JN0-351 Enterprise Routing and Switching Specialist (JNCIS-ENT) Questions and Answers

The DHCP snooping feature in Juniper Networks’ EX Series switches works by building a binding database that maps the IP address, MAC address, lease time, binding type, VLAN number, and interface information 1 .  This database is used to filter and validate DHCP messages from untrusted sources 1 .

However, there are certain conditions that could prevent entries from being automatically created in the snooping database for an interface:

MAC limiting: If MAC limiting is enabled on the interface, it could potentially interfere with the operation of DHCP snooping.  MAC limiting restricts the number of MAC addresses that can be learned on a physical interface to prevent MAC flooding attacks 1 . This could inadvertently limit the number of DHCP clients that can be learned on an interface, thus preventing new entries from being added to the DHCP snooping database.

Static IP address : If the device connected to the interface is configured with a static IP address, it will not go through the DHCP process and therefore will not have an entry in the DHCP snooping database 1 .  The DHCP snooping feature relies on monitoring DHCP messages to build its database 1 , so devices with static IP addresses that do not send DHCP messages will not have their information added.

Therefore, options B and C are correct.  Options A and D are not correct because performing a DHCPRELEASE would simply remove an existing entry from the database 1 , and Dynamic ARP inspection (DAI) uses the information stored in the DHCP snooping binding database but does not prevent entries from being created 1 .

In BGP, routing policies are used to control the flow of routing information between BGP peers 1 .

Option C suggests that import policies are applied after the RIB-In table.  This is correct because import policies in BGP are applied to routes that are received from a BGP peer, before they are installed in the local BGP Routing Information Base (RIB-In) 1 .  The RIB-In is a database that stores all the routes that are received from all peers 1 .

Option D suggests that export policies are applied after the RIB-Local table.  This is correct because export policies in BGP are applied to routes that are being advertised to a BGP peer, after they have been selected from the local BGP Routing Information Base (RIB-Local) 1 .  The RIB-Local is a database that stores all the routes that the local router is using 1 .

Therefore, options C and D are correct.

 A firewall filter is a Junos security solution to filter or control traffic at the data plane as they enter or exit an interface 1 .  A firewall filter consists of one or more terms, each with a set of conditions and actions 2 .  The device evaluates every packet against the firewall filter terms in the order they are defined, and performs the actions specified in the first term that matches the packet 3 .  If no term matches the packet, the device discards the packet by default 3 .

In the exhibit, the firewall filter  mac-address  is applied to the  family ethernet-switching  and involves filtering MAC addresses. There are two terms,  term one  and  term two , each with different conditions and actions. The  from  statement specifies the match conditions, and the  then  statement specifies the actions.  If the  from  statement is omitted, all packets are considered to match and the actions in the  then  statement are taken 2 .  If the  then  statement is omitted, the packets that match the conditions in the  from  statement are accepted by default 2 .

Term one  logs traffic from a specific source MAC address  88:05:00:29:3c:de/48 .  The  log  action is a nonterminating action, which means that the device continues to evaluate the packet against the remaining terms in the filter 2 . Therefore, traffic that matches the  from  statement in term one is not discarded, and option A is incorrect.

Term two  accepts all other traffic not specified in term one.  The  then accept  action is a terminating action, which means that the device stops evaluating the packet against the filter and forwards the packet 2 . Therefore, all traffic not matching the  from  statement in term one is accepted by term two, and option D is correct.

Term two  does not have a  from  statement, which means that all packets are considered to match this term. Therefore, traffic matching the  from  statement in term two is not logged, and option B is incorrect.

Term one  does not have a  then  statement other than the  log  action, which means that the packets that match the  from  statement are accepted by default. Therefore, traffic that matches the  from  statement in term one is accepted, and option C is correct.

A. The routes displayed have traversed one or more autonomous systems. True. The AS path attribute shows AS numbers (e.g., 65401 65520.), indicating the route ' s traversal through various autonomous systems. D. The routes displayed are being learned from an IBGP peer. True. The absence of the ' * > i ' marker before the prefixes indicates they are being learned from an IBGP peer. Therefore, the correct statements are A. The routes displayed have traversed one or more autonomous systems. and D. The routes displayed are being learned from an IBGP peer.

