JN0-351 Sample Questions Answers

The exhibit shows the configuration of BFD liveness detection for BGP at the global level, which applies to all BGP neighbors by default 1 .  However, this configuration does not specify the session mode, which determines whether BFD uses single-hop or multihop mode to communicate with a neighbor 2 .

For single-hop BGP neighbors, which are directly connected on the same subnet, the session mode can be either automatic or single-hop.  For multihop BGP neighbors, which are not directly connected and require multiple hops to reach, the session mode must be multihop 2 .

Since your BGP neighbors are in different countries, they are likely to be multihop neighbors.  Therefore, you need to configure the session mode as multihop for each neighbor individually at the [edit protocols bgp group group-name neighbor address bfd-liveness-detection] hierarchy level 2 . For example:

protocols { bgp { group usa { neighbor 192.0.2.1 { bfd-liveness-detection { session-mode multihop; } } } group france { neighbor 198.51.100.1 { bfd-liveness-detection { session-mode multihop; } } } } }

If you do not configure the session mode for multihop neighbors, BFD will use the default mode of automatic, which will try to use single-hop mode and fail to establish a BFD session with the remote neighbor 2 . This will prevent BGP from using BFD to detect liveliness and failover.

Therefore, the answer  B  is correct, as you need to configure the BFD liveness detection on the BGP neighbor level with the appropriate session mode for multihop neighbors.

According to the OSPF protocol, the designated router (DR) is the router that acts as the focal point for exchanging routing information on a multi-access network segment, such as a LAN 1 .  The DR election process is based on the following criteria, in order of precedence 1 :

The router with the highest OSPF priority becomes the DR. The default priority is 1, and a priority of 0 means the router will not participate in the election.

If there is a tie in priority, the router with the highest router ID becomes the DR. The router ID is a 32-bit number that uniquely identifies a router in an OSPF domain. It can be manually configured or automatically derived from the highest IP address of a loopback interface or a physical interface.

If there is a tie in router ID, the router that was first to become an OSPF neighbor becomes the DR.

In your scenario, two routers share the same highest priority and start time. This means that they have equal chances of becoming the DR based on the first and third criteria. Therefore, the second criterion will be used to break the tie, which is the router ID.  The router with the highest router ID will become the DR, and the other router will become the backup designated router (BDR), which is ready to take over the role of DR if it fails 1 .

Junos devices support various types of tunnels for different purposes 1 2 .

Option B is correct.  Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate a wide variety of network layer protocols inside virtual point-to-point links over an Internet Protocol network 1 .  Junos devices support GRE tunnels 1 .

Option D is correct.  IPsec (Internet Protocol Security) is a protocol suite for securing Internet Protocol (IP) communications by authenticating and encrypting each IP packet of a communication session 1 .  Junos devices support IPsec tunnels 1 .

Option A is incorrect. Spanning Tree Protocol (STP) is not a type of tunnel.  It’s a network protocol designed to prevent loops in a bridged Ethernet local area network 2 .

Option C is incorrect.  While Junos devices do support IP-IP (also known as IP tunneling), it’s not supported on all Junos devices 1 .

The two reasons for the failure to form an adjacency in a network running IS-IS could be:

B. There is no configured ISO address on any IS-IS interface. IS-IS requires each router interface to have an ISO address configured.  Without this address, the routers cannot form an adjacency 1 .

D. The family iso configuration is missing from the adjacency interface. The ‘family iso’ configuration is essential for IS-IS to function correctly.  If this configuration is missing from the adjacency interface, it could prevent the formation of an adjacency 1 .

These explanations are based on the Enterprise Routing and Switching Specialist (JNCIS-ENT) documents and learning resources available at Juniper Networks 2 3 .

The output shown in the exhibit is a brief display of the Ethernet switching table, which shows the learned Layer 2 MAC addresses for each VLAN and interface 1 .

The command  show ethernet-switching table  displays the Ethernet switching table with brief information, such as the destination MAC address, the VLAN name, the forwarding state, and the interface name 1 .

