Free ECP-206 Practice Test Questions and Answers (2026) | Cert Empire Practice Questions

Based on industry standard practice for implementing Quality of Service (QoS), the priority order from highest to lowest is established to protect the most delay-sensitive and critical traffic. Expedited Forwarding (EF) is given the highest priority as it is used for real-time applications like VoIP, which are extremely intolerant of delay and jitter. Network Control (NC) traffic (e.g., routing protocols) is critical for network stability and is prioritized next. Assured Forwarding (AF) provides multiple service levels for important data that is less sensitive than real-time traffic. Default Forwarding (DF), or best-effort, has the lowest priority and is used for all remaining traffic with no service guarantees.

B. EF, AF, NC, DF: This order incorrectly places Network Control (NC) traffic below Assured Forwarding (AF). NC is vital for network operation and is always prioritized higher than AF classes.

C. NC, EF, DF, AF: This order incorrectly places Default Forwarding (DF) above Assured Forwarding (AF). DF is the lowest-priority, best-effort service class by definition.

D. NC, EF, AF, DF: While Network Control traffic is critical, common implementation practice prioritizes user-facing real-time EF traffic in the highest-priority queue to prevent any impact from control plane processing.

1. IETF RFC 4594

"Configuration Guidelines for DiffServ Service Classes": This document provides the foundational recommendations for DiffServ service classes.

Section 3.1 (Network Control): Describes this class for traffic "essential for network operation" and recommends DSCP CS6.

Section 3.2 (Telephony): Recommends the EF PHB for real-time voice traffic

noting its intolerance to delay and jitter.

Section 3.4 (Assured Forwarding Classes) and Section 3.7 (Default Class): Detail the lower-priority AF and DF (Best-Effort) classes

establishing their place below EF and NC.

2. IETF RFC 3246

"An Expedited Forwarding PHB (Per-Hop Behavior)":

Section 1 (Introduction): Defines the EF PHB as a "building block for a low-loss

low-latency

low-jitter

assured-bandwidth service

" justifying its placement in a strict priority queue in network implementations.

3. Stallings

W. (2016). Foundations of Modern Networking: SDN

NFV

and Cloud Computing. Pearson Education.

Chapter 11

Section 11.3 (Differentiated Services): This academic textbook explains that EF is designed to be handled in a priority queue

receiving preferential treatment over other traffic classes like AF and Best-Effort (DF) to create a "virtual leased line." It also notes the high importance of network control traffic.

4. Case Western Reserve University

EECS 425/600: Intro to Computer Networks Courseware

"Differentiated Services" Lecture.

Slide "A Possible Implementation": This university lecture material illustrates a common queuing architecture where EF traffic is directed to a strict priority queue

while AF and Best-Effort (DF) traffic are handled by weighted fair queuing (WFQ) schedulers

demonstrating the top-priority treatment of EF in practice.

The IEEE 802.1Q standard defines the structure of a VLAN tag, which is inserted into an Ethernet frame. This tag contains a 3-bit field known as the Priority Code Point (PCP). The PCP field is used to mark frames with a specific Class of Service (CoS) priority at Layer 2. With 3 bits available, it is possible to represent 2^3, or 8, distinct priority levels. These levels are numbered from 0 (lowest priority, typically for best-effort traffic) to 7 (highest priority, typically for network control traffic).

B. 16: Incorrect. This would require a 4-bit priority field (2^4), but the IEEE 802.1Q standard specifies only 3 bits for the PCP.

C. 32: Incorrect. This would require a 5-bit priority field (2^5), which is not defined in the 802.1Q VLAN tag structure.

D. 64: Incorrect. This corresponds to a 6-bit field (2^6), which is used for Differentiated Services Code Point (DSCP) in the IP header (Layer 3), not the VLAN header (Layer 2).

1. IEEE Std 802.1Q-2018

IEEE Standard for Local and metropolitan area networks—Bridges and Bridged Networks

IEEE

2018

DOI: 10.1109/IEEESTD.2018.8623023.

Section 9.6

"Tag control information": This section explicitly defines the format of the Tag Control Information (TCI) field within the VLAN tag. It specifies that the Priority Code Point (PCP) is a 3-bit field

stating

"The 3-bit PCP field specifies a priority level for the frame."

