Nokia IS-IS Routing Protocol (4A0-112)

✅

Reasoning: Routers primarily use their routing table to determine the outgoing interface and next-hop for an IP packet. This decision is based on comparing the packet's destination IP address with entries in the routing table to find the longest prefix match. ❌ Why the other choices are incorrect:

• Option A is incorrect: The source IP address of a packet is not used by a router to make forwarding decisions. Its purpose is to identify the sender.
• Option C is incorrect: The ARP table maps IP addresses to MAC addresses. It is used to resolve the MAC address for a next-hop IP or final destination IP on a directly connected segment, not to match a source MAC address for routing.
• Option D is incorrect: While ARP is used to find a destination MAC address for the next hop, the initial Layer 3 forwarding decision itself is made using the routing table and the destination IP address, not by directly matching a destination MAC in the ARP table.

✅

Reasoning: Both ARP for IPv4 and Neighbor Discovery Protocol (specifically Neighbor Solicitation/Advertisement) for IPv6 are fundamental mechanisms designed to map a logical network address (IPv4 or IPv6) to a physical link-layer address (MAC address) on a local broadcast segment. This allows devices to encapsulate IP packets into MAC frames. ❌ Why the other choices are incorrect:

• Option A is incorrect: ARP does not detect duplicate MAC addresses. NDP's DAD function is for IP addresses, not MAC addresses.
• Option B is incorrect: While NDP includes Duplicate Address Detection for IPv6, ARP's primary function is not duplicate IP detection. Though some ARP-based methods exist, it's not a core, shared primary purpose.
• Option D is incorrect: This describes Reverse ARP (RARP) or similar protocols, which are not the primary function of either ARP or NDP in the context of host communication. They resolve IP-to-MAC, not MAC-to-IP.

✅

Reasoning: The Routing Table Manager (RTM), often called the RIB Manager in Nokia SR OS, is responsible for selecting the best path for each prefix from all routes learned by different routing protocols. It uses administrative distance (preference) and metric values to decide which single route to install as the active route into the global routing table. ❌ Why the other choices are incorrect:

• Option A is incorrect: The Routing Information Base (RIB) is a database that stores all learned routes. It's the repository, not the component that performs the route selection decision.
• Option C is incorrect: Equal Cost Multipath (ECMP) allows multiple paths with identical costs to be used simultaneously. It's a specific load-sharing feature, not the core mechanism for deciding the best route among different protocols or varied costs.
• Option D is incorrect: The Forwarding Information Base (FIB) contains the active routes selected by the RTM/RIB Manager, optimized for fast packet forwarding. It's the result of the decision, not the decision-maker itself.

Based on the networking topology provided, there will be 2 entries for the prefix 192.168.3.0/24 in router R3’s routing table.

Architectural Breakdown

To determine the number of routing table entries, we look at the paths from R3 to the source router R5:

Path 1 (via R1): R3 → R1 → R5. This path has a total cost of 2 hops.

Path 2 (via R4): R3 → R4 → R2 → R5. This path has a total cost of 3 hops.

Path 3 (Direct to R2): R3 → R2 → R5. This path also has a total cost of 2 hops.

Why the Answer is 2

Equal Cost Multi-Path (ECMP): The scenario states an ECMP value of 4 is configured, and all links have the same cost.

Best Path Selection: Interior Gateway Protocols (IGPs) only install the best (lowest cost) paths into the routing table.

The Result: There are exactly two paths with the minimum cost of 2 hops (via R1 and via R2). While the ECMP limit allows up to 4, only the 2 equal-cost best paths are utilized.

✅

Reasoning: The Input/Output Module (IOM) houses the forwarding ASICs, which are the data plane engines. These ASICs perform per-packet longest prefix match (LPM) lookups, QoS, and policing based on forwarding tables pushed by the control plane, handling packets arriving via MDAs. ❌ Why the other choices are incorrect:

• Option A is incorrect: The Control Processing Module (CPM) is the control plane, responsible for system management and building forwarding tables, not per-packet LPM lookups.
• Option B is incorrect: Media Dependent Adapters (MDAs) provide physical interfaces, performing basic L2 functions, but delegate complex L3 LPM lookups to the IOM.
• Option C is incorrect: The Switch Fabric (SF) interconnects IOMs for high-speed packet transfer, but does not perform data plane lookups or forwarding decisions itself.