According to the Juniper documentation1, when a switch receives a unicast frame destined to a MAC address that is not registered in the bridging table, it floods the frame out all ports that are part of the same VLAN, except the receiving port. This is the default behavior of a switch when it does not know the destination MAC address. The switch does not perform an ARP request, send an error message, or flood the frame to all VLANs. References: Understanding Unicast Flooding | Junos OS | Juniper Networks

Option A suggests that the MTU is not at least 1492 bytes.  This is correct because IS-IS requires a minimum MTU of 1492 bytes to establish adjacencies 1 .  If the MTU is less than this, IS-IS adjacencies will not be established 1 .

Option D suggests that the lo0 interface is not included as an IS-IS interface.  This is also correct because the loopback interface (lo0) is typically used as the router ID in IS-IS 1 .  If the loopback interface is not included in IS-IS, it could prevent IS-IS adjacencies from being established 1 .

Therefore, options A and D are correct.

An ISO NET address is a type of network address used by the IS-IS routing protocol.  It identifies a point of connection to the network, such as a router interface, and is also called a Network Service Access Point (NSAP) 1 .

An ISO NET address consists of three parts: an area ID, a system ID, and a selector 2 . The area ID identifies the IS-IS area to which the device belongs. The system ID uniquely identifies the device within the area.  The selector identifies a specific service or function on the device, such as routing or management 2 .

An ISO NET address must be unique for each device in the network, because it is used by IS-IS to establish adjacencies, exchange routing information, and compute shortest paths 2 . If two devices have the same ISO NET address, they will not be able to communicate with each other or with other devices in the network. Therefore, it is important to assign different ISO NET addresses to each device in the network.

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.

A RIB group is a configuration that allows a routing protocol to install routes into multiple routing tables in Junos OS. A RIB group consists of an import-rib statement, which specifies the source routing table, and an export-rib statement, which specifies the destination routing table or group.  A RIB group can also include an import-policy statement, which specifies one or more policies to control which routes are imported into the destination routing table or group 1 .

An import policy is a policy statement that defines the criteria for accepting or rejecting routes from the source routing table. An import policy can also modify the attributes of the imported routes, such as preference, metric, or community.  An import policy can be applied to a RIB group by using the import-policy statement under the [edit routing-options rib-groups] hierarchy level 1 .

Therefore, option A is correct, because an import policy is applied to the RIB group to control which routes are installed in the destination routing table or group. Option B is incorrect, because all routes in the source routing table are imported into the destination routing table or group, unless filtered by an import policy. Option C is incorrect, because a firewall filter is not used to install routes in the RIB groups; a firewall filter is used to filter packets based on various criteria. Option D is incorrect, because an export policy is not applied to the RIB group; an export policy is applied to a routing protocol to control which routes are advertised to other devices.

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.

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.

The BGP attribute that you would use to forward all Internet traffic through the R1 device is the  local preference 1 .

The local preference is an attribute that is used within an autonomous system (AS) and exchanged between iBGP routers 1 .  It is used to select an exit point from the AS 1 .  The path with the highest local preference is preferred 1 .  By setting a higher local preference for the routes received from R1, you can make R1 the preferred exit point for all Internet traffic 1 .

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.

The Graceful Routing Engine Switchover (GRES) feature in Junos OS enables a router with redundant Routing Engines to continue forwarding packets, even if one Routing Engine fails 1 .  GRES preserves interface and kernel information, ensuring that traffic is not interrupted 1 .  However, GRES does not preserve the control plane 1 .

To preserve routing during a switchover, GRES must be combined with either Graceful Restart protocol extensions or Nonstop Active Routing (NSR) 1 .  When GRES is combined with NSR, nearly 75 percent of line rate worth of traffic per Packet Forwarding Engine remains uninterrupted during GRES 1 .  Any updates to the primary Routing Engine are replicated to the backup Routing Engine as soon as they occur 1 .

Therefore, when GRES is combined with NSR, routing is preserved and the new master RE does not restart rpd 1 .

OSPF DR/BDR election is a process that occurs on multi-access data links. It is intended to select two OSPF nodes: one to be acting as the Designated Router (DR), and another to be acting as the Backup Designated Router (BDR). The DR and BDR are responsible for generating network LSAs for the multi-access network and synchronizing the LSDB with other routers on the same network 1 .

