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

The maximum allowable MTU size for a default GRE tunnel without IPv4 traffic fragmentation is 1476 bytes 1 .  This is because GRE packets are formed by the addition of the original packets and the required GRE headers 1 .  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 problems 1 .  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 use 1 .  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-Bytes 1 .

The routers that are directly connected to the Dallas and Denver subnets are not advertising the connected subnets.  This can be resolved by redistributing the connected subnets into OSPF 1 .

Option C suggests to configure and apply a routing policy that redistributes the connected Dallas and Denver subnets.  This is correct because redistribution allows routes from one routing protocol to be communicated to another, and in this case, it allows the connected subnets to be advertised through OSPF 1 .

Option D suggests enabling the passive option on the OSPF interfaces that are connected to the Dallas and Denver subnets.  This is also correct because in OSPF, a passive interface is an interface that belongs to the OSPF router, but does not send OSPF Hello packets 1 .  It’s typically used on an interface that you don’t want to use for OSPF adjacencies, but you still want to advertise its IP address 1 . Therefore, enabling passive interface can help in advertising the Dallas and Denver subnets.

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 degradation 1 .  A broadcast storm can occur when there are loops in the network topology, meaning that there are multiple paths between two devices 2 .

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 devices 3 .

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 congestion 3 .

The output of the show bgp neighbor command shows the following information:

The local AS number of R1 is 65400, as indicated by the line “Local: 10.32.1.1+179 AS 65400”.

The peer AS number of 10.32.1.2 is 63645, as indicated by the line “Peer: 10.32.1.2+179 AS 63645”.

The BGP session is established, as indicated by the line “State: Established”.

The routers are exchanging IPv4 routes, as indicated by the line “Type: External State: Established Flags: < Sync > ” and the table showing the number of prefixes received and sent for inet.0 routing table. References:  Enterprise Routing and Switching, Specialist (JNCIS-ENT) - Juniper Networks , [show bgp neighbor - Technical Documentation - Support - Juniper Networks]

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 port 1 .  However, an alternate port does not forward any traffic, as it is in a discarding state 2 . 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 port 1 .

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 ID 3 .  However, an alternate port receives a higher cost BPDU than the root port, otherwise it would be the root port itself 1 . Therefore, an alternate port provides an alternate higher cost path to the root bridge than the root port.

B  is correct because duplex and speed settings are required to match on both participating devices.  According to the Juniper Networks documentation 1 , 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 documentation 2 , 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.

Option A is correct. For traffic to be sent through a GRE tunnel, there must be a route that directs the traffic into the tunnel. This is typically accomplished through the use of a static route or a dynamic routing protocol.

Option B is correct. All intermediary devices must have a route to the tunnel endpoints34. In real-world scenarios, the tunnel endpoints for a tunnel going over the Internet must have globally reachable internet addresses. Otherwise, intermediate routers in the Internet cannot forward the tunneled packets.

Default BGP keepalive timer in Junos is 30 seconds

The default keepalive time for BGP is 30 seconds, which means that BGP peers send keepalive messages to each other every 30 seconds to verify the liveness of the BGP session. If no keepalive message is received from a peer within the hold time, which is 90 seconds by default, the peer is considered to be unavailable and the BGP session is closed. The keepalive time and the hold time can be configured manually for each BGP peer or peer group, but they must match on both ends of the BGP session. The keepalive time should be one third of the hold time for optimal performance. Therefore, option C is correct and the other options are incorrect. References:  BGP keepalive messages and hold time - Juniper Networks

When the MAC address limit feature with the shutdown action is implemented on a switch, if a violation occurs, the interface is disabled and a system log entry is generated 1 .  If the switch has been configured with the port-error-disable statement, the disabled interface recovers automatically upon expiration of the specified disable timeout 1 .  However, if the switch has not been configured for auto-recovery from port error disabled conditions, you must manually clear the violation by running the clear ethernet-switching port-error command for the interface to send and receive traffic again 1 .  This explanation is based on the Enterprise Routing and Switching Specialist (JNCIS-ENT) documents and learning resources available at Juniper Networks 1 .

In OSPF, the Designated Router (DR) and Backup Designated Router (BDR) are elected based on the priority of the routers 1 .  The router with the highest priority becomes the DR, and the router with the second highest priority becomes the BDR 1 .  If there is a tie in priority, then the router with the highest Router ID is chosen 1 .

In this scenario, R2 has a higher priority (64) than R1 (32), so R2 will become the DR 1 .  Since R1 has the second highest priority, it will become the BDR 1 . Therefore, option D is correct.

