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

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 default hello interval on an OSPF interface depends on the network type of the interface. For point-to- point and broadcast network types, the default hello interval is 10 seconds. For non-broadcast and point-to- multipoint network types, the default hello interval is 30 seconds. The hello interval is the time between sending hello packets to establish and maintain neighbor adjacencies. The hello interval can be configured manually on each interface using the ip ospf hello-interval command. References:
* OSPF Hello and Dead Interval – NetworkLessons.com
* OSPF Hello and Dead Interval: Operation and Configuration
* Adjusting OSPF Timers – Cisco IOS Cookbook, 2nd Edition
* ospf timer hello – WLAN AC V200R010C00 Command Reference – Huawei
* OSPF Timers – Hello and Dead Intervals – Packet Lab

Explanation
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 adjacency1.
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 adjacency1.
These explanations are based on the Enterprise Routing and Switching Specialist (JNCIS-ENT) documents and learning resources available at Juniper Networks23.

Explanation
IP-IP tunnels are a type of tunnels that use IP as both the encapsulating and encapsulated protocol. IP-IP tunnels are simple and easy to configure, but they do not provide any security or authentication features. IP-IP tunnels only support encapsulating IP traffic, which means that the payload of the inner packet must be an IP packet. IP-IP tunnels cannot encapsulate non-IP traffic, such as Ethernet frames or MPLS labels1.
Option A is correct, because IP-IP tunnels only support encapsulating IP traffic. Option B is incorrect, because IP-IP tunnels only support encapsulating non-IP traffic. Option C is incorrect, because the TTL in the inner packet is not decremented during transit to the tunnel endpoint. The TTL in the outer packet is decremented by each router along the path, but the TTL in the inner packet is preserved until it reaches the tunnel endpoint2.
Option D is incorrect, because there are 20 bytes of overhead with IP-IP encapsulation. The overhead consists of the header of the outer packet, which has a fixed size of 20 bytes for IPv43.
References:
1: IP-IP Tunneling 2: What is tunneling? | Tunneling in networking 3: IPv4 – Header

AS-path prepending is a traffic engineering technique used in a BGP export policy to artificially increase the length of the AS path attribute advertised to external peers. Since a shorter AS path is generally preferred during BGP best path selection, prepending makes a route appear less attractive to external autonomous systems, which can be used to influence inbound traffic engineering decisions.

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.
References:
1 : OSPF DR/BDR Election: Process, Configuration, and Tuning 2 : OSPF Designated Router (DR) and Backup Designated Router (BDR)

An aggregate route is a route that summarizes multiple more specific routes into a single route advertisement.
For example, an aggregate route to 10.0.0.0/8 can represent many routes to 10.x.x.x/24 subnets. An aggregate route has several purposes, such as:
To decrease the number of route advertisements: By consolidating multiple routes into one, an aggregate route reduces the size of the routing table and the amount of routing updates that need to be exchanged between routers. This can improve the scalability and efficiency of the network12.
To hide internal routes from external peers: By advertising only the aggregate route and not the more specific routes, an aggregate route can provide a level of abstraction and security for the internal network topology. This can prevent external peers from knowing the details of the internal routes and avoid potential attacks or misconfigurations13.
The other options are not correct because:
C). To allow external peers to see internal routes: This is the opposite of what an aggregate route does. An aggregate route hides the internal routes from external peers, not exposes them13.
D). To increase the number of route advertisements: This is also the opposite of what an aggregate route does. An aggregate route decreases the number of route advertisements, not increases them12.
References: Configuring Route Aggregation | Junos OS, JunOS Aggregate and Generate Routes – Network Direction, Aggregate Routes vs Generated Routes. | Junos OS – Juniper Networks

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.
References:
1 : [Configuring Bidirectional Forwarding Detection] 2 : [Configuring Bidirectional Forwarding Detection for BGP] 3 : [Configuring Bidirectional Forwarding Detection for OSPF] 4 : [Configuring Bidirectional Forwarding Detection for Link Aggregation Control Protocol] : [Configuring Bidirectional Forwarding Detection for IS-IS] : [Configuring Bidirectional Forwarding Detection for RIP] : [Config uring Bidirectional Forwarding Detection for VRRP] : [Understanding Rapid Spanning Tree Protocol] : [Understanding FTP]

Screen options provide protection against common network-layer and transport-layer attacks, such as SYN floods, ICMP floods, and IP address spoofing, by detecting anomalous traffic patterns. Screen options are applied at the zone level and inspect traffic as it enters the associated security zone, complementing but not replacing firewall filters, which provide more granular packet matching and filtering capabilities.

Explanation
On Juniper switches, the default VLAN is set to a VLAN ID of 1 by default12. This means that all interfaces on the switch are members of VLAN 1 until they are specifically assigned to another VLAN12. Therefore, option A is correct.
The default VLAN ID can be changed12. This allows network administrators to configure the switch to use a different VLAN as the default, if necessary12. Therefore, option D is correct.

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 commands 1 2 .
* 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) instance 1 . 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 traffic 1 .
* 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_junos 1 .
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 traffic 1 .

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.

MAC limiting protects against flooding of the Ethernet switching table (also known as the MAC forwarding table or Layer 2 forwarding table). This feature is enabled on interfaces (ports). MAC limiting sets a limit on the number of MAC addresses that can be learned on a single Layer 2 access port. The MAC limit value in the EX-series switch’s default configuration is five MAC addresses. You can configure the MAC limit on an interface, on an interface in a VLAN, or on all interfaces. You can also specify the action to be taken when the MAC limit is exceeded, such as dropping packets, logging messages, or shutting down the interface.
Additionally, you can configure specific “allowed” MAC addresses for the access port. Any MAC address that is not in the list of configured addresses is not learned. Allowed MAC binds MAC addresses to a VLAN so that the address does not get registered outside the VLAN. References:
[EX] How to control MAC address access through Junos OS for EX Series switches1 Configuring MAC Limiting2 Understanding MAC Limiting and MAC Move Limiting3

Graceful restart allows a router experiencing a Routing Engine restart or switchover (such as in a dual Routing Engine or high availability scenario) to continue forwarding traffic based on its existing forwarding table while routing protocol adjacencies are re-established, minimizing traffic disruption during the recovery process.