Free Cisco ENCOR (MPLS) Question and Answers — Questions and Answers

Question 1: What Cisco-exclusive technology is MPLS based on?

CEF (Correct answer)
PaGP
ISL
None of the above

Correct answer: CEF

MPLS (Multiprotocol Label Switching) is built upon Cisco Express Forwarding (CEF) for its high-speed packet forwarding capabilities. CEF pre-computes the entire forwarding path for IP packets, creating a Forwarding Information Base (FIB) and adjacency table. MPLS leverages these pre-computed paths, replacing slower IP lookups with efficient label lookups for faster data plane operations.

Question 2: What table does CEF create?

Arp table
FIB (Correct answer)
IP Route Table
Layer 3 label

Correct answer: FIB

Cisco Express Forwarding (CEF) is a high-performance, distributed switching mechanism that creates the Forwarding Information Base (FIB) table. The FIB is a database containing pre-computed next-hop information for all known routes, enabling routers to make rapid and efficient IP packet forwarding decisions without needing to consult the routing table for every packet.

Question 3: What ISP router, for example, adds labels to routes discovered by BGP?

LDP Label Discovery Protocol router
FIB Forwarding Information Base router
LSR Label Switching Router (Correct answer)
None of the above

Correct answer: LSR Label Switching Router

A Label Switching Router (LSR) is a core component in an MPLS network responsible for forwarding packets based on labels. Specifically, an Ingress LSR (the edge router where traffic enters the MPLS domain) will add (push) labels to IP packets based on routing information, such as routes discovered by BGP, before forwarding them into the MPLS cloud.

Question 4: What are the MPLS terms for adding and removing labels?

Wipe on wipe off
Plug and play
Push and pop (Correct answer)
All of the above

Correct answer: Push and pop

In MPLS, the terms 'push' and 'pop' describe the fundamental actions of adding and removing labels from packets. 'Push' refers to the process of adding a label to an IP packet as it enters the MPLS domain, typically performed by an ingress Label Edge Router. 'Pop' refers to the removal of a label from a packet, usually done by an egress LER or the penultimate hop router before the packet leaves the MPLS domain.

Question 5: Which MPLS plane contains label 3 routing and label exchange protocols?

Control plane (Correct answer)
Transport plane
Data plane
Sarengeti plane

Correct answer: Control plane

The control plane in MPLS is responsible for establishing and maintaining the label forwarding information. This includes running routing protocols (like OSPF, EIGRP, BGP) to learn network topology and IP routes, as well as label distribution protocols (like LDP or RSVP-TE) to exchange label mappings among Label Switching Routers (LSRs).

Question 6: ______ handles label swapping and transfers data based on L3 or L2 information.

Transport plane
Control plane
Control plane
Data plane (Correct answer)

Correct answer: Data plane

The data plane, also known as the forwarding plane, is responsible for the actual forwarding of packets based on the information established by the control plane. In MPLS, the data plane performs label swapping and transfers data packets using the Label Forwarding Information Base (LFIB), which maps incoming labels to outgoing labels and interfaces, enabling high-speed packet delivery.

Question 7: Where in a data frame does the MPLS label reside?

Between IP and MAC (Correct answer)
Within the IP layer
Between IP and data
None of the above

Correct answer: Between IP and MAC

The MPLS label is inserted into a data frame as a shim header, positioned between the Layer 2 (MAC/Ethernet) header and the Layer 3 (IP) header. This placement allows MPLS to operate independently of the underlying Layer 2 technology while still encapsulating the Layer 3 packet for label-based forwarding.

Question 8: What number of bits make up an MPLS label?

10
20 (Correct answer)
16
Label, TTL, and BS

Correct answer: 20

An MPLS label is a 20-bit field that is part of the MPLS header. This 20-bit value is used by Label Switching Routers (LSRs) to make forwarding decisions, enabling fast lookup and switching of packets across the MPLS network, which is more efficient than traditional IP address lookups.

Question 9: What constitutes a data frame's four components of MPLS information?

