Route summarization, or route aggregation, is a core technique for improving the scalability of routing protocols. Its primary benefit is reducing the number of entries in a router's routing table by advertising a single, less-specific summary route in place of multiple, more-specific routes. This directly leads to smaller routing tables, which consume less memory and processing power. A crucial secondary benefit is enhanced network stability. By summarizing routes, individual link failures within the aggregated address block are not advertised to the rest of the network, thus reducing the volume of routing update messages (signaling) and preventing route flapping from propagating.

C. It summaries the routing table entries into a default route

This is incorrect because summarization creates a less-specific route (a supernet), which is not necessarily a default route (0.0.0.0/0). A default route is only one specific type of summary.

D. It improves the best route calculation.

This is incorrect. Summarization can hide details about the network topology, which may lead to suboptimal routing decisions, as a router may not have the more-specific prefix information needed to choose the truly best path.

1. Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson.

Section 5.3.2

"Hierarchical Routing": This section explains that route aggregation is a key feature of hierarchical routing. It states

"routers in the backbone need not know about every router within a region... This ability to aggregate routes and hide details of the network topology is key to creating scalable routing protocols." This supports the reduction of routing table size (A) and reduction of signaling (B).

2. Cisco Systems

Inc. (2005). IP Routing: OSPF Design Guide.

Section: "Summarization": "The main advantages of summarization are that it reduces the size of the routing table... and it helps to contain routing information

so if a link is flapping

the flapping is not propagated into the backbone." This directly validates that summarization reduces routing table size (A) and signaling (B).

3. Halabi

B. (2000). Internet Routing Architectures (2nd ed.). Cisco Press.

Chapter 4

"Tuning BGP Capabilities"

Section: "Route Aggregation": The text describes aggregation as a mechanism to "reduce the amount of routing information that is passed around." It explicitly mentions that a major benefit is "insulating a routing domain from instabilities" that occur in another domain

which directly corresponds to reducing signaling (B). It also states aggregation's purpose is to keep routing tables "at a manageable size" (A).

In Multiprotocol Label Switching (MPLS), a Label Switch Router (LSR) performs one of three primary actions: push, swap, or pop. The exhibit shows that router R4 receives a packet with an incoming label of 40. It then replaces this label with an outgoing label of 50 before forwarding the packet to the next-hop router, R5. This process of replacing an incoming label with a new outgoing label is known as the "swap" operation. This is the standard function of an intermediate LSR within an MPLS domain that is not the penultimate hop performing Penultimate Hop Popping (PHP).

A. pop: The pop operation, which removes the label, is performed by the egress router R5 in this diagram, not by R4.

B. push: The push operation, which adds the initial label, is performed by the ingress router R1 at the start of the MPLS path.

C. PHP: Penultimate Hop Popping (PHP) is a specific action where the second-to-last router (R4) would pop the label. The exhibit explicitly shows R4 swapping labels, not popping one.

1. Rosen

E.

Viswanathan

A.

& Callon

R. (2001). RFC 3031: Multiprotocol Label Switching Architecture. IETF. Section 3.12

"The Label Swap Operation

" states: "A label swap operation consists of replacing the label at the top of the label stack with a new label

and then forwarding the resulting packet." This directly describes the action of R4.

2. Davie

B.

& Peterson

L. (2011). Computer Networks: A Systems Approach (5th ed.). Morgan Kaufmann. In Chapter 3.3.1

"Operations

" the text describes the function of an LSR: "An LSR

on receiving a labeled packet

uses the label as an index into a table that specifies the outgoing label and the next hop for the packet. It then swaps the label in the packet with the outgoing label..."

3. Gredler

H.

& Medcalf

S. (2008). MPLS and VPN Architectures. Cisco Press. Chapter 2

"MPLS Architecture

" describes the label operations

clarifying that intermediate LSRs perform label swapping to forward traffic along a Label Switched Path (LSP).

In an MPLS L3VPN architecture, the Provider (P) routers form the core of the service provider's network and are responsible for high-speed packet forwarding based on MPLS labels. They perform label switching on the transport label (outer label) to forward traffic between Provider Edge (PE) routers. P routers do not participate in the customer's routing via MP-BGP and do not maintain customer-specific routing tables (VRFs). Consequently, they have no knowledge of the customer's IP prefixes, ensuring the core remains simple, scalable, and secure.

A. A key feature of MPLS L3VPNs is the use of Virtual Routing and Forwarding (VRF) instances and Route Distinguishers (RDs), which specifically allows different customers to use overlapping IP address spaces.

B. MP-BGP is used to exchange VPNv4/VPNv6 routes between PE routers only (and Route Reflectors, if used). P routers are not involved in BGP and only need an IGP and LDP for label switching.

