📝 Pastry routing algorithm (27 MCQs)

📖 Explanation: The router matches the destination address with the static entry, determines that eth0 is the appropriate outgoing interface, and forwards the packet accordingly. Because the route is exact and directly connected, no additional processing such as broadcasting or ICMP generation is needed.

📖 Explanation: A mismatched hello interval prevents OSPF hello packets from being recognized, causing the adjacency to drop. When the adjacency disappears, the routers withdraw all OSPF-learned routes, so the routing table loses those entries immediately, while other protocol routes remain unchanged.

📖 Explanation: Link‑state protocols flood the entire network with LSAs, allowing each router to have a complete view and compute shortest‑path trees quickly. Distance‑vector protocols rely on incremental updates and can suffer from slow convergence and count‑to‑infinity problems, making link‑state generally faster.

📖 Explanation: A protocol aiming to minimize the bandwidth‑delay product evaluates the product of link bandwidth and propagation delay; the smaller this product, the more efficient the path. Therefore, the metric directly reflects the multiplication of bandwidth and delay values.

📖 Explanation: Hierarchical routing groups routers into areas and uses summary routes at area borders, allowing each router to store only aggregated entries instead of every individual subnet. This aggregation dramatically shrinks the number of entries each router must maintain.

📖 Explanation: CIDR, or Classless Inter‑Domain Routing, replaces the older class‑based addressing scheme. It allows variable‑length subnet masks, enabling more efficient allocation of IP address space and supporting route aggregation, which reduces the size of global routing tables.

📖 Explanation: Class B addresses occupy the first two octets for the network portion, leaving the last two for hosts. The default mask therefore has ones in the first sixteen bits (255.255) and zeros in the remaining sixteen bits, yielding 255.255.0.0.

📖 Explanation: The total path cost is the sum of individual link costs: $1 + 2 = 3$. Therefore, the route from Router A to Router C incurs a combined metric of three, which is the value presented in option D.

📖 Explanation: Metric type 1 indicates that the external route should be treated as internal for EIGRP calculations, causing EIGRP to adopt the OSPF‑derived cost directly as its own internal metric rather than applying a default external value.

📖 Explanation: When the TTL (Time‑to‑Live) counter decrements to zero, the router discards the packet and sends an ICMP Time Exceeded message back to the source. This informs the sender that the packet could not reach its destination because it traversed too many hops.

📖 Explanation: RIP is a distance‑vector protocol that restricts the network diameter to 15 hops, preventing routing loops. OSPF, a link‑state protocol, does not impose a hop‑count ceiling; it bases routing decisions on link costs, allowing arbitrarily large topologies.

📖 Explanation: A longer hello interval means routers wait longer before detecting neighbor loss, delaying the propagation of LSAs and the recomputation of shortest‑path trees. Consequently, the overall convergence process takes more time, which can affect network stability during failures.

📖 Explanation: Multiprotocol Label Switching (MPLS) enables explicit path control and traffic engineering, allowing operators to allocate bandwidth, prioritize traffic, and avoid congestion. These capabilities surpass the basic encapsulation offered by a plain IP VPN, making MPLS attractive for performance‑critical WAN deployments.

📖 Explanation: A /26 mask leaves $32 - 26 = 6$ host bits. The number of host addresses is $2^{6} - 2 = 64 - 2 = 62$. Therefore, 62 usable IPs can be assigned to devices within that subnet.

📖 Explanation: ECMP identifies multiple paths that have identical routing metrics and then spreads traffic among them, often on a per‑packet or per‑flow basis. This parallel utilization balances load, maximizes bandwidth, and provides resilience without needing additional configuration.

📖 Explanation: Routers apply the longest‑prefix match rule, preferring the most specific entry. The /32 host route is the longest match for 10.0.0.5, so the packet is forwarded out eth2 regardless of metric or learning order.

📖 Explanation: BGP first evaluates route specificity; a more specific prefix overrides MED values. Therefore, even though the more specific route carries a higher MED (200), its greater specificity makes it the preferred path.

📖 Explanation: Policy‑based routing can be configured to examine interface bandwidth attributes. When both paths have identical cost metrics, the router selects the one with the higher bandwidth—here Path A with 1 Gbps—because it offers greater capacity for traffic.

📖 Explanation: A /30 mask reserves 30 bits for the network, leaving 2 bits for host addresses. This yields $2^{2} - 2 = 2$ usable IPs, which perfectly fits a point‑to‑point connection requiring one address for each endpoint.

📖 Explanation: Routers implement the longest‑prefix match rule: they compare all matching entries and select the one with the most specific network mask, ensuring traffic is directed along the most precise route available.

📖 Explanation: OSPF summary routes carry the accumulated intra‑area cost plus any additional metric changes. When the internal link failure raises the cost by 5, the summary’s advertised metric becomes $20 + 5 = 25$, which other areas then use for path selection.

📖 Explanation: Applying Dijkstra, the cheapest route from A to D is A‑B‑C‑D with costs $2 + 1 + 2 = 5$. No alternative yields a lower sum, so the shortest‑path metric equals 5, corresponding to option B.

📖 Explanation: The control plane runs routing protocols, learns topology, and constructs the routing table. The forwarding plane (data plane) uses that table to move packets at line speed. Separating them allows the control plane to run complex computations without slowing packet forwarding, enabling high‑performance, scalable devices.

📖 Explanation: OSPF floods LSAs throughout the area, producing roughly O($N$) advertisements for each router, while RIP sends periodic distance‑vector updates to each neighbor, also O($N$). However, OSPF’s hierarchical design and incremental flooding make its message load more manageable than RIP’s full‑table broadcasts.

📖 Explanation: OSPF was standardized in RFC 2328, titled “OSPF Version 2”. This document specifies the protocol’s packet formats, state machines, and algorithmic behavior, forming the basis for OSPF implementations used in modern IP networks.

📖 Explanation: BGP decision process examines attributes in a defined order. The first comparison after weight is local‑preference; a higher local‑preference overrides a longer AS‑PATH. Only if local‑preference values are equal does the router consider AS‑PATH length.

📖 Explanation: Employing an SDN controller to manage primary forwarding rules enables rapid re‑configuration, while retaining conventional IP routing as a fallback ensures continuity if the controller fails. This dual‑layer approach leverages SDN’s agility and the robustness of legacy routing, delivering swift failover without sacrificing resilience.