Label, Experimental, TTL
Label, TTL, Checksum
Label, Experimental, BS (last label in stack), TTL (Correct answer)
None of the above

Correct answer: Label, Experimental, BS (last label in stack), TTL

An MPLS header consists of four distinct components: the 20-bit Label field for forwarding decisions, a 3-bit Experimental field often used for Quality of Service (QoS), a 1-bit Bottom of Stack (BS) flag indicating if it's the last label in a stack, and an 8-bit Time To Live (TTL) field to prevent loops and limit hop count.

Question 10: Name the hardware component that inserts or removes labels from frames as they enter or leave the provider's MPLS network.

Edge Switch
Edge Label Switch Router (Correct answer)
Core router
None of the above

Correct answer: Edge Label Switch Router

The Edge Label Switch Router (E-LSR), also known as a Label Edge Router (LER), is a critical hardware component located at the boundary of an MPLS network. It is responsible for 'pushing' (inserting) labels onto incoming IP packets as they enter the MPLS domain and 'popping' (removing) labels from packets as they exit the MPLS domain, translating between IP and MPLS forwarding.

Question 11: Official routing protocol of MPLS

FIB Forwarding Information Base
RIPv2
LDP Label Distribution Protocol. (Correct answer)
None of the above

Correct answer: LDP Label Distribution Protocol.

While MPLS relies on underlying IP routing protocols (like OSPF, EIGRP, BGP) to learn network topology, the Label Distribution Protocol (LDP) is the official and standard protocol specifically designed for distributing and exchanging label mappings between Label Switching Routers (LSRs) within an MPLS domain. LDP is crucial for establishing the label-switched paths that MPLS uses for forwarding.

Question 12: What is produced once labels are exchanged over LDP between two routers?

FIB
FLIB
LIB
LFIB (Correct answer)

Correct answer: LFIB

Once labels are exchanged between routers using the Label Distribution Protocol (LDP), each router builds a Label Information Base (LIB). From this LIB, the router then constructs the Label Forwarding Information Base (LFIB). The LFIB is the actual table used by the data plane to perform label-switched forwarding decisions, mapping incoming labels to outgoing labels and interfaces for efficient packet forwarding.

Question 13: Neighbor Finding Setting up the session Label Exchange and Distribution Which three are these?

LDP's three steps (Correct answer)
VTP's three steps
PAgP's three steps
None of the above

Correct answer: LDP's three steps

These three steps—Neighbor Finding, Setting up the session, and Label Exchange and Distribution—describe the fundamental process by which Label Distribution Protocol (LDP) operates in an MPLS network. LDP first discovers neighbors, then establishes a session with them, and finally exchanges and distributes label mappings for network prefixes. This allows routers to build their Label Forwarding Information Base (LFIB) for efficient packet forwarding.

Question 14: Which multicast address does LDP send its hellos to?

224.0.0.5
224.0.0.2 (Correct answer)
224.0.0.10
None of the above

Correct answer: 224.0.0.2

LDP uses the well-known IPv4 multicast address 224.0.0.2 for sending its Hello messages. This address is specifically reserved for all routers on a segment, allowing LDP-enabled routers to discover each other without prior configuration. This efficient discovery mechanism is crucial for establishing LDP adjacencies and building an MPLS domain.

Question 15: What UDP port does LDP send over?

22
464
646 (Correct answer)
643

Correct answer: 646

Label Distribution Protocol (LDP) uses UDP port 646 for its Hello messages, which are used for neighbor discovery. However, once a session is established between LDP peers, the actual label exchange and distribution messages are sent over TCP port 646. This dual-protocol approach ensures reliable communication for critical label information while using UDP for lightweight discovery.

Question 16: What distinguishes the FIB and LFIB from one another?

FIB forwards full label paths while LFIB forwards only limited path information.
FIB forwards IP packets while LFIB forwarded *labelled* IP packets. (Correct answer)
FIB forwards routing information while LFIB forwards label routing information.
None of the above

Correct answer: FIB forwards IP packets while LFIB forwarded *labelled* IP packets.

The Forwarding Information Base (FIB) is used in traditional IP routing to forward IP packets based on their destination IP address. In contrast, the Label Forwarding Information Base (LFIB) is specific to MPLS and is used to forward packets that have an MPLS label attached. The LFIB contains instructions for label switching operations like swap, push, or pop, enabling faster forwarding decisions than traditional IP lookup.

Free Cisco ENCOR (MPLS) Question and Answers

What Cisco-exclusive technology is MPLS based on?