2. Kurose

J. F.

& Ross

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

Chapter 6

Section 6.4.2

"Virtual Local Area Networks (VLANs)": This textbook

widely used in university networking courses

describes the 802.1Q frame format. It details the 4-byte VLAN tag

including the "3-bit priority field" which allows for the classification of frames into one of eight priority classes.

3. Ericsson. (2019). IP Operating System CLI Command Reference

Quality of Service. 2/221 04-FAP 130 319 Uen Rev C.

Section "QoS Overview"

Subsection "Classification": This official Ericsson documentation states

"Layer 2 classification is based on the 3-bit Priority Code Point (PCP) field in the IEEE 802.1Q VLAN header. The PCP field is also referred to as the 802.1p priority." This confirms the use of a 3-bit field

resulting in 8 priority levels.

The Link Aggregation Control Protocol (LACP) is the standard protocol used for the dynamic creation and management of Link Aggregation Groups (LAGs). LACP, defined in the IEEE 802.1AX standard (formerly IEEE 802.3ad), allows multiple physical Ethernet ports to be bundled together to form a single, high-bandwidth logical link. This aggregation not only increases the total available bandwidth but also provides link-level redundancy. If a physical link within the LAG fails, traffic is automatically redistributed across the remaining active links without disrupting the logical connection.

A. RSTP: Rapid Spanning Tree Protocol (RSTP) is a loop-prevention protocol that blocks redundant links in a switched network, which is contrary to bundling them for aggregated bandwidth.

C. ERP: Ethernet Ring Protection (ERP), defined in ITU-T G.8032, is a protocol designed for high-speed fault recovery and loop prevention in Ethernet ring topologies, not for link aggregation.

D. VRRP: The Virtual Router Redundancy Protocol (VRRP) is a Layer 3 protocol that provides default gateway redundancy by creating a virtual router, and it does not aggregate physical link bandwidth.

1. IEEE Standard 802.1AX-2014

"IEEE Standard for Local and metropolitan area networks--Link Aggregation

" IEEE Standards Association

2014.

Section 1

Clause 1.1 (Scope): "This standard defines a single MAC-independent Link Aggregation technology that allows one or more links to be aggregated together to form a Link Aggregation Group. The Link Aggregation Group operates as a single logical link..."

Section 6

Clause 6.4 (Link Aggregation Control Protocol): Details the operation of LACP for automatically configuring

maintaining

and monitoring Link Aggregation.

2. Ericsson IP Operating System CLI Reference

"Configuring Link Aggregation

" Document Number: 2/221 04-FAP 130 304 Uen Rev D.

Chapter: Link Aggregation: "Link aggregation is a method of grouping one or more physical links between two systems to form a single logical link

known as a Link Aggregation Group (LAG)... LACP is used to exchange information with the partner system to form a LAG."

3. Stallings

W.

"Data and Computer Communications

" 10th ed.

Pearson

2014. (Widely used in university curricula).

Chapter 16.4 (Link Aggregation): "The Link Aggregation Control Protocol (LACP) is defined in IEEE 802.1AX. LACP is designed to be used to manage the assignment of links to a link aggregation group." This text is a staple in networking courses at reputable institutions.

The BGP path selection process within an Autonomous System (AS) is primarily influenced by BGP attributes. The LOCALPREF attribute is specifically designed for this purpose. It is a well-known, discretionary attribute that is exchanged only between internal BGP (iBGP) peers. By setting a higher LOCALPREF value for routes learned from a specific external peer, an administrator can signal to all other routers within the same AS that this is the preferred exit point for that destination. A higher LOCALPREF value is always preferred, directly influencing the outbound routing policy for the entire AS.

A. BGP uses the ASPATH length, not the number of router hops (an IGP metric like in RIP), for its path selection process.

C. The link metric (e.g., cost) is a value used by Interior Gateway Protocols (IGPs) like OSPF, not by BGP for its primary path selection.

D. Administrative Distance (AD) is used by a router to choose between different routing protocols (e.g., BGP vs. OSPF), not to influence the BGP path selection algorithm itself.

1. RFC 4271: A Border Gateway Protocol 4 (BGP-4)

Y. Rekhter

T. Li

S. Hares

Eds.

January 2006.