The command  show route forwarding-table  displays the routing table information for each protocol family, such as inet, inet6, mpls, iso, and so on 2 . It does not show the Ethernet switching table or the MAC addresses.

The command  show ethernet-switching table extensive  displays the Ethernet switching table with extensive information, such as the destination MAC address, the VLAN name, the forwarding state, the interface name, the VLAN index, and the tag type 1 . It shows more details than the brief output shown in the exhibit.

The command  show route forwarding-table family ethernet-switching  displays the routing table information for the ethernet-switching protocol family, which shows the destination MAC address, the next-hop MAC address, and the interface name 3 . It does not show the VLAN name or the forwarding state.

BGP has a simple loop prevention mechanism for eBGP that checks the AS path for its own AS number. However, this mechanism does not work for iBGP, since all routers in the same AS have the same AS number. Therefore, iBGP has two additional rules to prevent routing loops within an autonomous system:

A router does not advertise a route learned from an iBGP neighbor to another iBGP neighbor. This rule ensures that a route does not loop back to the original iBGP router through another iBGP path.

A router does not accept a route learned from an iBGP neighbor if the next hop is not reachable via an IGP. This rule ensures that a router does not forward packets to an unreachable next hop.

These two rules imply that iBGP sessions must be configured as a logical full mesh, meaning that every iBGP router must have a direct iBGP session with every other iBGP router in the same AS. This way, every iBGP router can learn the best route from the eBGP routers and use the IGP to reach the next hop. A logical full mesh does not require a physical full mesh, meaning that iBGP routers do not have to be directly connected. They can use intermediate routers to establish iBGP sessions, as long as the IGP can reach the next hop.

A logical full mesh can be challenging to scale and maintain, especially for large ASes with many iBGP routers. Therefore, some alternatives to the full mesh requirement have been developed, such as route reflectors and confederations. These alternatives relax the iBGP rules and introduce additional attributes and mechanisms to prevent routing loops, such as originator ID and cluster list. References:

BGP Fundamentals

BGP - eBGP loop prevention mechanism same AS number

Loop Prevention in BGP Routing

https://www.juniper.net/documentation/us/en/software/junos/cli-reference/topics/ref/statement/lsa-refresh-interval-edit-protocols-ospf.html

The feature that enables an interface on an EX Series switch to receive both untagged traffic (from the computer) and tagged traffic (from the IP phone) is the  voice VLAN 1 2 .

The voice VLAN feature in EX-series switches enables access ports to accept both data (untagged) and voice (tagged) traffic and separate that traffic into different VLANs 1 2 .  This allows the switch to differentiate between voice and data traffic, ensuring that voice traffic can be treated with a higher priority 1 2 . Therefore, option D is correct.

Link-state update packets are used to flood link-state advertisements (LSAs) among OSPF routers. The destination IP address of these packets depends on the network type and the state of the interface. On broadcast and point-to-point networks, OSPF uses two IP multicast addresses: 224.0.0.5 for all OSPF routers and 224.0.0.6 for all designated routers (DRs) and backup designated routers (BDRs)12. If the interface state is DR or BDR, the link-state update packets are sent to 224.0.0.5. Otherwise, they are sent to 224.0.0.63. On non-broadcast networks, such as Frame Relay or ATM, link-state update packets are sent as unicasts to each adjacent neighbor3. Therefore, the correct answer is A and D. References: Understanding OSPF Areas | Junos OS, How OSPF Works - IP Routing [Book] - O’Reilly Media, Link state packet - Wikipedia

 In OSPF, a DR (Designated Router) and a BDR (Backup Designated Router) are elected on each multi-access network, such as Ethernet or Frame Relay. The DR and BDR are responsible for exchanging routing information with other routers on the same network segment. The election is based on two criteria: the OSPF priority and the router ID. The router with the highest priority becomes the DR, and the router with the second-highest priority becomes the BDR. If the priorities are equal, the router with the highest router ID becomes the DR, and the router with the second-highest router ID becomes the BDR. The default priority is 1, and the router ID is either manually configured or derived from the highest IP address on any interface12.

