Juniper Juniper Certifications JN0-351 Questions & Answers

• Question 131:

  After receiving a BGP route, which two conditions are verified by the receiving router to ensure that the received route is valid? (Choose two)

  A. The AS-path length is greater than 0.

  B. The loops do not exist.

  C. The next hop is reachable.

  D. The local preference is greater than 0.

  Correct Answer: BC

  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 inefficiency1. To prevent loops, BGP uses the AS-path attribute, which lists the AS numbers that a route has traversed from the origin to the destination2. The receiving router checks the AS-path attribute of the received route and discards it if it finds its own AS number in the list2. 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 network3. The receiving router checks the next hop attribute of the received route and verifies that it has a valid route to reach it3. If the next hop is not reachable, the received route is not usable and is rejected by the receiving router3. This way, BGP ensures that only feasible routes are accepted.

• Question 132:

  Exhibit

  

  You are troubleshooting an issue where traffic to 192.168.10.0/24 is being sent to R1 instead of your desired path through R2.

  Referring to the exhibit, what is the reason for the problem?

  A. R2's route is not the best path due to loop prevention.

  B. R2's route is not the best path due to a lower origin code.

  C. R1's route is the best path due to a higher local preference

  D. R1's route is the best path due to the shorter AS path.

  Correct Answer: C

  The exhibit shows the output of the command show ip bgp, which displays information about the BGP routes in the routing table1. The output shows two routes for the destination 192.168.10.0/24, one from R1 and one from R2. The route from R1 has a local preference of 200, while the route from R2 has a local preference of 100. Local preference is a BGP attribute that indicates the degree of preference for a route within an autonomous system (AS)2. A higher local preference means a more preferred route2. BGP uses a best path selection algorithm to choose the best route for each destination among multiple paths. The algorithm compares different attributes of the routes in a specific order of precedence3. The first attribute that is compared is weight, which is a Cisco-specific attribute that is local to the router3. If the weight is equal or not set, the next attribute that is compared is local preference3. In this case, both routes have the same weight of 0, which means that they are learned from external BGP (eBGP) peers3. Therefore, the next attribute that is compared is local preference. Since R1's route has a higher local preference than R2's route, it is chosen as the best path and installed in the routing table3. The other attributes, such as origin code and AS path, are not considered in this case.

• Question 133:

  Exhibit.

  

  Why is this OSPF adjacency remaining in this state?

  A. A subnet mask mismatch exists between the OSPF neighbors.

  B. An MTU mismatch exists between the OSPF neighbors.

  C. A hello interval mismatch exists between the OSPF neighbors.

  D. An area ID mismatch exists between the OSPF neighbors

  Correct Answer: B

  The exhibit shows the output of the command show ospf neighbor, which displays information about the OSPF neighbors on a router1. The output shows that the OSPF neighbor with the address 172.26.1.1 and the interface ge-0/0/3.0 is in the Exstart state1. The Exstart state is the fourth state in the OSPF neighbor formation process, after Down, Init, and 2-Way states2. In this state, the OSPF neighbors establish a master-slave relationship and exchange database description (DBD) packets, which contain summaries of their link-state databases2. The most common reason for OSPF neighbors to be stuck in the Exstart state is an MTU mismatch between the interfaces3. MTU stands for maximum transmission unit, which is the largest size of a packet that can be transmitted on a network segment4. If the MTU values of two OSPF neighbors are different, theymay not be able to exchange DBD packets successfully, as some packets may be dropped or fragmented due to their size exceeding the MTU limit3. To solve this problem, you need to ensure that the MTU values of both OSPF neighbors are the same or compatible. You can use the command show interfaces to display the MTU value of an interface5. You can also use the command ping with the do-not-fragment option to test the MTU size between two routers. You can change the MTU value of an interface by using the command set interfaces interface-name mtu mtu-value in configuration mode5.

• Question 134:

  You have DHCP snooping enabled but no entries are automatically created in the snooping database for an interface on your EX Series switch. What are two reasons for the problem? (Choose two.)

  A. The device that is connected to the interface has performed a DHCPRELEASE.

  B. MAC limiting is enabled on the interface.

  C. The device that is connected to the interface has a static IP address.

  D. Dynamic ARP inspection is enabled on the interface.

  Correct Answer: BC

  Explanation: 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 information1. This

  database is used to filter and validate DHCP messages from untrusted sources1.