Section 9.1.2.2

"Decision Process": This section details the BGP best-path selection algorithm. Step (c) explicitly states: "The route with the highest local preference is preferred." This confirms that LOCALPREF is a primary factor in the decision process. The attribute is defined in Section 5.1.4 as being used "by a BGP speaker to inform its other internal peers of the advertising speaker's degree of preference for an advertised route."

2. Kurose

J. F.

& Ross

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

Chapter 5.3

"The Border Gateway Protocol (BGP) in the Internet": This chapter explains BGP route selection policies. It describes how Local Preference is used to select a specific gateway router for a destination among multiple options within an AS

stating

"The local preference attribute is used to indicate a preference for one route over another... and is thus an AS-internal-only BGP attribute."

3. MIT OpenCourseWare

Course 6.829

Computer Networks

Fall 2002.

Lecture 14

"BGP": The lecture notes describe the BGP decision process. They list the attributes in order of evaluation

placing "Highest Local Preference" as the first major policy-based step after basic reachability checks

reinforcing its role in influencing route selection within an AS. (Available via MIT OCW website).

In an IS-IS network, a Level 1 (L1) router maintains a link-state database only for its own area. To route packets to destinations outside this area, it requires a default route. This default route points to the nearest Level 1-Level 2 (L1/L2) router, which acts as a gateway to the Level 2 backbone. An L1/L2 router advertises its capability to act as this gateway by setting the Attach (ATT) bit in its L1 Link State PDUs (LSPs). When other L1 routers in the area receive an LSP with the ATT bit set, they identify the sender as a path out of the area and install a default route pointing to it.

A. options bit: This is a generic term. The specific mechanism in IS-IS for identifying an L2 gateway is the Attach bit, not a general "options bit."

B. status bit: This is not a standard term used in the IS-IS protocol for identifying an L1/L2 router's gateway function.

D. overload bit: The overload (OL) bit signals that a router should not be used for transit traffic, typically during maintenance, which is unrelated to identifying an area gateway.

1. Internet Engineering Task Force (IETF) RFC 1195

"Use of OSI IS-IS for routing in TCP/IP and dual environments

" Section 3.10.2

"Generation of L1 LSPs by L1/L2 routers." The document states: "An L1/L2 router MUST set the 'attached' bit in its L1 LSPs. This bit indicates that the L1/L2 router is attached to the L2 backbone

and can be used as a default route by L1 routers in its area."

2. Cisco Systems Official Documentation

"IS-IS Design Guide

" Section: "IS-IS Areas and Levels." This guide explains: "The L1/L2 router sets a special bit in its L1 LSP called the attached bit. This bit tells the L1 routers that it is attached to an L2 router and can be used to reach other areas."

3. Alwayn

V. (2002). Advanced MPLS Design and Implementation. Cisco Press. Chapter 2

"IS-IS Protocol Basics

" under the section "IS-IS Hierarchical Routing

" explains that the ATT bit is set by the L1/L2 router to attract L1 routers to send it inter-area traffic.

A fundamental architectural difference between OSPF and IS-IS lies in their transport mechanism. OSPF was designed specifically for the TCP/IP protocol suite and operates at the network layer. Its packets are encapsulated directly within IP datagrams, identified by IP protocol number 89.

Conversely, IS-IS was developed for the OSI protocol stack and functions at the data link layer (Layer 2). Its control messages, known as Protocol Data Units (PDUs), are encapsulated directly into Layer 2 frames (e.g., Ethernet). This design makes IS-IS independent of the network layer protocol it is routing, as it does not rely on IP for its own transport.

B. Both OSPF and IS-IS are link-state routing protocols. They both build a topological map of the network and use Dijkstra's algorithm to compute shortest paths.

C. This statement incorrectly reverses the encapsulation methods. OSPF uses IP for transport, not Ethernet directly, while IS-IS uses Layer 2 frames, not IP.

D. OSPF is a link-state protocol, not a distance vector protocol. Distance vector protocols, such as RIP, operate on different principles (e.g., Bellman-Ford algorithm).

1. Internet Engineering Task Force (IETF). (1998). RFC 2328: OSPF Version 2. Section 1.2

"Protocol overview". This document states

"OSPF runs directly over the internet protocol layer... OSPF does not use a transport protocol (such as UDP or TCP). OSPF has been assigned IP protocol number 89."