In this scenario, R1 has a priority of 32 and a router ID of 192.168.1.2, and R2 has a priority of 64 and a router ID of 192.168.1.1. Since R2 has a higher priority than R1, it will become the BDR. The router ID does not matter in this case, because the priority is different. Therefore, the correct answer is C.

The other options are not correct because:

A. At least two routers are required for a DR/BDR election, not three2.

B. Router IDs do not have to match for an adjacency to form, they just have to be unique3.

D. R1 will not be the BDR, because it has a lower priority than R212.

B  is correct because the loops do not exist is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid.  A loop in BGP means that a route has been advertised by the same AS more than once, which can cause routing instability and inefficiency 1 .  To prevent loops, BGP uses the AS-path attribute, which lists the AS numbers that a route has traversed from the origin to the destination 2 .  The receiving router checks the AS-path attribute of the received route and discards it if it finds its own AS number in the list 2 . This way, BGP avoids accepting routes that contain loops.

C  is correct because the next hop is reachable is one of the conditions that are verified by the receiving router to ensure that the received BGP route is valid.  The next hop is the IP address of the next router that is used to forward packets to the destination network 3 .  The receiving router checks the next hop attribute of the received route and verifies that it has a valid route to reach it 3 .  If the next hop is not reachable, the received route is not usable and is rejected by the receiving router 3 . This way, BGP ensures that only feasible routes are accepted.

Aggregate routes are used for advertising summarized network prefixes 1 2 .  They help minimize the number of routing tables in an IP network by consolidating selected multiple routes into a single route advertisement 1 .  This approach is in contrast to non-aggregation routing, in which every routing table contains a unique entry for each route 1 .

Therefore, option D is correct. Options A, B, and C are not correct because:

Aggregate routes can be used with both static routing and dynamic routing protocols 1 .

Aggregate routes are not automatically generated for all of the subnets in a routing table.  They need to be manually configured 1 .

Aggregate routes are not always preferred over more specific routes.  The route selection process in Junos OS considers several factors, including route preference and metric, before determining the active route 1 .

 References:

Enterprise Routing and Switching, Specialist (JNCIS-ENT) - Juniper Networks

Enterprise Routing and Switching, Specialist (JNCIS-ENT) - Juniper Networks

Juniper Networks Certified Specialist, Enterprise Routing & Switching …

Juniper Networks Certified Specialist, Enterprise Routing & Switching …

[Configuring Layer 2 Firewall Filters - TechLibrary - Juniper Networks]

[Understanding Firewall Filter Terms - TechLibrary - Juniper Networks]

The correct answer to your question is  C. Option C . Here is why:

Option C shows the configuration of the  chassis  statement, which defines the properties of the router chassis, such as the number of aggregated Ethernet interfaces, the number of FPCs, and the number of PICs 1 .

To enable aggregated Ethernet interfaces on a router, you need to specify the  aggregated-devices  statement under the  chassis  statement and set the  ethernet  parameter to the desired number of interfaces 2 . For example, to enable two aggregated Ethernet interfaces, you can use the following configuration:

chassis { aggregated-devices { ethernet { device-count 2; } } }

Option C shows this configuration with the device-count set to 2, which will enable two aggregated Ethernet interfaces on the router. The other options do not show this configuration and will not enable any aggregated Ethernet interfaces on the router.

Therefore, option C is the correct answer to your question.

The solution that should be implemented in this scenario is to create a default generate route that includes an import policy to match BGP routes from ISP1 and assign a preference value of four or less. This way, the default route will be advertised to the internal OSPF neighbors only when there is a BGP route from ISP1 in the routing table, and it will have a higher preference than any other default route from ISP2 or ISP3. If ISP1 is not available, the default generate route will be withdrawn and the traffic will use the next available default route from ISP2 or ISP3.

Option A is incorrect because creating a default static route with ISP1’s address as the next hop while specifying the addresses for ISP2 and ISP3 as qualified next hops with a preference value of six or higher will not ensure that the default route is advertised to the internal OSPF neighbors only when ISP1 is available. The default static route will always be in the routing table regardless of the availability of ISP1, and it will have a lower preference than any other default route from ISP2 or ISP3.

