Practice Free JN0-351 Enterprise Routing and Switching Specialist (JNCIS-ENT) Exam Questions Answers With Explanation

In Rapid Spanning Tree Protocol (RSTP), there are several port roles that determine the behavior of the port in the spanning tree123. The roles include root, designated, alternate, backup, and disabled123.

The discarding state is associated with the backup, alternate, and disabled roles123. 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 state2. Disabled ports are also in the discarding state3.

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

• 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 RSTP1. 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 sequence2.
• The other switches in the network are likely to be running RSTP, as it is the default protocol for EX Series switches3. 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 neighbor1.
• 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 mode1.

• A is correct because the interface is configured as a disabled port. A disabled port does not forward any traffic, including DHCP packets. Therefore, DHCP snooping cannot learn any MAC addresses or lease information from a disabled port1.
• C is correct because the interface is configured as a trunk port. By default, all trunk ports on the switch are trusted for DHCP snooping2. This means that DHCP snooping does not inspect or filter any DHCP packets received on a trunk port. Therefore, DHCP snooping does not add any entries to the snooping database for a trunk port2.

• Option A is correct. The storm control feature is enabled in the factory-default configuration on EX Series switches12. On EX2200, EX3200, EX3300, EX4200, and EX6200 switches, the factory default configuration enables storm control for broadcast and unknown unicast traffic on all switch interfaces2. On EX4300 switches, the factory default configuration enables storm control on all Layer 2 switch interfaces1.
• Option B is incorrect. The storm control feature does not require a special license on EX Series switches34.
• Option C is incorrect. There’s no information available that suggests the storm control feature is not supported on aggregate Ethernet interfaces.
• Option D is incorrect. The storm control configuration applies to traffic at the ingress of an interface5, not just between the forwarding and control plane.

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 network1.

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 interface2.

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.

References:

1:OSPF DR/BDR Election: Process, Configuration, and Tuning2:OSPF Designated Router (DR) and Backup Designated Router (BDR)

• A broadcast storm is a network condition where a large number of broadcast packets are sent and received by multiple devices, causing congestion and performance degradation1. A broadcast storm can occur when there are loops in the network topology, meaning that there are multiple paths between two devices2.
• A spanning tree protocol is a network protocol that prevents loops from being formed when switches or bridges are interconnected via multiple paths. It does this by creating a logical tree structure that spans all the devices in the network, and disabling or blocking the links that are not part of the tree, leaving a single active path between any two devices3.
• By eliminating loops, a spanning tree protocol also eliminates broadcast storms, as broadcast packets will not be forwarded endlessly along the looped paths. Instead, broadcast packets will be sent only along the tree structure, reaching each device once and avoiding congestion3.

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 adjacencies1. If the MTU is less than this, IS-IS adjacencies will not be established1.

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-IS1. If the loopback interface is not included in IS-IS, it could prevent IS-IS adjacencies from being established1.

Therefore, options A and D are correct.

• B is correct because duplex and speed settings are required to match on both participating devices. According to the Juniper Networks documentation1, all the interfaces in a LAG must have the same speed and be in full-duplex mode. This ensures that the LAG can operate as a single logical link without any performance or compatibility issues.
• C is correct because member links are not required to be contiguous ports. According to the Juniper Networks documentation2, you can group any Ethernet interfaces on a switch into a LAG, regardless of their physical location or slot number. This provides flexibility and scalability for configuring LAGs on switches.

• A is correct because IBGP updates the next-hop attribute to ensure reachability within an AS. This is because the next-hop attribute is the IP address of the router that advertises the route to a BGP peer. If the next-hop attribute is not changed by IBGP, it would be the IP address of an external router, which may not be reachable by all routers within the AS. Therefore, IBGP updates the next-hop attribute to the IP address of the router that received the route from an EBGP peer1.
• B is correct because IBGP peers advertise routes received from EBGP peers to other IBGP peers. This is because BGP follows the rule of advertising only the best route to a destination, and EBGP routes have a higher preference than IBGP routes. Therefore, IBGP peers advertise routes learned from an EBGP peer to all BGP peers, including both EBGP and IBGP peers1.

• 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 interface1.
• 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 name1.
• The command show route forwarding-table displays the routing table information for each protocol family, such as inet, inet6, mpls, iso, and so on2. 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 type1. 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, whichshows the destination MAC address, the next-hop MAC address, and the interface name3. It does not show the VLAN name or the forwarding state.

• A is correct because RSTP alternate ports block traffic while receiving superior BPDUs from a neighboring switch. An alternate port is a backup port for a root port, which means it receives better BPDUs from another bridge than the current root port1. However, an alternate port does not forward any traffic, as it is in a discarding state2. It only listens to BPDUs and waits for the root port to fail. If the root port fails, the alternate port can immediately transition to a forwarding state and become the new root port1.
• C is correct because RSTP alternate ports provide an alternate higher cost path to the root bridge. An alternate port is selected based on the same criteria as the root port, which are the lowest bridge ID, the lowest path cost, the lowest sender port ID, and the lowest receiver port ID3. However, an alternate port receives a higher cost BPDU than the root port, otherwise it would be the root port itself1. Therefore, an alternate port provides an alternate higher cost path to the root bridge than the root port.

• 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.

BGP (Border Gateway Protocol) is a protocol designed to exchange routing and reachability information among autonomous systems (AS) on the internet1.

• Option A is correct. When an EBGP router advertises routes to its neighbors, it appends its AS number to the AS_PATH attribute1. This is a key mechanism in BGP to prevent routing loops1.
• Option C is correct. BGP has a built-in loop prevention mechanism whereby if a BGP router detects its own AS in the AS_PATH attribute, it will drop the prefix and will not continue to advertise it2. This helps to prevent routing loops2.
• Option B is incorrect. EBGP routers do not accept routes that contain their own AS number in the AS_PATH2. Instead, they drop such routes as part of the loop prevention mechanism2.
• Option D is incorrect. While it’s true that EBGP routers append their AS number when advertising routes, they do not prepend their AS number1. The term “prepend” in BGP usually refers to a technique used to influence path selection by artificially lengthening the AS_PATH3.

To isolate management traffic in a non-default routing-instance on Junos-based devices, you can use the set system management-instance and set routing-instances mgmt_junos commands12.

• set system management-instance: This command associates the management interface (usually named fxp0 or em0 for Junos OS, or re0:mgmt-* or re1:mgmt-* for Junos OS Evolved) with the non-default virtual routing and forwarding (VRF) instance1. After you configure the non-default management VRF instance, management traffic no longer has to share a routing table with other control traffic or protocol traffic1.
• set routing-instances mgmt_junos: This command creates a new routing instance named mgmt_junos. The name of the dedicated management VRF instance is reserved and hardcoded as mgmt_junos; you cannot configure any other routing instance by the name mgmt_junos1.

Therefore, options C and D are correct. Options A and B are not correct because they attempt to assign an interface to the mgmt_junos routing instance, which is not necessary for isolating management traffic1.