MPLS

Question 1

What is the size of the MPLS label shim header?

Answer 1

4 bytes (32 bits)

The MPLS label shim header is added between the Layer 2 frame header and the Layer 3 packet header.

Question 2

What are the four fields contained in the MPLS label shim header?

Answer 2

• Label (20 bits)
• TC (3 bits)
• S (1 bit)
• TTL (8 bits)

Each field serves a specific purpose in the MPLS framework.

Question 3

The first 20 bits of the MPLS label shim header define the ________.

Answer 3

label number

This number is crucial for identifying the MPLS path.

Question 4

The TC field in the MPLS label shim header is used for ________.

Answer 4

quality of service (QoS)

This field helps manage the performance of the network.

Question 5

True or false: The S field in the MPLS label shim header indicates whether the label is the last label in the stack.

Answer 5

TRUE

This is important for packets that use multiple labels, such as in MPLS VPNs.

Question 6

What does the TTL field in the MPLS label shim header represent?

Answer 6

time to live

Similar to IP’s TTL, it ensures MPLS frames are discarded if they do not reach their destination in time.

Question 7

What is inbound label binding filtering used for?

Answer 7

To control the label bindings that an LSR accepts from other peered LSRs

It involves using access control lists (ACLs) to identify what will and will not be accepted from peers.

Question 8

What is the purpose of the entropy label in MPLS?

Answer 8

Used for load balancing

It allows deep packet inspection (DPI) to occur once at the ingress router to create an entropy label for load balancing decisions.

Question 9

What value indicates an implicit NULL label in the MPLS header?

Answer 9

3

This label is used for PHP, indicating that the label must be popped before forwarding the packet.

Question 10

What is the value of an explicit NULL label in the MPLS header?

Answer 10

0

This label preserves the QoS EXP bits and copies them into the QoS IP precedence or DiffServ bits after PHP occurs.

Question 11

What does the MPLS LDP autoconfiguration feature simplify?

Answer 11

MPLS configuration

It enables LDP on every interface associated with a specified Interior Gateway Protocol (IGP) without manual configuration.

Question 12

What are the only supported IGPs for MPLS LDP autoconfiguration?

Answer 12

• Open Shortest Path First (OSPF)
• Intermediate System-to-Intermediate System (IS-IS)

These protocols are necessary for enabling LDP on interfaces automatically.

Question 13

What does MPLS Traffic Engineering (TE) focus on?

Answer 13

Manipulating traffic as it flows through the MPLS environment

The goal is to ensure that traffic conforms to a particular need.

Question 14

What does TE enable the service provider to accommodate?

Answer 14

the needs (SLAs) of their different customers

TE stands for Traffic Engineering, which is essential for managing quality of service (QoS) in MPLS environments.

Question 15

What control plane protocol is used for MPLS-TE?

Answer 15

Resource Reservation Protocol (RSVP)

RSVP is integral for managing resource reservations in MPLS Traffic Engineering.

Question 16

What is the function of RSVP?

Answer 16

a signaling protocol used to request resource reservations

RSVP operates between systems to manage network resources effectively.

Question 17

True or false: RSVP can generate protocol message requests and process received protocol request messages.

Answer 17

TRUE

This capability allows RSVP to manage resource reservations dynamically.

Question 18

What does RSVP ensure regarding resources?

Answer 18

resources are reserved for data flows

This reservation is done from a source to a destination in the form of an LSP.

Question 19

Fill in the blank: RSVP ensures that resources are reserved for data flows from a source to a destination in the form of a _______.

Answer 19

LSP

LSP stands for Label Switched Path, which is used in MPLS networks.

Question 20

What do MPLS Layer 3 VPNs provide?

Answer 20

Peer-to-peer connectivity

This connectivity is established between private customer sites across a shared network.

Question 21

What do PE routers need to support multiple customers?

Answer 21

VRF instances

VRF stands for Virtual Routing and Forwarding, which allows multiple instances of a routing table to exist on the same router.

Question 22

What is the purpose of using VRF instances on PE routers?

Answer 22

To isolate customer information and traffic from other customers

This isolation is crucial for maintaining security and privacy among different customers.

Question 23

How many VRF instances need to be created on the PE routers for each customer?

Answer 23

A different VRF instance for each customer

This ensures that each customer’s data is kept separate and secure.

Question 24

What must be associated with the correct VRF on the PE routers?

Answer 24

The interface that connects to the customer’s CE router

This association is necessary for proper routing of customer traffic.

Question 25

How does **BGP** handle identical **network prefixes** that belong to different customers?