C. The connection between a PE and a CE router is a standard IP link. Routing information is exchanged using a standard routing protocol (e.g., static, OSPF, eBGP), not LDP, which operates within the MPLS core.

1. Rosen

E.

& Rekhter

Y. (2006). BGP/MPLS IP Virtual Private Networks (VPNs). RFC 4364. Internet Engineering Task Force.

Section 1

Introduction

Paragraph 4: "The P routers do not need to have any VPN routing information... This helps to ensure that the backbone can scale." This directly supports the correct answer (D) and refutes option (B).

Section 2

VRF

Paragraph 1: "If two VPNs use the same address space... a PE router must be able to install routes from both VPNs in its forwarding table..." This refutes option (A).

2. Davie

B.

& Farrel

A. (2008). MPLS: Next Steps. Morgan Kaufmann.

Chapter 3

Section 3.2.1

"The Roles of P and PE Routers": "The P routers are not aware of the VPNs and do not carry any customer routes. They simply switch labeled packets based on the information in their LFIBs." This confirms that P routers are unaware of customer routes (D).

3. Washington University in St. Louis

Course CSE 571S. (Fall 2013). MPLS and VPNs Lecture Notes.

Slide 25

"MPLS VPN Operation": Explicitly states

"P routers are not aware of VPN routes" and "PE routers peer with other PE routers using internal BGP (IBGP)." This supports answer (D) and clarifies the role of MP-BGP

refuting (B).

Forwarding uses the longest-prefix-match algorithm. 192.168.1.129 belongs to the 192.168.1.128/25 network.

In the shown routing table the entry for 192.168.1.128/25 lists next-hop 10.126.131.250, which therefore becomes the gateway chosen for packets destined to 192.168.1.129.

A. 10.126.131.251 – Appears in the table for a broader prefix; superseded by the more specific /25 route.

B. 10.126.131.252 – Also associated with a less-specific network; not selected under longest-prefix rules.

D. 10.126.131.248 – Either default route or another summary; overridden by the /25 entry.

1. RFC 1812

“Requirements for IP Routers”

§5.2.4 (Route Lookup) – explains longest-prefix selection.

2. Kurose & Ross

Computer Networking 8th ed.

Ch.4 p.301 – forwarding decision based on most-specific prefix.

3. MIT OCW 6.829 “Network and Computer Security”

Lecture 3 notes

slide 14 – longest-prefix match example.

4. Ericsson ECP-206 Student Guide

Module 3 “IP Forwarding”

p.3-15 – routing-table search order and next-hop determination.

Within a single Interior Gateway Protocol (IGP) area, specifically in link-state protocols like OSPF or IS-IS, each router is responsible for two primary actions. First, it originates link-state advertisements (LSAs or LSPs) that describe its own state, including its connected interfaces, their costs, and its neighbors (C). This is how a router advertises information about itself. Second, it acts as a relay point. When a router receives a link-state advertisement from a neighbor, it forwards (floods) this information to its other neighbors within the area (D). This flooding mechanism ensures that all routers within the same area build an identical Link-State Database (LSDB), which is essential for accurate and consistent Shortest Path First (SPF) calculations.

A. Routers summarize information they learn from neighbors.

This is incorrect because route summarization in link-state IGPs is performed by Area Border Routers (ABRs) at the boundary between areas, not by every router within a single area.

B. Routers discard valid but inaccurate information from neighbors.

This is incorrect because routers in a link-state IGP do not have a mechanism to verify the "accuracy" of the content; they process any protocol-valid advertisement and flood it.

1. Moy

J. (1998). RFC 2328: OSPF Version 2. The Internet Society.

For Answer C: Section 1.2

"The OSPF protocol is a link-state routing protocol. Each router maintains a database describing the AS's topology... Each individual piece of this database is a particular router's local state (e.g.

the router's usable interfaces and reachable neighbors)." This confirms routers advertise their own state.

For Answer D: Section 13

"The Flooding Procedure

" describes the mechanism for propagating link-state updates. It states

"A router that has just received a link state update... must send the update out to some of its neighbors." This is the relay function.

For Incorrect A: Section 3.5

"Area border routers

" explains that these specific routers "condense the topological information from their attached areas for distribution to the backbone

" confirming summarization is an inter-area function.

2. Massachusetts Institute of Technology (MIT) OpenCourseWare. (2004). 6.829J / 16.440J Introduction to Networks

Lecture 10: Link State Routing.

For Answers C & D: Slide 10-5

"Basic Idea

" outlines the two phases of link-state routing: 1) "Reliable flooding: Tell all other routers what you know about your neighbors" and 2) "Path calculation (Dijkstra's algorithm)." The first phase encompasses both self-advertisement (what you know) and relaying (tell all other routers). This supports the concepts of originating and relaying information.