load balancing.

Available Next Hops (Option C): The output explicitly lists two functional next hops for the active path: 192.168.10.1 via ge-1/0/0.2 and 192.168.20.1 via ge-1/0/1.3. Both show an outgoing interface, confirming that this route has two next hops available for traffic forwarding.

IBGP vs. EBGP (Option A): The BGP routes shown have an AS path of 200 I. This indicates the routes were learned from an external Autonomous System (AS 200). Furthermore, the protocol preference is 170. In Junos OS, the default preference for External BGP (EBGP) is 170, whereas the default preference for Internal BGP (IBGP) is 200. Therefore, this configuration is for EBGP, not IBGP.

Hidden Next Hops (Option D): The summary line at the top of the exhibit mentions that there are "2 hidden" routes in the inet.0 table. However, these hidden routes are not the next hops for the 172.16.10.1/32 prefix. A hidden route is a prefix that was rejected by policy or has an unreachable next hop; it is not a "hidden next hop" belonging to an active route.

In Junos OS, the supplicant-mode configuration under protocols dot1x determines how the switch handles multiple MAC addresses on a single physical port. According to the exhibit, the current mode is set to Single, and the Number of connected supplicants is 2. This indicates that the port is currently allowing multiple devices, which contradicts the goal of limiting access to only one device at a time.

Here is the breakdown of why Option C is the correct solution based on Juniper’s standard behavior:

Supplicant Mode: Single (Current State): In this mode, the first device to authenticate opens the port for all subsequent devices. As long as the first device remains authenticated, other devices can send traffic through the port without individual authentication. This is why the exhibit shows 2 connected supplicants despite the mode being "Single."

Supplicant Mode: Single-Secure (The Solution): This mode strictly limits the port to only one MAC address. Once a device successfully authenticates via 802.1X, the switch drops any traffic coming from any other MAC address on that port. If the authenticated device logs off or the session times out, the port becomes available for a new device, but never more than one simultaneously. * Supplicant Mode: Multiple (Option B): This mode allows multiple supplicants to authenticate individually. Each MAC address must go through its own authentication process. This would allow more than one device, which is the opposite of the user's requirement.

MAC RADIUS Restrict (Option A): This feature is used to force MAC-based authentication and does not inherently limit the number of devices to one in the same way that changing the supplicant mode does.

Maximum EAPOL requests (Option D): This parameter defines how many times the switch will send an EAP-Request/Identity frame to a supplicant before giving up. Changing this to 1 does not restrict the number of devices allowed on the port; it only changes the retry logic for a single authentication attempt.

Configuration Example for Junos OS 24.4: To implement this change, you would use the following command: set protocols dot1x edit interface ge-0/0/10.0 supplicant-mode single-secure

The exhibit shows a network topology where Multiple VLAN Registration Protocol (MVRP) is used to dynamically manage VLANs across switches Core-1, Access-1, and Access-2.

Identifying the Missing Configuration: According to the diagram, the trunk link between Core-1 and Access-2 uses interface ge-0/0/16.0 on the Core-1 side.

Output Analysis: However, the show protocols mvrp output for Core-1 only lists interfaces ge-0/0/14.0 and ge-0/0/15.0.

MVRP Requirements: For MVRP to successfully propagate VLAN information between Access-2 and the rest of the network, the protocol must be enabled on every trunk interface in the path.

Solution: Because ge-0/0/16.0 is missing from the Core-1 MVRP configuration, VLAN registration messages from Access-2 are not being processed by Core-1. Configuring this interface under MVRP on Core-1 will solve the communication problem.