The DR/BDR election is based on two criteria: the OSPF priority and the router ID. The OSPF priority is a value between 0 and 255 that can be configured on each interface participating in OSPF. The default priority is 1. A priority of 0 means that the router will not participate in the election and will never become a DR or BDR. The router with the highest priority will become the DR, and the router with the second highest priority will become the BDR. If there is a tie in priority, then the router ID is used as a tie-breaker. The router ID is a 32-bit number that uniquely identifies each router in an OSPF domain. It can be manually configured or automatically derived from the highest IP address on a loopback interface or any active interface 2 .

In this scenario, all routers have the same priority of 1, so the router ID will determine the outcome of the election. The router IDs are shown in the exhibit as RID values. The highest RID belongs to R4 (10.10.10.4), so R4 will become the DR. The second highest RID belongs to R3 (10.10.10.3), so R3 will become the BDR.

The exhibit shows the configuration of RSTP on EX-4, which has the command  force-version stp .  This command forces the switch to use the legacy STP protocol instead of RSTP, even though the switch supports RSTP 1 .  This means that EX-4 will not be able to take advantage of the faster convergence and enhanced features of RSTP, such as edge ports, link type, and proposal/agreement sequence 2 .

The other switches in the network are likely to be running RSTP, as it is the default protocol for EX Series switches 3 . Therefore, there will be a compatibility issue between EX-4 and the other switches, which will result in longer convergence times and suboptimal performance.  The switch will also generate a warning message that says “Warning: STP version mismatch with neighbor” when it receives a BPDU from a RSTP neighbor 1 .

To solve this problem, the force-version command must be removed from EX-4, so that it can run RSTP natively and interoperate with the other switches in the network. This will enable faster convergence and better stability for the network topology.  To remove the command, you can use the  delete protocols rstp force-version  command in configuration mode 1 .

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.

A. On R1, enable Level 1 on the ge-0/0/1 interface.  In IS-IS, both levels (Level 1 and Level 2) are enabled by default when you enable IS-IS on an interface 1 .  Level 1 systems route within an area 2 .  If the destination is outside an area, Level 1 systems route toward a Level 2 system 2 . Therefore, enabling Level 1 on the ge-0/0/1 interface on R1 would allow packets to reach from R1 to R3.

D. On R3 enable Level 1 on the ge-0/0/4 interface Similarly, enabling Level 1 on the ge-0/0/4 interface on R3 would allow packets to reach from R1 to R3.

These explanations are based on the IS-IS configuration documents and learning resources available at Juniper Networks 1  and Cisco 3 4 .

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.

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 .

BFD is a protocol that can be used to quickly detect failures in the forwarding path between two adjacent routers or switches. BFD can be integrated with various routing protocols and link aggregation protocols to provide faster convergence and fault recovery.

According to the Juniper Networks documentation, the following protocols support BFD on Junos OS devices 1 :

BGP: BFD can be used to monitor the connectivity between BGP peers and trigger a session reset if a failure is detected.  BFD can be configured for both internal and external BGP sessions, as well as for IPv4 and IPv6 address families 2 .

OSPF: BFD can be used to monitor the connectivity between OSPF neighbors and trigger a state change if a failure is detected.  BFD can be configured for both OSPFv2 and OSPFv3 protocols, as well as for point-to-point and broadcast network types 3 .

LACP: BFD can be used to monitor the connectivity between LACP members and trigger a link state change if a failure is detected.  BFD can be configured for both active and passive LACP modes, as well as for static and dynamic LAGs 4 .

Other protocols that support BFD on Junos OS devices are:

IS-IS: BFD can be used to monitor the connectivity between IS-IS neighbors and trigger a state change if a failure is detected. BFD can be configured for both level 1 and level 2 IS-IS adjacencies, as well as for point-to-point and broadcast network types.

RIP: BFD can be used to monitor the connectivity between RIP neighbors and trigger a route update if a failure is detected. BFD can be configured for both RIP version 1 and version 2 protocols, as well as for IPv4 and IPv6 address families.

VRRP: BFD can be used to monitor the connectivity between VRRP routers and trigger a priority change if a failure is detected. BFD can be configured for both VRRP version 2 and version 3 protocols, as well as for IPv4 and IPv6 address families.