2. Internet Engineering Task Force (IETF). (1990). RFC 1195: Use of OSI IS-IS for routing in TCP/IP and dual environments. Section 3

"Encapsulation". This standard specifies

"IS-IS protocol data units (PDUs) shall be encapsulated in the data link frames... On broadcast links such as Ethernet... IS-IS PDUs are transmitted in standard LAN frames."

3. Doyle

J.

& Carroll

J. (2005). Routing TCP/IP

Volume 1 (2nd ed.). Cisco Press. Chapter 11

"IS-IS for IP Networks"

p. 638. The text explicitly contrasts the two protocols: "Whereas OSPF packets are encapsulated in IP packets

IS-IS PDUs are encapsulated directly into data-link frames." This book is a standard reference in advanced networking courses.

The abbreviation of IPv6 addresses follows two main rules as defined in RFC 5952:

1. Zero Compression: The longest contiguous sequence of 16-bit zero-value hextets is replaced by a double colon (::). This can only be done once. If there are multiple blocks of zeros of the same longest length, the first one is compressed.

2. Leading Zero Omission: Leading zeros within any 16-bit hextet can be removed.

Applying these rules to BFEA:DACA:0000:0000:9390:0000:0000:D91:

1. There are two blocks of consecutive zeros (0000:0000), both of equal length. The rule dictates compressing the first block. This yields BFEA:DACA::9390:0000:0000:D91.

2. Next, leading zeros in the remaining hextets are omitted. The 0000 fields become 0. This results in the final abbreviated address: BFEA:DACA::9390:0:0:D91.

A. BFEA:DACA::9390::D91 is incorrect because the double colon (::) can only be used once in an address to avoid ambiguity.

B. BFEA:DACA::9390:0:D91 is incorrect as it expands to BFEA:DACA:0000:0000:0000:9390:0000:D91, which is not the original address.

C. BFEA:DACA::D91/6 is incorrect. The /6 notation indicates a network prefix length, not part of the host address abbreviation.

1. Hinden

R.

& Deering

S. (2006). IP Version 6 Addressing Architecture. RFC 4291

Section 2.2. IETF.

This document outlines the fundamental rules for representing IPv6 addresses

including the use of "::" to compress zeros and the omission of leading zeros in a field.

2. Kawamura

S.

& Kawashima

M. (2010). A Recommendation for IPv6 Address Text Representation. RFC 5952

Section 4.2.3. IETF.

This RFC provides the canonical representation rules. Section 4.2.3 explicitly states: "If the length of the runs of zero bits are equal

the first run of zero bits MUST be shortened." This directly supports the choice of compressing the first 0000:0000 block.

3. Stanford University. (2021). CS144: Introduction to Computer Networking

Lecture 10: The Internet Protocol. Courseware.

Lecture slides and notes for this course typically cover IPv6 addressing

explaining the two main rules for abbreviation: compressing the longest string of zeros with :: (only once) and dropping leading zeros from each hextet. This aligns with the logic used to derive the correct answer.

An OSPF stub area is a special area type designed to reduce the size of the link-state database (LSDB) and routing tables on internal routers. This is achieved by preventing the flooding of Type 5 LSAs (AS-External-LSAs), which advertise routes external to the OSPF Autonomous System (AS). Instead of numerous external routes, the Area Border Router (ABR) connected to the stub area injects a single default route (via a Type 3 Summary LSA) into the area. This allows routers within the stub area to reach external destinations without needing to maintain information about each specific external prefix.

A. Type 1 LSAs (Router LSAs) are fundamental for describing router links within an area and are not restricted in any area type.

B. Type 2 LSAs (Network LSAs) describe multi-access network segments within an area and are essential for intra-area routing.

C. Type 3 LSAs (Summary LSAs) are used by ABRs to advertise routes from other areas and are permitted in stub areas to ensure inter-area reachability.

1. IETF RFC 2328 - OSPF Version 2:

Section 3.6

"Stub areas": "Stub areas are areas that do not import AS-external-LSAs. Stub areas are protected from the complexities of external routing. Instead

a default route (IP destination 0.0.0.0) is used to handle all traffic to external destinations." This directly confirms that Type 5 LSAs (AS-external-LSAs) are blocked.

Section 12.4.5

"AS-external-LSAs": Defines these as Type 5 LSAs

which "describe routes to destinations external to the Autonomous System."

