4A0-100 Exam Dumps - Nokia SRC Certification Questions and Answers

In MPLS architecture, theIngress LER (Label Edge Router)is responsible for:

Pushing labelsonto unlabeled packets and injecting them into the MPLS domain.

Acting as thestarting point of the Label Switched Path (LSP).

From the diagram:

R1is thestarting point (Ingress LER)for both LSP A and LSP B.

R4is theEgress LER, which wouldpopthe labels at the end of the LSP.

So,R1 is responsible for label push (imposing)at the start of both LSPs.

Option A is correct– R1 pushes labels for both LSP A and LSP B.

Option Bis incorrect – R4 is an egress router, not ingress.

Option C/Dare incorrect – the LSPs both start at R1.

AVPRN (Virtual Private Routed Network)provides aLayer 3 VPN serviceover a service provider’s network.

Each customer site connects to aPE router.

Sites are connected via arouted networkusingseparate routing instances.

Customer routers seerouted connectivity, as if connected to their own private WAN.

Option C is correct– VPRN providesrouted connectivitybetween sites.

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.

Thesystem interfacein Nokia SR OS is aloopback-style logical interfacethat represents therouter itself. It is used for:

Router ID assignment (if not manually set)

Source IP for control-plane protocols (OSPF, BGP)

Reachability testing

Important properties:

It istype loopback✔

Itexists by default✔

If router ID is unset, it uses thesystem IP address✔

Butthe IP address on the system interfacedoes not need to be /32❌

It can haveother prefix lengths, especially in service provider scenarios.

Thus, Option B is FALSE.

The Link layer, also known as the Data Link layer in the OSI model, is responsible for encapsulating packets from the Network layer into frames suitable for transmission over the physical medium. This process includes adding headers and trailers to the packet to create a frame, which contains necessary information for error detection, addressing, and control.​

According to the OSI model, the Data Link layer serves as the intermediary between the Network layer and the Physical layer. It ensures that data is packaged into frames and manages the access to the physical transmission medium.​

Option A correctly describes this function.​

Option B pertains to the Network layer, which is responsible for encapsulating data into IP datagrams and determining the routing of these datagrams across networks.​

Option C relates to the Transport layer, which handles the encapsulation of application data into transport layer segments, such as TCP or UDP messages.​

Option D is a function of the Physical layer, which deals with the transmission and reception of raw bitstreams over a physical medium, including the timing and synchronization of signals.​

VLAN Stacking, also known asQ-in-Q tunneling, is a technology that allowsmultiple customers to use overlapping VLAN IDswhile being transported over ashared service provider network.

Key Concepts:

Customer VLAN (C-VLAN):The VLAN used by the customer in their own network.

Service VLAN (S-VLAN):The VLAN used by the service provider to encapsulate the customer VLAN.

When VLAN stacking is applied:

A customer's Ethernet frame already tagged with aC-VLANisencapsulated with an additional outer S-VLAN tag.

This way,multiple customers can use the same C-VLAN ID, but remain isolated by theunique S-VLANassigned to them by the service provider.

Why Other Options Are Incorrect:

A. VLAN Trunking:Used to carry multiple VLANs over a single physical link but doesn’t allow reuse of VLAN IDs across customers.

C. VLAN Tagging:Refers to the general process of adding a VLAN tag; it doesn't provide the isolation needed across shared infrastructure.

D. VLAN Routing:Refers to Layer 3 routing between VLANs, not to ID reuse or transport over a shared provider backbone.

In a typical MPLS/VPN service architecture:

PE (Provider Edge) router: Directly connects to thecustomer edge (CE)device.

P (Provider) router: Only forwards MPLS-labeled packets, doesnot touch customer routes.

LSR (Label Switch Router): General term, can be either PE or P depending on placement.

Correct Answer: A – PE

Itinterfaces with CE devicesandruns customer routing protocols(e.g., OSPF, BGP).

Tier 1 ISPs: Haveglobal reachabilityand exchange traffic via peering. Theydo not payfor transit.

Tier 2 ISPs: Purchasetransit services from Tier 1and provide transit to Tier 3 or other Tier 2 ISPs.

Tier 3 ISPs: Providelast-mileconnectivity to end users and donot provide transit services.

Option A is correct– Tier 1 and Tier 2provide transit services.

In Nokia SR OS:

Aphysical interfacerefers to a physical port on the router (e.g., port 1/1/1).

It can be configured withIP addresses, associated with services (e.g., IES, VPRN), and connects to other routers or customer devices.

Systemandloopbackinterfaces arelogical interfaces, not physical.

Option D is FALSE, because a physical interfacecannotbe of type "system" or "loopback" — those arevirtual/logicalinterfaces in the Nokia router architecture.

When a routerforwards a packet, it performs the following operations:

Looks up the destination IP addressin therouting tableto determine thenext hoporegress interface.

Uses theARP tableto resolve theMAC address of the next-hop IP.

Option Ais incorrect – source IP is not used for route lookup.

Option Cis incorrect – routers don’t resolve thesource MAC.

Option Dis partially true, butrouting begins with the routing table, makingBthe most accurate.