The protocols that do not support BFD on Junos OS devices are:

RSTP: RSTP is a spanning tree protocol that provides loop prevention and rapid convergence in layer 2 networks. RSTP does not use BFD to detect link failures, but relies on its own hello mechanism that sends BPDU packets every 2 seconds by default.

FTP: FTP is an application layer protocol that is used to transfer files between hosts over a TCP connection. FTP does not use BFD to detect connection failures, but relies on TCP’s own retransmission and timeout mechanisms.

A keepalive OAM probe is a mechanism that can be used to monitor the state of a GRE tunnel and detect any failures in the tunnel path. A keepalive OAM probe consists of sending periodic packets from one end of the tunnel to the other and expecting a reply.  If no reply is received within a specified time, the tunnel is considered down and the line protocol of the tunnel interface is changed to down 1 .

To configure a keepalive OAM probe for a GRE tunnel, you need to specify two parameters: the keepalive-time and the hold-time. The keepalive-time is the interval between each keepalive packet sent by the local router.  The hold-time is the maximum time that the local router waits for a reply from the remote router before declaring the tunnel down 2 .

According to the Juniper Networks documentation, the hold-time value must be two times the keepalive-time value for a GRE tunnel 2 . This is because the hold-time value must account for both the round-trip time of the keepalive packet and the processing time of the remote router. If the hold-time value is too small, it may cause false positives and unnecessary tunnel flaps.

In the exhibit, the configuration shows that the keepalive-time is set to 10 seconds and the hold-time is set to 15 seconds for the gr-1/1/10.1 interface. This means that the local router will send a keepalive packet every 10 seconds and will wait for 15 seconds for a reply from the remote router. However, this hold-time value is not two times the keepalive-time value, which violates the recommended configuration. This may cause traffic drops if the remote router takes longer than 15 seconds to reply.

Therefore, option D is correct, because the hold-time value must be two times the keepalive-time value for a GRE tunnel.  Option A is incorrect, because BFD is not required for GRE tunnels; BFD is another protocol that can be used to monitor tunnels, but it is not compatible with GRE keepalives 3 .  Option B is incorrect, because the “event link-adjacency-loss” option is not related to GRE tunnels; it is an option that can be used to trigger an action when a link goes down 4 .  Option C is incorrect, because LLDP does not need to be removed from the gr-1/1/10.1 interface; LLDP is a protocol that can be used to discover neighboring devices and their capabilities, but it does not interfere with GRE tunnels 5 .

In BGP, the path selection process is based on a set of attributes 1 .  The process starts by preferring the path with the highest weight, then the highest local preference, then the locally originated routes, and so on 1 .  If all these attributes are the same, then it prefers the path with the shortest AS path 1 .

Referring to the exhibit, all four ISPs have the same weight, local preference, and origin 1 .  However, ISP 4 has the shortest AS path 1 . Therefore, ISP 4 will be selected as the active path. So, option C is correct.

A  is correct because you need to configure BGP multihop to enable redundancy for the EBGP peering between the two routers.  BGP multihop is a feature that allows BGP peers to establish a session over multiple hops, instead of requiring them to be directly connected 1 .  By default, EBGP peers use a time-to-live (TTL) value of 1 for their packets, which means that they can only reach adjacent neighbors 1 .  However, if you configure BGP multihop with a higher TTL value, you can allow EBGP peers to communicate over multiple routers in between 1 . This can provide redundancy in case of a link failure or a router failure between the EBGP peers.

B  is correct because you need to configure loopback interface peering to enable redundancy for the EBGP peering between the two routers.  Loopback interface peering is a technique that uses loopback interfaces as the source and destination addresses for BGP sessions, instead of physical interfaces 2 .  Loopback interfaces are virtual interfaces that are always up and reachable as long as the router is operational 2 .  By using loopback interface peering, you can avoid the dependency on a single physical interface or link for the BGP session, and use multiple paths to reach the loopback address of the peer 2 . This can provide redundancy and load balancing for the EBGP peering.

C  is correct because you need to configure routes for the peer loopback interface IP addresses to enable redundancy for the EBGP peering between the two routers.  Routes for the peer loopback interface IP addresses are necessary to ensure that the routers can reach each other’s loopback addresses over multiple hops 2 .  You can use static routes or dynamic routing protocols to advertise and learn the routes for the peer loopback interface IP addresses 2 . Without these routes, the routers will not be able to establish or maintain the BGP session using their loopback interfaces.