2. Kurose

J. F.

& Ross

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

Chapter 5.3

"Routing in the Internet": In the discussion of OSPF

the text explains that to reduce overhead

an OSPF autonomous system can be configured into areas

and a "stub area" is defined as an area that does not receive link-state advertisements for routes external to the AS. This refers to the filtering of Type 5 LSAs.

3. Doyle

J.

& Carroll

J. (2006). Routing TCP/IP

Volume 1 (2nd ed.). Cisco Press.

Chapter 8

"OSPF

" Section "Stub Areas": "A stub area is an area into which AS External LSAs (Type 5) are not flooded... By filtering Type 5 LSAs

the LSDBs of internal routers are significantly reduced." This explicitly states the restriction of Type 5 LSAs.

Priority Queuing (PQ) is a Quality of Service (QoS) scheduling algorithm that services queues based on a strict priority hierarchy. It always dequeues packets from the highest-priority queue first. As long as there are packets in the highest-priority queue, no other queue is serviced. This ensures that the most critical traffic experiences the lowest possible latency and jitter. However, a major drawback of this method is "starvation," where a continuous flow of high-priority traffic can prevent lower-priority queues from ever being serviced, denying them any bandwidth. PQ does not provide minimum bandwidth guarantees.

B. The highest priority queue is not inherently reserved for voice traffic; its use is determined by network policy, though delay-sensitive traffic like voice is a common candidate.

D. PQ provides no minimum bandwidth guarantees. This is a feature of other queuing mechanisms like Class-Based Weighted Fair Queuing (CBWFQ).

1. Cisco Systems

Inc.

"Congestion Management Overview" in QoS: Congestion Management Configuration Guide

Cisco IOS XE Release 3S. The section on "Priority Queuing" states: "PQ ensures that important traffic gets the quickest handling at the expense of other traffic... If traffic in a higher-priority queue is constant

traffic in all lower-priority queues can be completely blocked (a condition called starvation)." This directly supports answers A and C.

2. Kurose

J. F.

& Ross

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

Section 7.2.2 "Scheduling

" the text describes Priority Queuing: "a packet from a low-priority queue can be transmitted only when all of the higher-priority queues are empty... a continuous stream of high-priority packets can block out all low-priority packets." This confirms that high-priority traffic gets minimal delay (A) at the risk of starving low-priority traffic (C).

3. Stallings

W. (2014). Data and Computer Communications (10th ed.). Pearson. In Chapter 19

Section 19.2 "Resource Allocation

" the discussion on "Priority Queuing" explains that this method gives preferential treatment to high-priority packets

which are always chosen for transmission over low-priority packets. It highlights the risk of lower-priority queues being starved for service. This supports both A and C.

The Dynamic Host Configuration Protocol (DHCP) is a network management protocol used on IP networks for automatically assigning IP addresses and other communication parameters to devices. Its fundamental role (A) is to provide a client with a unique IP address, subnet mask, and a lease for that address. Additionally, DHCP is designed to deliver other critical configuration options, including the IP address of the default gateway (B), which enables the host to communicate with devices outside its local subnet. It also commonly provides DNS server addresses.

C. Hop count information is a function of routing protocols like RIP or diagnostic tools such as traceroute, not the DHCP configuration protocol.

D. Network authorization is handled by dedicated security protocols like RADIUS, TACACS+, or IEEE 802.1X, which control access, not by DHCP.

1. IETF RFC 2131

"Dynamic Host Configuration Protocol": The abstract and introduction (Section 1) state that DHCP's purpose is to provide a framework for passing configuration information

including the automatic allocation of network addresses

to hosts on a TCP/IP network. This directly supports option A.

2. IETF RFC 2132

"DHCP Options and BOOTP Vendor Extensions": Section 3.3

"Router Option

" explicitly defines an option (Code 3) for DHCP to provide a list of IP addresses for routers on the client's subnet. The first address is typically used as the default gateway

supporting option B.

3. MIT OpenCourseWare

6.033 Computer System Engineering

Spring 2018

Lecture 11 Notes: The section on "Dynamic Host Configuration Protocol (DHCP)" states

"DHCP can also tell the client the IP address of the local DNS resolver

the default gateway

etc." This confirms that providing the default gateway is a standard role of DHCP.

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.