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 selector2. 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 management2.

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 paths2. 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.

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.

1:IP-IP Tunneling2:What is tunneling? | Tunneling in networking3: IPv4 - Header

The default keepalive time for BGP is60 seconds1.The keepalive time is the interval at which BGP sends keepalive messages to maintain the connection with its peer1.If the keepalive message is not received within the hold time, the connection is considered lost1.By default, the hold time is three times the keepalive time, which is180 seconds1.

A tunnel is a connection between two computer networks, in which data is sent from one network to another through an encrypted link. Tunnels are commonly used to secure data communications between two networks or to connect two networks that use different protocols.

Option B is correct, because tunnel endpoints must have a valid route to the remote tunnel endpoint. A tunnel endpoint is the device that initiates or terminates a tunnel connection. For a tunnel to be established, both endpoints must be able to reach each other over the underlying network.This means that they must have a valid route to the IP address of the remote endpoint1.

Option D is correct, because tunnels add additional overhead to packet size. Tunnels work by encapsulating packets: wrapping packets inside of other packets. This means that the original packet becomes the payload of the surrounding packet, and the surrounding packet has its own header and trailer. The header and trailer of the surrounding packet add extra bytes to the packet size, which is called overhead.Overhead can reduce the efficiency and performance of a network, as it consumes more bandwidth and processing power2.

Option A is incorrect, because BFD can be used to monitor tunnels. 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. BFD can also be used to monitor the connectivity of tunnels, such as GRE, IPsec, or MPLS.

Option C is incorrect, because IP-IP tunnels are stateless. 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 are stateless, which means that they do not keep track of the state or status of the tunnel connection. Stateless tunnels do not require any signaling or negotiation between the endpoints, but they also do not provide any error detection or recovery mechanisms.

1:What is Tunneling? | Tunneling in Networking2:What Is Tunnel In Networking, Its Types, And Its Benefits?: [Configuring Bidirectional Forwarding Detection] : [IP-IP Tunneling]

The exhibit shows the output of the commandshow 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, 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 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 commandshow interfacesto display the MTU value of an interface5. You can also use the commandpingwith thedo-not-fragmentoption to test the MTU size between two routers.You can change the MTU value of an interface by using the commandset interfaces interface-name mtu mtu-valuein configuration mode5.