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

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 .

A  is correct because dynamic ARP inspection (DAI) is a Layer 2 security feature that prevents ARP spoofing attacks. ARP spoofing is a technique that allows an attacker to send fake ARP messages to associate a spoofed MAC address with a legitimate IP address. This can result in traffic redirection, man-in-the-middle attacks, or denial-of-service attacks.  DAI validates ARP packets by checking the source MAC address and IP address against a trusted database, which is usually built by DHCP snooping 1 .  DAI discards any ARP packets that do not match the database or have invalid formats 1 .

C  is correct because DHCP snooping is a Layer 2 security feature that prevents DHCP spoofing attacks. DHCP spoofing is a technique that allows an attacker to act as a rogue DHCP server and offer fake IP addresses and other network parameters to unsuspecting clients. This can result in traffic redirection, man-in-the-middle attacks, or denial-of-service attacks. DHCP snooping filters DHCP messages by classifying switch ports as trusted or untrusted.  Trusted ports are allowed to send and receive any DHCP messages, while untrusted ports are allowed to send only DHCP requests and receive only valid DHCP replies from trusted ports 2 .  DHCP snooping also builds a database of MAC addresses, IP addresses, lease times, and binding types for each client 2 .

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 .

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.

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 .

On Juniper switches, the default VLAN is set to a VLAN ID of 1 by default 1 2 .  This means that all interfaces on the switch are members of VLAN 1 until they are specifically assigned to another VLAN 1 2 . Therefore, option A is correct.

The default VLAN ID can be changed 1 2 .  This allows network administrators to configure the switch to use a different VLAN as the default, if necessary 1 2 . Therefore, option D is correct.

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

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.