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Question 21

Which MPLS label operation replaces the top label with another label?

1. Pop
2. Swap
3. Push
4. Remove

Correct Answer: 2

Explanation

The MPLS swap operation replaces the top label in an MPLS label stack with another label. This operation is commonly performed by transit Label Switch Routers (LSRs) as packets move through an MPLS network. The incoming label identifies the forwarding treatment, and the router swaps it with the outgoing label before forwarding the packet. A push operation adds a label, while pop removes the top label. These operations allow MPLS routers to forward packets efficiently using labels rather than performing a traditional IP routing lookup at every hop.

Question 22

Which MPLS component assigns a label to an incoming packet based on its forwarding equivalence class?

1. Label Switch Router
2. DNS server
3. DHCP server
4. Syslog server

Correct Answer: 1

Explanation

A Label Switch Router (LSR) is an MPLS-enabled router that performs label operations on packets. Depending on the packet’s Forwarding Equivalence Class (FEC), an LSR can impose, swap, or remove MPLS labels. In an MPLS network, routers use labels to efficiently determine forwarding behavior. An ingress router may impose a label, transit routers typically swap labels, and an egress router removes MPLS labels before forwarding traffic toward its destination. DNS, DHCP, and Syslog servers perform completely different functions and are not responsible for MPLS label forwarding.

Question 23

What does the MPLS Forwarding Equivalence Class (FEC) represent?

1. A group of packets that receive the same forwarding treatment
2. A physical interface speed
3. A BGP authentication method
4. A customer VLAN database

Correct Answer: 1

Explanation

A Forwarding Equivalence Class (FEC) represents a group of packets that receive the same forwarding treatment within an MPLS network. Packets belonging to the same FEC can be assigned the same label and follow the same forwarding behavior. The FEC concept allows MPLS networks to separate forwarding decisions from traditional destination-based IP lookup processes. FECs can be associated with destinations, VPNs, or other forwarding characteristics depending on the MPLS implementation. They are not physical interface specifications, authentication methods, or VLAN databases.

Question 24

Which protocol can be used to establish label-switched paths using explicit traffic-engineering constraints?

1. ARP
2. RSVP-TE
3. DHCP
4. SNMP

Correct Answer: 2

Explanation

Resource Reservation Protocol with Traffic Engineering extensions (RSVP-TE) can establish MPLS Label Switched Paths (LSPs) based on traffic-engineering constraints. It can signal paths through the network while considering requirements such as bandwidth and explicit route information. RSVP-TE was widely used for MPLS Traffic Engineering before Segment Routing became increasingly common in modern service provider networks. ARP resolves IP-to-MAC mappings, DHCP provides address configuration, and SNMP is primarily used for network monitoring and management. Therefore, RSVP-TE is the correct protocol.

Question 25

Which MPLS label field indicates the position of a label at the bottom of the label stack?

1. EXP
2. Label
3. TTL
4. Bottom-of-Stack bit

Correct Answer: 4

Explanation

The Bottom-of-Stack (BoS) bit in an MPLS label stack indicates whether a particular label is the last label in the stack. MPLS packets can contain multiple labels, such as a transport label and a VPN service label. The BoS field helps routers identify the bottom label in that stack. The label field contains the actual label value, while the TTL field supports loop prevention and packet lifetime handling. The former EXP field is associated with traffic-class information in traditional MPLS terminology. Therefore, the Bottom-of-Stack bit is correct.

Question 26

Which BGP address family is used to carry IPv4 VPN routes in an MPLS Layer 3 VPN?

1. IPv4 unicast
2. VPNv4
3. IPv6 unicast
4. L2VPN only

Correct Answer: 2

Explanation

The BGP VPNv4 address family is used to carry IPv4 VPN routes across an MPLS Layer 3 VPN provider network. VPNv4 combines an IPv4 prefix with a Route Distinguisher, allowing overlapping customer address spaces to coexist within the provider’s BGP control plane. Route Targets are then used to determine which VPN routes should be imported into particular VRFs. Regular IPv4 unicast BGP does not provide the same VPN-specific route separation. Therefore, VPNv4 is the correct address family.

Question 27

What is the primary purpose of a VRF in a service provider router?