  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 attacks1. 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 database1. The DHCP snooping feature relies

  on monitoring DHCP messages to build its database1, 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 database1, and Dynamic ARP inspection (DAI) uses the information stored in the

  DHCP snooping binding database but does not prevent entries from being created1.

• Question 135:

  You are troubleshooting a BGP routing issue between your network and a customer router and are reviewing the BGP routing policies. Which two statements are correct in this scenario? (Choose two.)

  A. Export policies are applied to routes in the RIB-ln table.

  B. Import policies are applied to routes in the RIB-Local table.

  C. Import policies are applied after the RIB-ln table.

  D. Export policies are applied after the RIB-Local table.

  Correct Answer: CD

  Explanation: In BGP, routing policies are used to control the flow of routing information between BGP peers1.

  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 peers1. 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 using1.

  Therefore, options C and D are correct.

• Question 136:

  Two routers share the same highest priority and start time.

  A. In this situation, what is evaluated next when determining the designated router? The router with the lowest router ID become the DR.

  B. The router with the highest router ID becomes the DR

  C. The routers perform another DR election.

  D. The router with the highest MAC address become the DR

  Correct Answer: B

  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 LAN1. The DR election process is based on the following

  criteria, in order of precedence1:

  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 fails1.

• Question 137:

  Exhibit

  

  Referring to the exhibit, which statement is correct?

  A. The local device is using a bridge priority of 4k.

  B. The root bridge is using a bridge priority of 4k.

  C. The root bridge has not been elected for this RSTP topology.

  D. The local device is the root bridge for this RSTP topology.

  Correct Answer: D

  Explanation: In a Rapid Spanning Tree Protocol (RSTP) topology, the root bridge is determined by the switch with the lowest bridge priority value12. If all switches have the same priority, then the root bridge is assigned to the switch whose MAC address's hex value is the lowest2. The default bridge priority value is 3276832. However, without the actual exhibit, it's difficult to definitively determine which device is the root bridge. But based on the options provided, if we assume that the local device has a lower bridge priority or a lower MAC address than other devices in the network, then it could be considered as the root bridge for this RSTP topology45.

• Question 138:

  Refer to the exhibit.

  

  Referring to the output shown in the exhibit, which statement is correct?

  A. The state is normal for a DR neighbor.

  B. The state is normal for a DRother neighbor

  C. An MTU mismatch exists between the OSPF neighbors.

  D. An area ID mismatch exists between the OSPF neighbors

  Correct Answer: B

  Explanation: In OSPF, the state of the neighbor relationship is determined by the exchange of OSPF packets between routers1. The state "2Way" as shown in the exhibit indicates that bi-directional communication has been established

  between the two OSPF routers1. 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 network1. These

  neighbors are often referred to as "DRothers"1.

  Therefore, option B is correct.

• Question 139:

  What is the maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation?

  A. 1496 bytes

  B. 1480 bytes

  C. 1500 bytes

  D. 1476 bytes

  Correct Answer: D

  Explanation: The maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation is 1476 bytes1. This is because GRE packets are formed by the addition of the original packets and the required GRE headers1. These headers are 24- bytes in length and since these headers are added to the original frame, depending on the original size of the packet we may run into IP MTU problems1. The most common IP MTU is 1500-bytes in length (Ethernet)1. When the tunnel is created, it deducts the 24-bytes it needs to encapsulate the passenger protocols and that is the IP MTU it will use1. For example, if we are forming a tunnel over FastEthernet (IP MTU 1500)the IOS calculates the IP MTU on the tunnel as: 1500-bytes from Ethernet - 24-bytes for the GRE encapsulation = 1476-Bytes1.

• Question 140:

  What is the default MAC age-out timer on an EX Series switch?

  A. 30 minutes

  B. 30 seconds

  C. 300 minutes

  D. 300 seconds

  Correct Answer: D

  Explanation: The default MAC age-out timer on an EX Series switch is 300 seconds12. The MAC age-out timer is the maximum time that an entry can remain in the MAC table before it "ages out," or is removed31. This configuration can influence efficiency of network resource use by affecting the amount of traffic that is flooded to all interfaces1. When traffic is received for MAC addresses no longer in the Ethernet routing table, the router floods the traffic to all interfaces1.