Pass the Nokia SRC Certification 4A0-100 Questions and answers with ExamsMirror

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.​

TheIngress Label Edge Router (LER)is the first router that:

Receives an unlabeled IP packet.

Applies an MPLS labelbased on its forwarding decision.

Forwards it into theMPLS core.

Option A is correct– Ingress LER does MPLS label push.

Option B (Egress LER)– removes labels at the end of the path.

Option C (LSR)– switches labels, but doesn’t push them onto unlabeled packets.

Option D– is not a specific MPLS role.

ICMP (Internet Control Message Protocol)is used for network diagnostics and error messaging. Key uses include:

Ping (Echo Request/Reply)– tests end-to-end connectivity✔

Traceroute– uses ICMP time exceeded messages to identify hops✔

Error reporting– returns various error types with differentcodes✔

However,ICMP does not re-route traffic. If a destination is unreachable, ICMP reports the error back to the sender; it doesn’t instruct routers to re-route.

Option D is FALSE– ICMPdoes not perform routing.

Astatic routeis a manually configured route added by a network administrator. It does not rely on dynamic routing protocols or metrics.

Itdoes not exchangeroutes with other routers (A – false).

It can be used bothwithin or across ASes(B – false).

Itdoes not use metricslike hop count (C – false).

Option D is correct– the operator explicitly defines the next-hop IP or egress interface manually.

Static routes are often used for:

Default routing

Backup routing

Simple or small networks

AMAC address(Media Access Control address) is a 48-bit identifier typically expressed in six groups of two hexadecimal digits (e.g., 00-20-60-CE-2B-28). It consists of two key parts:

OUI (Organizationally Unique Identifier)– The first24 bits(or first 3 octets, e.g., 00-20-60) are assigned by theIEEE to hardware manufacturers. This identifies the vendor or manufacturer.

Device Identifier (NIC Specific)– The remaining24 bits(e.g., CE-2B-28) are assigned uniquely by the vendor to each device/interface.

So in the address 00-20-60-CE-2B-28:

00-20-60 is theOUI

CE-2B-28 is thedevice-specific portion

Thus, the correct answer isD. 00-20-60.

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.

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 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.

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.

A/31 subnetin IPv4 provides exactlytwo IP addresses(e.g., 192.168.1.0/31 → usable: 192.168.1.0 and 192.168.1.1). Traditionally, these would represent anetwork address and a broadcast address, butRFC 3021redefines /31 subnets forpoint-to-point links, such as router-to-router connections.

In a /31 subnet:

Broadcast address is not used.

Both IPs are treated as usable host addresses.

It'sideal for point-to-point links, saving address space.

This is widely supported andstandardized in Nokia IP and routing platforms.