ECP-206 Practice Questions

The Border Gateway Protocol (BGP) decision algorithm is a multi-step process used to select the single best path to a destination. A fundamental prerequisite is that the next-hop router for a path must be reachable; if the next-hop is inaccessible (i.e., no route to it exists in the Interior Gateway Protocol), the BGP path is considered invalid and is not eligible for selection. Among the valid paths, the LOCALPREF attribute is one of the first criteria evaluated. This attribute is used within an Autonomous System (AS) to influence the exit point for outbound traffic. A path with a higher LOCALPREF value is always preferred over a path with a lower value.

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

D. This statement is the opposite of the correct BGP behavior; a higher Local Preference value is preferred, not a lower one.

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

Section 9.1.2.2

Decision Process: "The following rules are used to select a route for a given destination... (c) If the route is learned from an internal peer

and the value of the LOCALPREF attribute is higher for one route than for the others

prefer the route with the higher LOCALPREF." This directly supports answer A.

Section 9.1.2.1

Phase 2: Route Resolution: This section details the process of resolving the BGP NEXTHOP attribute. It states that if the BGP NEXTHOP for a route is not resolvable

the route cannot be installed in the routing table. This supports answer C.

2. MIT OpenCourseWare

6.829 Computer Networks

Fall 2002

Lecture 15: BGP:

Slide 15-21

"BGP Decision Process": The lecture slide lists the BGP decision process steps in order. The first step listed is "1. Highest Local Preference

" confirming that a higher value is preferred. The process implicitly requires a valid

reachable next-hop before these steps are applied.

3. Ericsson IP Operating System

CLI Command Reference:

In documentation for Ericsson routers (such as the Router 6000 series)

the description of the BGP decision process consistently aligns with RFC 4271. The show router bgp routes command output and configuration guides for route-policies confirm that a higher local-preference is preferred and that routes with an unresolved next-hop are marked as invalid and not used. (e.g.

Section: "BGP Route Selection" in relevant configuration guides).

The maximum number of unique VLAN IDs is defined by the IEEE 802.1Q standard. The standard specifies a 12-bit field for the VLAN Identifier (VID) within the Ethernet frame's VLAN tag. A 12-bit field can represent 2^12, or 4096, unique values. This creates a theoretical range of VLAN IDs from 0 to 4095. While IDs 0 and 4095 are reserved for specific functions and are generally not usable for user-configured VLANs, the total address space defined by the 12-bit field is 4096.

A. 2048 is the result of an 11-bit field (2^11), which is not the size of the IEEE 802.1Q VID field.

B. 3192 is an arbitrary value with no technical correlation to the IEEE 802.1Q VLAN standard.

D. 16384 is the result of a 14-bit field (2^14), which is incorrect for the standard VLAN ID.

1. IEEE Std 802.1Q-2018

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

Section 9.6

"VLAN tag format": This section explicitly defines the VLAN Identifier (VID) as a 12-bit field. The text states

"The VLAN Identifier (VID) is a 12-bit field that uniquely identifies the VLAN to which the frame belongs." This directly supports the calculation of 2^12 = 4096 possible IDs.

2. Ericsson

"Layer 2 Switching and VPLS Configuration Guide" for Smart Services Router (SSR) running IPOS.

Chapter: "VLANs"

Section: "VLAN Tagging": The documentation states

"The VLAN ID is a 12-bit field

which specifies the VLAN to which the frame belongs. The range of VLAN IDs is 0 to 4095." This official vendor documentation confirms the 4096 total IDs.

3. University of Cambridge

Computer Laboratory

"L32: Virtual LANs (VLANs)" Course Material.

Slide 5

"802.1Q VLAN Tag": The course slide deck illustrates the 802.1Q tag format and explicitly labels the VLAN ID (VID) field as "12 bits

4096 VLANs possible." This serves as a reputable academic source confirming the total number.

The IS-IS Network Service Access Point (NSAP) address is structured into three parts: the Area ID, the System ID, and the N-Selector (NSEL). According to the ISO/IEC 10589 standard, the NSEL is the last byte (octet), and the System ID is a fixed length of six bytes that precedes the NSEL. The remainder of the address at the beginning constitutes the Area ID.

Given the address 49.00F0.0100.5012.3010.00:

NSEL: 00 (the last byte)

System ID: 0100.5012.3010 (the preceding six bytes)

Area ID: 49.00F0 (the remaining part)

B. 49.00F0.0100: This is incorrect because it would imply the System ID is only four bytes long (5012.3010), which violates the standard six-byte length for a router's System ID.

