4A0-100 Questions and Answers

TheShortest Path Tree (SPT)is calculated byLink-State routing protocolslikeOSPFandIS-ISusing:

Acomplete map of the network topology.

Each router uses algorithms likeDijkstra’sto compute the shortest paths to all destinations.

Option C is correct– fullnetwork topologyis used.

Option Arefers to Distance Vector methods.

Option BandDare results of topology analysis but not the input used.

AVirtual Local Area Network (VLAN)is a logical separation of devices atLayer 2, even ifthose devices are connected to the same physical switch. Each VLAN forms itsown broadcast domain, and traffic cannot cross from one VLAN to anotherwithout Layer 3 routing.

Option Ais incorrect – devices on the same switch but in different VLANs still cannot communicate directly.

Option Bis incorrect – VLANs explicitlycreate separate broadcast domains.

Option Dis technically incorrect – communication is possible with the right setup.

Option Cis correct – to enable communication between different VLANs, arouteror aLayer 3 switchwith inter-VLAN routing capability is required.

This process is calledinter-VLAN routing, and it's a fundamental task in enterprise networks using VLANs.

Spanning Tree Protocol (STP)is used specifically inEthernet Layer 2 networkstoprevent loopscaused byredundant paths.

STPdetects loops and blocks one or more redundant pathsto ensure there is aloop-free topologyin a bridged network.

STP dynamicallyrecalculates the topologyif a link fails, reactivating previously blocked paths.

Explanation of other options:

A. MPLS with LSPs:MPLS uses its own mechanisms for path management, such as RSVP or Segment Routing. STP is not applicable here.

C. OSPF with ECMP (Equal-Cost Multi-Path):OSPF is a Layer 3 protocol that inherently supports multiple paths without loops; STP is not involved.

D. eBGP (Exterior Border Gateway Protocol):Operates at Layer 3 for inter-AS communication; STP is not used in BGP routing.

Therefore,Option Bis correct, as STP is explicitly designed forLayer 2 Ethernet networkswithredundant paths.

BGP (Border Gateway Protocol)uses amulti-step decision processto select the best route among multiple paths. It doesnot use SPF or simple metrics like hop count.

Key BGP selection criteria include:

Local preference

AS path length

Origin type

MED (Multi-Exit Discriminator)

eBGP over iBGP

IGP metric to next hop

Option C is correct– BGP evaluatesmultiple attributesfor route selection.

In Nokia SR OS, theMD-CLIprovides multiple configuration edit modes to allow different types of user interaction with the configuration system:

Private modeallows multiple users to create individual configuration candidates, which are then merged with the global candidate during commit.

Global modeenables multiple users to edit the same candidate configuration at the same time, with a shared commit session.

Read-only modeallows the user to view the configuration without making changes.

Exclusive moderestricts access to the global candidate configuration such thatonly one user at a timecan enter and make changes. This prevents conflicts and is useful in environments where strict control over configuration changes is necessary.

Therefore, the correct answer isExclusive mode, as itexclusively locks access to the global candidatefor a single user.

AP router (Provider router)is locatedin the core of the service provider’s network. Its functions include:

ForwardingMPLS-labeled packetsbetweenPE routers.

Not interacting directly with customer routes or routing instances.

Having no knowledge ofVPN routing or customer-specific services.

Option C is correct– P routers sit in theMPLS core, isolated from customer edge.

Abroadcast IP addressis used tosend a message to all deviceson a specific IP subnet. This address is thehighest address in the subnet(i.e., all host bits set to 1).

Example: In 192.168.1.0/24 → broadcast is 192.168.1.255.

It is used for purposes like ARP requests or DHCP discovery.

Option A is correct– refers to all hosts in a subnet.

Option Brefers tomulticast, not broadcast.

Option Cmight refer toanycast, which is different.

Option Ddescribes aunicastaddress.