Option C is incorrect because creating a default static route with each neighbor address as the next hop will not ensure that all traffic uses ISP1 as the primary connection and only uses ISP2 and ISP3 when ISP1 is not available. The default static route will always be in the routing table regardless of the availability of ISP1, and it will load balance the traffic among the three ISPs.

Option D is incorrect because creating a default aggregated route will not ensure that the default route is advertised to the internal OSPF neighbors only when ISP1 is available. The default aggregated route will always be in the routing table regardless of the availability of ISP1, and it will not have any preference value associated with it. References:

Enterprise Routing and Switching, Specialist (JNCIS-ENT) - Juniper Networks

Enterprise Routing and Switching, Specialist (JNCIS-ENT) - Juniper Networks

BGP attributes are properties that BGP uses for route advertisement, path selection, and loop prevention 1 .  There are four categories of BGP attributes 1 2 3 :

Well-known mandatory: Must be recognized by all BGP routers, present in all BGP updates, and passed on to other BGP routers 1 2 3 .

Well-known discretionary: Supported by all BGP implementations, and are optionally included in BGP updates 1 .

Optional transitive: May not be supported by all implementations of BGP 1 .

Optional non-transitive: May not be supported by all implementations of BGP 1 .

The well-known mandatory attributes must be supported by all BGP implementations and must be included in every update 1 2 3 .  These include the AS path and next hop attributes 2 3 . Therefore, options A and C are correct.

In OSPF, the state of the neighbor relationship is determined by the exchange of OSPF packets between routers 1 .  The state “2Way” as shown in the exhibit indicates that bi-directional communication has been established between the two OSPF routers 1 .  This is the normal state for a neighbor that is not the Designated Router (DR) or Backup Designated Router (BDR) on a broadcast, non-broadcast multi-access (NBMA), or point-to-multipoint network 1 .  These neighbors are often referred to as " DRothers " 1 . Therefore, option B is correct.

In a trunk port configuration with multiple VLANs, the native VLAN carries untagged traffic. This means frames sent or received on the port without a VLAN tag are automatically assigned to the native VLAN.

Here ' s why the other options are incorrect:

A. The native VLAN must be the default VLAN on the port.  This is not necessarily true. The native VLAN and the default VLAN can be different. The default VLAN is the VLAN assigned to untagged traffic on access ports, while the native VLAN applies to trunk ports.

B. The native VLAN is automatically configured through DHCP.  This is incorrect. The native VLAN needs to be manually configured on the switch port. DHCP is used for assigning IP addresses, not VLAN IDs.

D. The native VLAN is the highest tagged VLAN ID on the port.  This is also incorrect. The native VLAN and tagged VLANs are independent. The native VLAN is not related to the highest or lowest tagged VLAN ID.

Therefore, the only true statement is that the native VLAN is an untagged VLAN ID on the port.

According to the Juniper documentation 1 , the source and destination addresses of a GRE tunnel interface must be in different subnets. If they are in the same subnet, the router will not be able to route the GRE packets correctly. In the exhibit, the source and destination addresses are both in the 192.168.197.0/24 subnet, which is invalid for GRE tunneling. Therefore, the correct answer is D. References:  Configuring GRE Tunnel Interfaces | Junos OS

IS-IS (Intermediate System to Intermediate System) is a routing protocol that is designed to move information efficiently within a computer network 1 2 .  It supports multiple protocol topologies, including IPv4, IPv6, and multicast 1 2 . Therefore, options C, E, and D are correct.

In Rapid Spanning Tree Protocol (RSTP), there are several port roles that determine the behavior of the port in the spanning tree 1 2 3 .  The roles include root, designated, alternate, backup, and disabled 1 2 3 .

The discarding state is associated with the backup, alternate, and disabled roles 1 2 3 .  In a stable topology with consistent port roles throughout the network, RSTP ensures that every root port and designated port immediately transition to the forwarding state while all alternate and backup ports are always in the discarding state 2 .  Disabled ports are also in the discarding state 3 .

Therefore, options B, C, and D are correct.