C. 5012.3010.00: This is incorrect as it represents the last four bytes of the System ID combined with the one-byte NSEL, not the Area ID.

D. 00F0 0100: This is incorrect as it is a fragment containing parts of both the Area ID (00F0) and the System ID (0100).

1. Medhi

D.

& Ramasamy

K. (2017). Network Routing: Algorithms

Protocols

and Architectures (2nd ed.). Morgan Kaufmann.

Reference: Chapter 5

Section 5.4.1

"Addressing in an OSI Environment

" describes the NSAP address format used by IS-IS. It specifies that the System ID is a 6-byte identifier and the NSEL is a 1-byte field

with the variable-length Area ID preceding them.

2. ISO/IEC 10589:2002

"Information technology -- Telecommunications and information exchange between systems -- Intermediate System to Intermediate System intra-domain routeing information exchange protocol..."

Reference: Section 7.4

"NSAP addresses

" defines the components of the NSAP address. It specifies that the System ID field has a fixed length throughout a routing domain (conventionally 6 octets) and is followed by a single octet NSEL.

3. Ericsson IP Operating System (IPOS) Software

Command Reference.

Reference: In the documentation for the router isis command set

the net command syntax is defined as area-id.system-id.sel. Examples provided

such as net 49.0001.0000.0000.1001.00

consistently show a 6-byte System ID (0000.0000.1001)

reinforcing the standard structure.

OSPF (Open Shortest Path First) is a link-state routing protocol that uses the Shortest Path First (SPF) algorithm, also known as Dijkstra's algorithm, to determine the best path to a destination. The metric used in this calculation is "cost." The cost of a path is the cumulative value of the costs assigned to each outgoing interface along the path from the source to the destination. The route with the lowest total cost is considered the best path and is installed in the IP routing table. The interface cost is typically calculated based on the interface's bandwidth, with higher bandwidth links having a lower cost.

A. jitter: Jitter, the variation in packet delay, is a quality of service (QoS) metric, not a routing metric used by OSPF for path selection.

B. packet loss: Packet loss is a measure of link reliability and is not directly used by the standard OSPF algorithm to calculate the best path.

D. delay: Delay is a component of the composite metric used by other protocols like EIGRP, but it is not the metric used by OSPF.

1. Moy

J. (1998). RFC 2328: OSPF Version 2. The Internet Engineering Task Force (IETF). Section 2.2

"The Shortest-Path Tree

" states

"The cost of a path is the sum of the costs of the links in the path. The algorithm calculates the shortest-path tree

with the calculating router as the root." Available at: https://doi.org/10.17487/RFC2328

2. Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Chapter 5

Section 5.4.2

"The Link-State (LS) Routing Algorithm

" the text explains that OSPF uses Dijkstra's algorithm where the cost of a link is the metric used for path calculation.

3. Stanford University. (n.d.). CS144: Introduction to Computer Networking - Lecture 13: Intra-domain Routing. Lecture Slides. Slide 21 discusses OSPF

explicitly stating

"Link cost configured by administrator... Router computes least cost path to all nodes." Retrieved from the Stanford CS144 course materials archive.

Link-state routing protocols, such as OSPF, are designed for efficiency and accuracy. They achieve this by having each router build a complete map of the network topology. Updates, known as Link-State Advertisements (LSAs), are not sent periodically but are triggered by an event, such as a link failure or a change in link cost. This makes the protocol efficient in stable networks. To ensure every router maintains an identical and accurate topology database, these updates are flooded throughout the network using a reliable transport mechanism that includes acknowledgments, ensuring all neighbors receive the information correctly.

C. Link-state protocols use a metric called 'cost,' typically derived from bandwidth, not hop-count, which is characteristic of distance-vector protocols like RIP.

D. Dijkstra's algorithm, used by link-state protocols, is more processor-intensive than distance-vector algorithms because it calculates a complete topology map.

1. Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Chapter 5.2

"The Link-State (LS) Routing Algorithm

" the text states

"LS is an algorithm with global state information... The algorithm is often referred to as Dijkstra's algorithm... When a link's cost changes

the link-state broadcast algorithm is initiated." This confirms that updates are event-triggered (A) and that Dijkstra's is the link-state algorithm (relevant to D).