Answer 25

It uses a **route distinguisher (RD)** to expand the customer's IP prefix ## Footnote The RD includes a unique value that distinguishes it from other identical prefixes.

Question 26

What is the purpose of the **route distinguisher (RD)** in BGP?

Answer 26

To include a unique value that distinguishes identical prefixes from different customers ## Footnote The RD is generated and used by the PE routers on a per-customer VRF instance basis.

Question 27

True or false: The **route distinguisher (RD)** is only used when there are overlapping address spaces.

Answer 27

FALSE ## Footnote The RD is used regardless of whether there are overlapping address spaces.

Question 28

The **route distinguisher (RD)** is generated and used by which routers?

Answer 28

PE routers ## Footnote The RD is used on a per-customer VRF instance basis.

Question 29

What is prepended to the **32-bit customer prefix** to create a **96-bit unique prefix**?

Answer 29

64-bit RD ## Footnote This process results in a VPNv4 address.

Question 30

What is the full term for the **96-bit unique prefix** created from the 64-bit RD and 32-bit customer prefix?

Answer 30

VPNv4 address ## Footnote This address is used in routing protocols.

Question 31

The **VPNv4 address** is exchanged by the _______ neighboring routers.

Answer 31

MP-IBGP ## Footnote MP-IBGP stands for Multi-Protocol Internal Border Gateway Protocol.

Question 32

What command is used to configure **RDs** in VRF configuration mode?

Answer 32

rd rd-value ## Footnote RDs are configured for the respective customer VRF.

Question 33

What does the provider use to ensure that what has been exported from a customer **VRF instance** on a PE router is sent across the **MPLS environment**?

Answer 33

route target (RTs) ## Footnote Route targets are used to control the import and export of routes between different VRF instances.

Question 34

What are **RTs** used for in the context of customer VRF instance routes?

Answer 34

To control the import and export process ## Footnote RTs are similar to RDs and are configured in the VRF configuration for the customer VRF instance.

Question 35

An **RT** is configured in the VRF configuration for the _______.

Answer 35

customer VRF instance ## Footnote RTs help manage route distribution for specific customer instances.

Question 36

On PE_R5, what command is used to configure an **export RT** of 653:27?

Answer 36

route-target export 653:27 ## Footnote This command is part of the configuration for route-targets in a VRF instance.

Question 37

What command is used to configure an **import RT** of 123:123 for the Customer A VRF instance?

Answer 37

route-target import 123:123 ## Footnote This command allows the VRF instance to import routes associated with the specified route-target.

Question 38

What is required for traffic to be successfully forwarded through the **MPLS domain**?

Answer 38

A **label stack** ## Footnote A label stack is essential for the forwarding process in MPLS.

Question 39

How many labels are required for traffic to be forwarded through the **MPLS domain**?

Answer 39

Two labels ## Footnote The two labels are necessary for successful traffic forwarding.

Question 40

What is the **first label** that is attached to the packet in the MPLS domain?

Answer 40

**VPN label** ## Footnote This label is crucial for identifying the virtual private network.

Question 41

What is the **second label** that is attached to the packet in the MPLS domain?

Answer 41

**LDP label** ## Footnote This label is used for label distribution protocol purposes.

Question 42

What happens when the **IP packet** arrives at the **ingress PE router**?

Answer 42

The PE router attaches both labels ## Footnote This process is crucial for managing the packet within the MPLS domain.

Question 43

What does the **egress router** use the **VPN label** for?

Answer 43

To determine customer specifics about the packet and what should be done with it ## Footnote This ensures that the packet is handled according to the customer's requirements.

Question 44

What is the purpose of the **LDP label**?

Answer 44

Used for label switching from PE to PE in the MPLS domain ## Footnote This facilitates efficient routing of packets within the MPLS network.

Question 45

How are **VPN labels** learned?

Answer 45

From PE routers over the **MP-IBGP peering** ## Footnote This method allows for the distribution of VPN labels among routers.

Question 46

In OSPF, how do you configure LDP autoconfig?

Answer 46

router ospf 1 mpls ldp autoconfig

Question 47

What is appended to a VPNv4 BGP route to indicate membership in an MPLS VPN?

Answer 47

RT

Question 48

What is an MPLS optimization technique where the second-to-last router removes the MPLS label before sending the packet to the final router?

Answer 48

Penultimate Hop Popping (PHP)

Question 49

What additional information is displayed with show ip vrf detail compared to just show ip vrf?

Answer 49

import and export RT information for a vrf instance

Question 50

What UDP port does LDP use?

Answer 50

UDP 646