1. To provide multiple independent routing tables
2. To increase Ethernet frame size
3. To replace MPLS labels
4. To synchronize device clocks

Correct Answer: 1

Explanation

Virtual Routing and Forwarding (VRF) allows a single router to maintain multiple independent routing and forwarding tables. Service providers commonly use VRFs to isolate customer traffic and maintain separate routing contexts on the same PE router. This is fundamental to MPLS Layer 3 VPN implementations. Different customers can even use overlapping IP address ranges because their routes are maintained within separate VRFs. VRF does not modify Ethernet frame size, replace MPLS labels, or provide time synchronization. Therefore, providing multiple independent routing tables is its primary purpose.

Question 28

Which BGP attribute is normally propagated within an autonomous system to influence outbound path selection?

1. MED
2. Local Preference
3. Origin
4. Next-Hop

Correct Answer: 2

Explanation

Local Preference is a BGP attribute used within an autonomous system to influence outbound path selection. When multiple external routes are available, routers generally prefer the path with the higher Local Preference value, assuming the paths reach the relevant stage of BGP selection. This allows administrators to influence which exit point traffic should use. MED is primarily used to provide a neighboring AS with information about preferred entry points. Origin and Next-Hop have different functions in BGP path selection. Therefore, Local Preference is correct.

Question 29

Which BGP attribute is commonly associated with selecting the shortest AS path?

1. MED
2. Weight
3. AS_PATH
4. Local Preference

Correct Answer: 3

Explanation

The AS_PATH attribute records the sequence of autonomous systems that a BGP route has traversed. BGP generally prefers a shorter AS_PATH over a longer one when comparing otherwise applicable routes. The attribute also plays an important role in preventing routing loops because a router can reject a route if its own AS number already appears in the AS_PATH. Local Preference influences outbound path selection, Weight is a Cisco-specific local attribute, and MED can influence route selection between neighboring autonomous systems. Therefore, AS_PATH is correct.

Question 30

Which BGP feature helps prevent routing loops between autonomous systems?

1. AS_PATH
2. VLAN ID
3. MPLS TTL
4. MAC address

Correct Answer: 1

Explanation

The BGP AS_PATH attribute helps prevent routing loops between autonomous systems. Each AS that advertises a route through eBGP normally adds its AS number to the AS_PATH. When a BGP router receives a route containing its own AS number in the path, it can reject that route, preventing the route from being accepted as a valid path back into the same AS. MPLS TTL serves a different purpose related to packet lifetime and loop prevention at the forwarding level. VLAN IDs and MAC addresses do not perform BGP loop prevention. Therefore, AS_PATH is correct.

Question 31

Which BGP session type is normally used between routers belonging to different autonomous systems?

1. iBGP
2. eBGP
3. MP-BGP only
4. Static BGP

Correct Answer: 2

Explanation

External BGP (eBGP) is normally used to establish BGP sessions between routers in different autonomous systems. Service providers use eBGP to exchange routing information with customers, peers, and other providers. Internal BGP (iBGP) is used between BGP speakers within the same autonomous system. MP-BGP extends BGP to support multiple address families, including VPNv4 and VPNv6, but the terms iBGP and eBGP describe the relationship between BGP speakers based on their AS numbers. Therefore, eBGP is the correct answer.

Question 32

Which BGP session type is normally used between BGP routers within the same autonomous system?

1. eBGP
2. iBGP
3. LDP
4. RSVP

Correct Answer: 2

Explanation

Internal BGP (iBGP) is used to exchange BGP routing information between BGP speakers within the same autonomous system. In service provider networks, iBGP or MP-iBGP can distribute routes and VPN information between PE routers and other BGP speakers. iBGP has specific rules regarding route propagation, which is why techniques such as route reflectors are often used to improve scalability. eBGP is used between different autonomous systems. LDP and RSVP are MPLS-related signaling protocols rather than BGP session types. Therefore, iBGP is correct.

Question 33

What is the primary purpose of a BGP Route Reflector?

1. To eliminate all routing protocols
2. To reduce the need for a full iBGP mesh
3. To encrypt BGP updates
4. To replace MPLS labels

Correct Answer: 2

Explanation

A BGP Route Reflector improves iBGP scalability by reducing the requirement for a full mesh of iBGP sessions. Instead of every BGP router needing a direct session with every other BGP router, selected routers can act as route reflectors and distribute routes to their clients according to the route-reflection rules. This reduces the number of BGP sessions that must be maintained in large service provider networks. Route reflectors do not encrypt BGP updates or replace MPLS labels. Therefore, reducing the need for a full iBGP mesh is the correct answer.

Question 34

Which protocol is commonly used to exchange link-state information within an MPLS service provider core?