2. Moy

J. (1998). RFC 2328: OSPF Version 2. IETF. Section 13

"The Flooding Procedure

" describes the reliable mechanism for disseminating link-state advertisements using Link State Update and Link State Acknowledgment packets

which supports the claim of a reliable update mechanism (B).

3. Stanford University. (2013). CS144: Introduction to Computer Networking

Lecture 13 - Routing. [Lecture Slides]. Slide 23 compares Distance Vector and Link State

noting that Link State has a "high computation cost" (Dijkstra's algorithm) and that Distance Vector (RIP) uses "hop count" as a metric (relevant to C). This confirms the reasoning for D and C being incorrect. Available at: https://web.stanford.edu/class/cs144/lectures/13-routing.pdf

In the exhibited MPLS network, router R3 is the penultimate hop Label Switching Router (LSR) for the path ending at the egress Label Edge Router (LER), R4. The standard and most efficient operation performed by a penultimate router is Penultimate Hop Popping (PHP). With PHP, R3 removes the transport MPLS label from the packet before forwarding it to R4. This action is an optimization that reduces the processing load on the egress router (R4), as R4 receives a native IP packet and can perform a single lookup in its IP Forwarding Information Base (FIB) instead of two lookups (one in the LFIB and one in the FIB).

A. push: A label push is performed by the ingress LER (R1) when it encapsulates an IP packet into an MPLS frame for the first time.

B. pop: While PHP is a type of pop operation, "PHP" is the more precise and specific term for the default action occurring at the penultimate hop.

C. swap: A label swap is the typical action of an intermediate LSR (like R2), where it replaces an incoming label with a new outgoing label.

1. IETF RFC 3031: Rosen

E.

Viswanathan

A.

& Callon

R. (2001). RFC 3031: Multiprotocol Label Switching Architecture. IETF. In Section 3.14

"Penultimate Hop Popping

" the document states

"The penultimate LSR pops the label from the packet's label stack and then forwards the packet to the egress LSR. This is useful because it means that the egress LSR does not need to look up the label

pop it

and then look up the packet's network layer header." Available at: https://doi.org/10.17487/RFC3031

2. University Courseware Textbook: Kurose

J. F.

& Ross

K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). Pearson. In Section 4.6.2

"MPLS Operation

" the text describes the forwarding process: "The penultimate router... pops the MPLS label from the datagram and forwards the resulting IP datagram to the egress router."

3. Academic Publication: Ghein

D. (2006). MPLS Fundamentals. Cisco Press. Chapter 3

"MPLS Operation

" explains the label forwarding process

detailing that the penultimate router performs PHP by default to relieve the egress router from the burden of label processing.

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).

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).

The Forwarding Information Base (FIB), also known as a forwarding table, is the specific data structure used by a router's data plane to make forwarding decisions for IP packets. While routing protocols (operating in the control plane) populate a Routing Information Base (RIB) with all known routes, the FIB is a streamlined, optimized version containing only the best, unambiguous routes. This optimization allows for the extremely fast lookups required for line-rate packet processing. When a packet arrives, its destination IP address is looked up in the FIB to determine the next-hop and egress interface.

A. The Management Information Base (MIB) is used by network management protocols like SNMP for monitoring and configuration, not for per-packet forwarding decisions.

B. The Label Information Base (LIB) is used in MPLS networks to map labels to next-hop label forwarding entries, specifically for forwarding labeled packets.

C. The Traffic Engineering Data Base (TED) is a control plane database used by protocols like OSPF-TE to calculate constrained paths, not for direct packet forwarding.

1. IETF RFC 3222: Terminology for Forwarding Information Base (FIB) based Router Performance. Section 2

"Definitions

" states: "The Forwarding Information Base (FIB) is a data structure that is used to make forwarding decisions... It is populated with information from the Routing Information Base (RIB)."

2. Stallings

W. (2017). Data and Computer Communications (10th ed.). Pearson. In Chapter 19

"Routing

" the distinction is made clear: "The routing protocol populates a routing table (or RIB)... From the RIB

a forwarding table (or FIB) is created

which is used to make forwarding decisions." (Paraphrased from concepts in Section 19.2).

3. Kurose

J. F.

& Ross

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

Section 4.4

"Inside a Router

" describes the separation of the control plane (routing algorithms building the RIB) and the data plane (forwarding function using the FIB). The forwarding table (FIB) is explicitly identified as the structure consulted during the input port processing for packet forwarding.

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.