 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.

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.

Option A is correct. Creating multiple areas in OSPF can help to reduce the convergence time . This is because changes in one area do not affect other areas, so fewer routers need to run the SPF algorithm in response to a change.

Option D is correct. Creating multiple areas in OSPF can help to reduce Link State Advertisement (LSA) flooding across the network. This is because LSAs are not flooded out of their area of origin.

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.

The exhibit shows the output of the command  show ospf neighbor , which displays information about the OSPF neighbors on a router 1 .

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 state 1 .

The Exstart state is the fourth state in the OSPF neighbor formation process, after Down, Init, and 2-Way states 2 .  In this state, the OSPF neighbors establish a master-slave relationship and exchange database description (DBD) packets, which contain summaries of their link-state databases 2 .

The most common reason for OSPF neighbors to be stuck in the Exstart state is an MTU mismatch between the interfaces 3 .  MTU stands for maximum transmission unit, which is the largest size of a packet that can be transmitted on a network segment 4 .  If the MTU values of two OSPF neighbors are different, they may not be able to exchange DBD packets successfully, as some packets may be dropped or fragmented due to their size exceeding the MTU limit 3 .

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 interface 5 . 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 mode 5 .

BGP is a routing protocol that operates between autonomous systems (AS). An AS is a group of routers under a single administrative control. BGP can be classified into two types: internal BGP (IBGP) and external BGP (EBGP). IBGP is the BGP communication between routers within the same AS, while EBGP is the BGP communication between routers in different AS. BGP uses the AS_PATH attribute to record the AS numbers that the route has passed through, and uses it to prevent routing loops and select the best path.

In this scenario, you want to add a second ISP connection and remove the default route to the existing ISP. This means that you want to have more control over the routing decisions and use BGP to exchange routes with both ISPs. To do this, you need to deploy the BGP protocol in your network and configure both IBGP and EBGP sessions. The correct statements about BGP in this scenario are:

IBGP peers advertise routes received from EBGP peers to other IBGP peers. This is the default behavior of IBGP, as it allows the routers within the same AS to learn the routes from different EBGP peers and select the best exit point. However, IBGP has a rule that it does not advertise routes received from IBGP peers to other IBGP peers, to avoid creating routing loops. Therefore, option B is correct and option C is incorrect. To overcome this rule, IBGP requires a full mesh topology, where every IBGP router is directly connected to every other IBGP router, or a route reflector or confederation design, where some IBGP routers act as intermediaries to reflect or aggregate the routes to other IBGP routers.

EBGP peers advertise routes received from IBGP peers to other EBGP peers. This is the default behavior of EBGP, as it allows the routers in different AS to exchange routes and reachability information. However, EBGP has a rule that it does not advertise routes received from EBGP peers to other EBGP peers, to avoid creating routing loops. Therefore, option D is correct and option A is incorrect. To overcome this rule, EBGP uses the AS_PATH attribute to filter out the routes that contain its own AS number, or uses route maps or policies to control the route advertisement. IBGP does not update the next-hop attribute to ensure reachability within an AS, as the next-hop attribute is preserved by IBGP. Instead, IBGP relies on an underlying IGP (Interior Gateway Protocol) to provide reachability to the next-hop.

Both aggregate and generated routes would work, but the best choice is generated routes.

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.

Option A is correct.  The storm control feature is enabled in the factory-default configuration on EX Series switches 1 2 .  On EX2200, EX3200, EX3300, EX4200, and EX6200 switches, the factory default configuration enables storm control for broadcast and unknown unicast traffic on all switch interfaces 2 .  On EX4300 switches, the factory default configuration enables storm control on all Layer 2 switch interfaces 1 .

Option B is incorrect.  The storm control feature does not require a special license on EX Series switches 3 4 .

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 interface 5 , not just between the forwarding and control plane.

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 .

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 .

Redundant Trunk Groups (RTGs) on EX Series switches provide a simple solution for network recovery when a trunk port on a switch goes down 1 .  They are configured on the access switch and contain two links: a primary or active link, and a secondary link 1 .  Therefore, option B is correct because if the active link fails, the secondary link automatically starts forwarding data traffic without waiting for normal spanning-tree protocol convergence 1 .

Option D is also correct.  In a typical enterprise network composed of distribution and access layers, RTGs are used where one Access switch is connected to two different uplink switches 2 .  This implies that RTGs must be connected to the same aggregation switch 2 .

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.