1. OSPF
2. HTTP
3. FTP
4. DHCP

Correct Answer: 1

Explanation

OSPF is a link-state interior gateway protocol that can be used to exchange topology information within an MPLS service provider core. It calculates routes based on a link-state database and can provide the underlying IP reachability needed by MPLS technologies. IS-IS is another commonly deployed link-state IGP in service provider networks. HTTP and FTP are application-layer protocols, while DHCP provides address configuration. The specific IGP selected depends on the provider’s architecture and operational requirements. Therefore, OSPF is a valid and commonly used choice.

Question 35

Which IS-IS level is responsible for routing between different IS-IS areas?

1. Level 0
2. Level 1
3. Level 2
4. Level 3

Correct Answer: 3

Explanation

IS-IS uses Level 1 and Level 2 routing domains. Level 1 routers provide routing within an area, while Level 2 routers provide routing between different areas. A router can also operate as both Level 1 and Level 2, allowing it to participate in intra-area and inter-area routing. Unlike OSPF, IS-IS does not use an Area 0 backbone area. Level 2 therefore provides the inter-area connectivity in an IS-IS topology. Level 0 and Level 3 are not the standard IS-IS routing levels. Thus, Level 2 is correct.

Question 36

Which OSPF area is commonly used as the backbone for inter-area routing?

1. Area 0
2. Area 10
3. Area 100
4. Area 255

Correct Answer: 1

Explanation

OSPF Area 0 is the backbone area and is used as the central area for inter-area routing. Other OSPF areas normally connect to the backbone either directly or through supported mechanisms such as virtual links in specific designs. The backbone design helps OSPF scale by separating routing domains into areas. Area numbers such as 10, 100, or 255 can also be valid OSPF areas, but they do not have the special backbone role assigned to Area 0. Therefore, Area 0 is the correct answer.

Question 37

Which technology can provide rapid detection of a BGP peer failure without relying solely on BGP hold timers?

1. NAT
2. BFD
3. STP
4. DHCP

Correct Answer: 2

Explanation

BFD can provide rapid detection of forwarding-path failures for protocols such as BGP. Without BFD, BGP normally relies on its keepalive and hold timers to detect an unresponsive neighbor. BFD can detect failures much faster and then notify the associated routing protocol so that it can react. This can improve convergence in service provider environments. NAT translates addresses, STP prevents Layer 2 loops, and DHCP provides network configuration information. Therefore, BFD is the appropriate technology for rapid failure detection associated with a BGP peer.

Question 38

Which service provider technology uses BGP as its control plane to advertise Ethernet VPN information?

1. EVPN
2. LDP
3. RSVP-TE
4. OSPF

Correct Answer: 1

Explanation

Ethernet VPN (EVPN) uses BGP as its control plane to distribute information required for Ethernet VPN services. EVPN can advertise MAC addresses, IP reachability, Ethernet segments, and other information depending on the implementation and route type. This BGP-based control plane provides a scalable way to distribute customer Ethernet information across a service provider network. LDP and RSVP-TE are MPLS signaling technologies, while OSPF is an IGP. Therefore, EVPN is the correct technology.

Question 39

Which EVPN route type is commonly associated with MAC/IP advertisement?

1. Route Type 1
2. Route Type 2
3. Route Type 4
4. Route Type 5 only

Correct Answer: 2

Explanation

EVPN Route Type 2 is the MAC/IP Advertisement route. It is used to advertise MAC address reachability and can also include associated IP information. This allows EVPN devices to distribute endpoint reachability through the BGP control plane rather than relying solely on data-plane learning. EVPN has multiple route types serving different purposes, including Ethernet Auto-Discovery routes, Inclusive Multicast Ethernet Tag routes, Ethernet Segment routes, and IP Prefix routes. Therefore, Route Type 2 is the correct answer for MAC/IP advertisement.

Question 40

Which component in an MPLS Layer 3 VPN architecture typically carries customer VPN routes using MP-BGP?

1. PE router
2. P router
3. Access switch only
4. DHCP server

Correct Answer: 1

Explanation

Provider Edge (PE) routers exchange customer VPN routes using Multiprotocol BGP (MP-BGP) in an MPLS Layer 3 VPN architecture. PE routers maintain customer VRFs and advertise VPNv4 or VPNv6 routes through the provider control plane. The core Provider (P) routers generally forward MPLS packets based on transport labels and do not need to maintain individual customer VPN routing tables. This separation allows the MPLS core to remain scalable while PE routers handle customer-specific routing information. Therefore, the PE router is the correct answer.