4A0-112 Practice Questions

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 ”

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 Routing Table Manager (RTM) is the control plane process responsible for maintaining the routing information base (RIB). When multiple routing protocols, such as IS-IS, OSPF, or BGP, provide a route to the same destination prefix, the RTM selects the single best route. This selection is primarily based on the configured preference (administrative distance) for each protocol. The route from the protocol with the lowest preference value is chosen as the active route and is then installed into the forwarding information base (FIB) for use in packet forwarding.

A. The routing information base (RIB) is a database that stores all learned routes from all protocols; it is the repository of information, not the decision-making entity itself.

C. The equal cost multipath (ECMP) configuration is a feature that allows multiple best paths from a single, selected protocol to be used simultaneously, not for choosing between different protocols.

D. The forwarding information base (FIB) contains only the active routes that have already been selected by the RTM. It is the result of the decision, not the decision-maker.

1. Nokia 7750 SR OS Router Configuration Guide

Release 21.10.R1

Section: Routing Table Manager. This section describes the RTM's role in selecting the best route from various protocols based on preference and installing it into the forwarding table.

2. Nokia 7750 SR OS Basic System Configuration Guide

Release 21.10.R1

Section: Route Tables. This guide explains that the RTM manages the route tables (RIB) and is responsible for resolving best paths to be offered to the forwarding engine (FIB).

In a dual-stack IPv4/IPv6 IS-IS environment, the SPF (Shortest Path First) algorithm is only run once

per router for each protocol (IPv4 or IPv6). The topology is shared, but the SPF calculations for IPv4

and IPv6 are separate. This means that the SPF algorithm will run twice (once for IPv4 and once for

IPv6) for each L1/L2 router, not four times.

The metric values between the routers dictate the routing paths, and IS-IS will calculate the shortest

path based on these values.

R6 to R3: The path from R6 to R3 will go through R5 and R2, as this route has the least cumulative

metric (10 + 10 + 15 = 35).

R3 to R6: The reverse path from R3 to R6 will follow R3 → R2 → R5 → R6 because this route is also

the shortest with a total metric of 10 + 10 + 10 = 30.

In the configuration on router R1, the static route is defined with the next-hop IP address of

139.120.121.1.

However, the next-hop IP address 139.120.121.1 does not belong to the same subnet as the directly

connected interface on R1, which is 172.31.1.1/30. For the static route to work properly, the next-

hop IP address must be reachable via a directly connected interface, meaning it must be within the

same subnet.

Therefore, this mismatch in subnet adjacency is likely causing the failure to reach the destination

(139.120.121.2).

When an IS-IS router receives a newer LSP (Link-State PDU) than the one already in its database, it

takes the following actions:

Updates its link-state database with the new LSP.

Acknowledges the LSP with a PSNP (Partial Sequence Number PDU), which is sent to the router that

originated the LSP, confirming the receipt of the newer LSP.

Floods a copy of the updated LSP to its neighbors on other interfaces to ensure all routers in the

network have the updated topology information.

The fundamental purpose of the IP forwarding process on a router is to determine the next hop for a packet on its path to the final destination. When a router receives an IP packet, it examines the destination IP address in the packet header. It then performs a lookup in its routing table to find the most specific, or longest prefix, match. This matching routing table entry provides the necessary information, such as the outgoing interface and the IP address of the next-hop router, to forward the packet correctly. This destination-based lookup is the core of the IP forwarding decision.

A. The forwarding decision is based on the packet's destination IP address, not its source. The source address is used for return traffic.

C. The ARP table maps IP addresses to MAC addresses; it is not used to look up a source MAC address for forwarding decisions.

D. The ARP table is used to find the next-hop MAC address only after the routing table lookup has already determined the next-hop IP address.

1. Nokia. (2021). Nokia Service Routing Architect (SRA) Self-Study Guide: IP/MPLS and Ethernet Services. Section: The IP Forwarding Process. This guide details that the primary step in IP forwarding is the lookup of the destination IP address in the routing table.

2. Kurose

J. F.

& Ross

K. W. (2021). Computer Networking: A Top-Down Approach (8th ed.). Pearson. Chapter 4

Section 4.2

"The Router". The text explicitly states

"the router forwards the datagram based on the datagram's destination address" and describes the process of consulting the forwarding table (routing table).

3. Nokia. (2022). SR OS R22.10.R1 Basic System Configuration Guide. Part: IP Routing

Section: Unicast Routing Overview. The documentation describes the forwarding plane's reliance on the routing table

which is populated by routing protocols

to make decisions based on destination prefixes.

The static route configured on router R1 uses an indirect next-hop, which is 10.10.10.3 (R3). While

the echo request from R1 reaches the server through the IGP, the problem lies in the return path for

the echo response.

The route 10.4.100.0/24 is reachable through R3, but there is no reciprocal route in R3's routing table

that allows the response to flow back towards R1. This results in a failure to return the echo response

to R1, causing the ping to fail.

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".

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.