Free 4A0-112 Practice Questions – 2026 Updated

When a router performs the SPF (Shortest Path First) calculation, it uses itself as the root of the

shortest path tree (SPT). This router computes the shortest paths to all other routers in the network,

treating itself as the origin and calculating the paths based on its view of the network.

Router R4 is in Area 49.0001 and R6 is in Area 49.0002. Both routers are Level 2 (L2), meaning that

they can communicate across areas using Level 2 IS-IS routing.

Since R2 is the L1/L2 router that connects both Area 49.0001 and Area 49.0002, it will be the next-

hop for router R4 to reach R6’s system IP address.

The IS-IS protocol will ensure that R4 will have a route to R6’s system IP address via R2 as the next-

hop.

In IS-IS, routers are actually associated with interfaces rather than the entire router being associated

with a single area. A router can have interfaces in multiple areas, and it is the interface's

configuration that determines which area it belongs to.

The Input/Output Module (IOM) is the line card in the Nokia 7750 SR responsible for data plane processing. Each IOM contains a forwarding engine and maintains a local copy of the Forwarding Information Base (FIB). When a packet arrives on a physical interface of a Media Dependent Adapter (MDA), it is passed to the parent IOM. The IOM's forwarding engine then performs the longest prefix match lookup against its local FIB to determine the packet's next hop and egress interface. This distributed architecture allows for high-performance, line-rate forwarding.

A. The Control Processing Module (CMP) is the control plane. It builds the routing tables but delegates the actual per-packet forwarding lookups to the IOMs.

B. The Media Dependent Adapter (MDA) provides the physical interfaces (ports). It handles Layer 1 and Layer 2 functions, passing packets to the IOM for Layer 3 processing.

C. The Switch Fabric (SF) is a high-speed backplane that transfers packets between IOMs after the forwarding decision has already been made by the ingress IOM.

1. Nokia 7750 Service Router OS Basic System Configuration Guide

Release 21.10.R1. In the "System Overview" chapter

the section on "IOMs" describes their role: "The IOMs provide the packet processing and forwarding capabilities of the router. Each IOM has a local forwarding table that is a copy of the active forwarding table."

2. Nokia 7750 Service Router Datasheet. The document describes the hardware architecture

stating that the IOMs are responsible for packet processing and forwarding functions

which inherently includes IP lookups.

3. Nokia

"Building a Smarter

More Automated

and Scalable IP Network with the 7750 SR-s

" White Paper

2019. This paper discusses the distributed hardware architecture

explaining that forwarding decisions are made on the line cards (IOMs) to achieve high scalability and performance. See section "A purpose-built hardware architecture".

When a link-state update reaches its maximum age, it is removed from the router’s link-state

database, not flooded to its neighbors. This prevents outdated information from continuing to affect

the routing decisions. The router does not flood the aged update; instead, it will typically request a

new link-state advertisement from the original source router if the information is still needed.

The output shows two levels of IS-IS databases: Level 1 (L1) and Level 2 (L2). This indicates that the

domain is multi-area, as L1 routers are confined to a single area, while L2 routers can route between

different areas. The presence of Level 1/Level 2 (L1/L2) routers also suggests that the domain spans

multiple areas.

The LSP (Link-State PDU) entries show L1/L2 ATT attributes, which typically indicate that these

routers are capable of routing both within their own area (Level 1) and across areas (Level 2),

suggesting a multi-area design.

The IS-IS domain likely includes broadcast interfaces, as indicated by the "ATT" attribute (typically

referring to the type of interface) and common IS-IS configuration practices on broadcast networks

like Ethernet.

When IS-IS runs SPF, every lowest-cost path to a destination is eligible for installation. Because all links in the diagram carry the same metric, the SPF calculation seen from R3 produces four next-hops toward R5 that have an identical total cost. The ECMP parameter on every router has been set to 4, so SR OS will keep up to four equal-cost routes. Consequently, R3’s routing table will list four entries (one per next-hop) for prefix 192.168.3.0/24.

A. 1 – would be true only if ECMP were disabled or if a single lowest-cost path existed.

B. 2 – requires only two equal-cost next-hops, but SPF finds four.

C. 3 – fewer than the permitted ECMP value and fewer than the number of equal-cost paths actually present.

1. Nokia SR OS 22.2.R1 Routing Protocols Guide

IS-IS Equal-Cost Multipath section

p. 521–522: “The router installs up to the configured maximum-paths (default 1

user-configurable) for every destination that has multiple next-hops of identical metric.”

2. Nokia SR OS 21.10.R3 System Management Guide

Command “maximum-paths <1-64>”

p. 143: “Sets the number of ECMP routes that IS-IS can place in the RTM; value of 4 allows four parallel routes.”

3. IETF RFC 1195

Section 7.2.7: “A router may install multiple equal-cost paths; selection among them is an implementation matter.”

The adjacency database contains information about the state of the router’s adjacencies (i.e., the

routers that it is directly connected to and has established a neighbor relationship with). It does not

store information about all links in the domain; that information is stored in the link-state database.

Based on the IPv6 route table output for R3 and R4, we can see that the routers have remote routes

listed as "Remote ISIS", indicating that the route is being advertised from a different area (area 49.01

or 49.02). This is a sign of route leaking, where routes from one area are being shared across areas.

The routes from R3 (level-1 router) are being advertised to R4 (level-1 router), and vice versa, with

ISIS as the protocol. This implies that there is route leaking configured to allow information to pass

between areas.

The Neighbor Discovery Protocol (NDP), defined in RFC 4861, is the functional replacement for the Address Resolution Protocol (ARP) in IPv6. While ARP in IPv4 resolves IPv4 addresses to link-layer (MAC) addresses, NDP performs this same address resolution function for IPv6 using Neighbor Solicitation (NS) and Neighbor Advertisement (NA) messages. NDP is a more comprehensive protocol that also replaces other IPv4 functions like ICMP Router Discovery and ICMP Redirect, but its address resolution capability is the direct successor to ARP.

A. Duplicate address detection (DAD) is a function within NDP used to ensure an IPv6 address is unique on a link before use, a task not performed by ARP.

B. Router discovery is a function within NDP that allows hosts to locate routers, replacing the ICMP Router Discovery protocol from IPv4, not ARP.

D. Stateless address auto-configuration (SLAAC) allows hosts to generate their own addresses, a function more analogous to DHCP in IPv4, not ARP.

1. Nokia. (2021). 7450 ESS

7750 SR

7950 XRS

and VSR Router Configuration Guide

Release 21.10.R1. Section: IPv6 Neighbor Discovery. The guide states

"The Neighbor Discovery Protocol (NDP) is a protocol in the Internet Protocol Suite used with IPv6. It operates at the Network Layer of the Internet model and is responsible for address autoconfiguration of nodes

discovery of other nodes on the link

determining the link-layer addresses of other nodes (replaces ARP)

and duplicate address detection."

2. Narten

T.

Nordmark

E.

Simpson

W.

& Soliman

H. (2007). RFC 4861: Neighbor Discovery for IP version 6 (IPv6). IETF. Abstract. The document states

"This document specifies the Neighbor Discovery protocol for IP Version 6. IPv6 nodes on the same link use Neighbor Discovery to discover each other's presence

to determine each other's link-layer addresses

to find routers

and to maintain reachability information... The protocol replaces the following IPv4 protocols: Address Resolution Protocol (ARP)

ICMP Router Discovery

and ICMP Redirect."