4A0-100 Nokia IP Networksand Services Fundamentals Questions and Answers

The Shortest Path Tree (SPT) is calculated by Link-State routing protocols like OSPF and IS-IS using:

A complete map of the network topology .

Each router uses algorithms like Dijkstra’s to compute the shortest paths to all destinations.

Option C is correct – full network topology is used.

Option A refers to Distance Vector methods.

Option B and D are results of topology analysis but not the input used.

Spanning Tree Protocol (STP) is used specifically in Ethernet Layer 2 networks to prevent loops caused by redundant paths .

STP detects loops and blocks one or more redundant paths to ensure there is a loop-free topology in a bridged network.

STP dynamically recalculates the topology if 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 B is correct, as STP is explicitly designed for Layer 2 Ethernet networks with redundant paths .

Address summarization (also called route aggregation) is a technique used in IP routing to combine several contiguous network prefixes into a single summarized route .

This reduces the number of routes that need to be advertised and stored in routing tables, improving scalability and performance .

Option A is correct – summarization replaces many specific routes with a single aggregated route.

Option B refers to best path selection (not summarization).

Option C is incorrect; duplicate advertisements are still processed and filtered based on metrics.

Option D is unrelated to summarization; directly connected routes are always advertised unless specifically filtered.

In the diagram:

R2 and R4 are both within AS 65540 , which indicates that they are in the same Autonomous System .

eBGP (External Border Gateway Protocol) is designed for routing between different ASes (inter-AS routing).

Since R2 and R4 are in the same AS , you would use:

OSPF or IS-IS (both interior gateway protocols – IGPs) ✔

Static routing is also valid between any two routers if manually configured ✔

eBGP ❌ is invalid between routers in the same AS .

➡ The correct protocol in that case would be iBGP (internal BGP) if BGP were to be used within the AS.

Therefore, the correct answer is: D. eBGP

A layered protocol stack is designed to simplify network communication by separating functionalities into distinct layers. Each layer has a defined role and communicates with the layers directly above and below it. The main benefits of this model include:

Simplified design and implementation : Developers can focus on one layer without needing to understand the entire system.

Flexibility : New protocols can be developed and inserted at specific layers without redesigning the whole stack.

Interoperability : Standard interfaces allow different systems and vendors to work together.

Isolation of functions : Errors and updates can be managed within one layer without impacting others.

However, reducing protocol overhead is not a primary goal of this model. In fact, the encapsulation and interaction between layers can sometimes introduce extra overhead . This overhead is a trade-off accepted to gain the other benefits listed above.

VLAN Stacking , also known as Q-in-Q tunneling , is a technology that allows multiple customers to use overlapping VLAN IDs while being transported over a shared 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 a C-VLAN is encapsulated with an additional outer S-VLAN tag .

This way, multiple customers can use the same C-VLAN ID , but remain isolated by the unique S-VLAN assigned 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.

BGP (Border Gateway Protocol) uses a multi-step decision process to select the best route among multiple paths. It does not 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 evaluates multiple attributes for route selection.

VPWS (Virtual Private Wire Service) on Nokia 7750 SR:

Emulates a Layer 2 point-to-point connection.

Does not require MAC learning , unlike VPLS.

Uses MPLS encapsulation , not IP datagrams for transport.

Option D is FALSE – VPWS uses MPLS labels , not IP encapsulation.

Link-State routing protocols (e.g., OSPF , IS-IS ) operate by:

Each router flooding link-state advertisements (LSAs) throughout the network.

Every router constructs a complete map of the network topology .

Then each router runs the SPF algorithm to compute optimal paths.

Option C is correct – routers know the entire network topology .

Option A is Distance Vector behavior (e.g., RIP).

Option B is not efficient or accurate for Link-State .

Option D – hop-count is used in Distance Vector (e.g., RIP).