1-8 - Layer 3 MPLS VPN Data Plane Operation

Question 1

Describe what the data plane is.

Answer 1

• it manages the forwarding process for actual packets transiting through the device
• it forwards traffic based on either destination address or labels
• its a simple forwarding engine that is independent of the type of routing protocol or label exchange protocol being used

Question 2

What are the two components of the data plane?

Answer 2

• Forwarding Information Base (FIB)
• Label Forwarding Information Base (LFIB)

Question 3

Describe the structure of an MPLS header.

Answer 3

• It gets inserted between L2 and L3 header of the PDU
• Components:
  *20-bit label: actual label for switching; values 0-15 are reserved
  *3-bit EXP field: undefined by RFC; used by Cisco for CoS or IP Precedence
  *Bottom-of-stack bit: signifies which label is the bottom of the label stack (bit == 1 means its the bottom label)
  *8-bit TTL field: time-to-live; same purpose as TTL in IP header

Question 4

T/F: MPLS can be used on virtually any media and L2 encapsulation.

Answer 4

True

Question 5

What types of applications typically use stacks of labels, and how many labels do they typically use?

Answer 5

• MPLS VPNs (two labels)
• MPLS TE (two or more labels)
• MPLS VPNs combined with MPLS TE (3 or more labels)

Question 6

For MPLS VPNs, what is the top label in the stack referred to as, and what does it do?

Answer 6

It is the transport label, and it is used to forward that traffic to the egress PE.

Question 7

For MPLS VPNs, what is the second label in the stack referred to as, and what does it do?

Answer 7

It is the VPN label, and it is used to tell the egress PE how to forward the incoming packet once its received (i.e. which VRF does that packet belong to)

Question 8

Which of the following forwards IP packets based on decisions that routing protocols make?

A. RIB
B. FIB
C: LFIB
D: LSR

Answer 8

B

Question 9

Which of the following contains the incoming and outgoing label, outgoing interface, and next-hop router information?

A. LFIB
B. FIB
C. RIB
D. LSR

Answer 9

A

Question 10

Which of the following scenarios creates two labels in the label stack?

A. MPLS
B. MPLS VPNs
C. Cisco MPLS TE
D. MPLS VPNs combined with Cisco MPLS TE

Answer 10

B