JMF-Ch2: MPLS Fundamentals

Question 1

What concerns does MPLS address?

Answer 1

• Scalability of the IP Control plane
• Necessity of integrating with (and eventually replacing) contemporary layer-2 technologies like ATM and Frame Relay
• The need for fine-grained traffic engineering capabilities much more accurate than any approach based on conventional IP Routing

Question 2

How does MPLS address the traffic engineering & routing scalability concerns?

Answer 2

• Prepend each packet with a small, fixed-length header with a numerical value
• Forward the packet using only the label value
• The possibility of setting arbitrary LSPs to solve complex traffic engineering problems

Question 3

What are some other MPLS applications?

Answer 3

• Pseudowires for transport of arbitrary layer 2 frames
• Virtual Private Networks
  
  • VPLS, EVP, L2VPN, L3VPN, Multicast technologies
• P2MP LSP for multicast distribution

Question 4

Describe the MPLS Header

Answer 4

• a 32-bit header that is prepended to each packet as it enters the MPLS domain, and is typically removed at the end of the label-switched-path.
• ‘label’ is commonly used to refer to the entire header

Question 5

What are the components of the MPLS Header?

Answer 5

• 20-bit Label: used to associate a packet with an LSP
• 3-bit Traffic Class: Formerly ‘Experimental bits’, used for CoS
• 1-bit: Bottom-of-Stack bit: Some applications require multiple stacked labels. This is set to 1 if this is the last label before the payload
• 8-bit Time to Live (TTL): Decremented at every hop
  
  • By default, the IP TTL is copied into the MPLS TTL

Question 6

What is the significance of labels?

Answer 6

By default, labels only have local significance. At every hop the label will typically change

Labels are generally global to a router, and not tied to the ingress interface. Filters are applied based upon the label & prefix filters, not on interface based filters

Question 7

What, if any, MPLS labels are reserved?

Answer 7

Labels 0-15 are reserved

4-6,8-12, & 15 are unassigned at this time.

Question 8

What are explicit & implicit null labels?

Answer 8

• Explicit Null (Label 0): Always assigned an action of ‘decapsulate’ (pop)
• Implicit Null (Label 3): Never in actual MPLS Frames, only within MPLS signaling. Used by the egress router to request the previous router remove the MPLS header. i.e. ‘penultimate-hop popping’ which is the Junos OS Default

Question 9

What is an LSR?

Answer 9

• Label-switched router
• Any router which participates in MPLS forwarding, including both ingress & egress nodes.

Question 10

What are the MPLS Label Operations?

Answer 10

• Push: add an MPLS label to a packet (Ingress LSR)
• Swap: replaces the incoming label value with another (Transit LSr)
• Pop: Removes an MPLS label from a packet (Egress/penultimate LSR)

Question 11

What is an LSP?

Answer 11

• Label-Switched Path
• A unidirectional path through the network defined in terms of label operations
  
  • Typically assigned by a label distribution protocol, but can be configured statically

Question 12

What is an iLSR?

Answer 12

• Ingress Label-Switching Router
• Performs the initial label operation of ‘push’, and encapsulates non-MPLS traffic by adding one or more labels prior to forwarding it down a label-switched path

Question 13

What is a tLSR?

Answer 13

• Transit Label-Switching Router
• Typically performs swap operations
• Forwarding decisions exclusively based on the label value

Question 14

What is the LIB?

Answer 14

• Label Information Base
• Contains the actual label switching table
• Maps incoming labels to forwarding actions

Question 15

What is penultimate-hop popping?

Answer 15

• MPLS header is removed by the 2nd to lats router in an LSP
• Facilitates forwarding on the last hop and is enabled by default for most MPLS applications
• Avoids having to both remove the MPLS header, and do an additional route lookup

Question 16

What is an eLSR?

Answer 16

• Egress Label-Switching Router, or tail-end
• Takes a forwarding decision based on the packet payload

Question 17

What are two scenarios where penultimate-hop popping must be disabled?

Answer 17

• Class of Service applications
• Applications that need to preserve the ‘identity’ of the LSP even at the last hop. ex. Multicast in a VPN, Circuit-cross-connect (CCC)

Question 18

Where is the LIB stored?

Answer 18

in the mpls.0 table

Question 19

What needs to be configured to allow an interface to process MPLS frames?

Answer 19

enable family mpls at the logical unit level

set interfaces ge-1/0/0 unit 0 family mpls

Question 20

What configuration steps must be performed to configure the MPLS protocol?

Answer 20

Specify which interfaces will be running MPLS

Question 21

What command would verify the MPLS interface configuration?

Answer 21

‘show mpls interface’

Question 22

What are possible reasons why an MPLS interface might be missing from ‘show mpls interface’?

Answer 22

• family mpls not configured at the logical unit level
• interface has not been listed at the [edit protocols mpls] level

Question 23

What routing tables are directly affected by MPLS?

Answer 23

• mpls.0
  
  • mpls label-switching table
  • maps incoming labels to forwarding actions
• inet.3
  
  • table of LSP egress points
  • contains signaled LSPs at ingress node

Question 24

What command allows for verification of the LSP state?

Answer 24

‘show mpls static-lsp ingress’

Question 25

What command could be used to check the state of an LSP on a transit router?

Answer 25

‘show mpls static-lsp transit’

Question 26

What is the function of the mpls.0 table?

Answer 26

the mpls.0 table contains the label-switching table, which associates each incoming label its forwarding operation

Question 27

Identify the configuration in the screenshot provided

Answer 27

MPLS LSP - Ingress Router