18. VRF, MPLS & MPLS L3 VPN

Question 1

What is VRF?

Answer 1

Virtual Routing & Forwarding is a technology for creating seperate virutal routers on a single physical routers

Question 2

What is the difference between VRF and VRF-Lite?

Answer 2

VRF-lite provides VRF without MPLS

Question 3

Are the VRF instances names CASE sensitive?

Answer 3

Yes

Question 4

Why must subinterfaces be created with VRFs?

Answer 4

Only one physical interface can belong to a single VRF

Question 5

What encapsulation must be set on the subinterface?

Answer 5

DOT1Q with a matching VLAN

Question 6

What is MPLS?

Answer 6

Multiprotocol Label Switching, a packet forwarding method based on labels instead of L3 destination

Question 7

How does MPLS work upon receiving an unlabeled packet?

Answer 7

The FIB is used to make the forwarding decision. If it indicates the outgoing interface non-MPLS, it will be forwarded unlabeled.

If the interface is MPLS, it will label the packet and forward.

Question 8

How does MPLS work upon receiving a labeled packet?

Answer 8

The LFIB is used to make the forwarding decision. If it indicates the outgoing interface is non-MPLS, it will remove the packet and forward unlabeled.

If the interface is MPLS, it will forward the packet labeled

Question 9

What is the LSR in MPLS?

Answer 9

The label switching router

Question 10

What are the 2 types of LSRs in MPLS?

Answer 10

• Edge LSR

- Intermidiate LSR

Question 11

What is the LSP in MPLS?

Answer 11

Label Swichted Path, the cumulative unidirectional labeled path that a packet takes through the MPLS domain

Question 12

Where is the MPLS label placed in a packet?

Answer 12

Between the L2 header and the L3 packet 4-bytes are placed.

Question 13

Where does the S-bit stand for in a MPLS label?

Answer 13

This indicates if the label is the last label when multiple labels are used in a single packet

Question 14

What is LDP in MPLS?

Answer 14

Label Distribution Protocol

Question 15

What is the multicast address used by MPLS?

Answer 15

224.0.0.2

Question 16

What is the UDP port used by MPLS?

Answer 16

646

Question 17

What is the TCP port used by MPLS?

Answer 17

646

Question 18

Which router will become the active router in MPLS?

Answer 18

The router with the higher LDP ID

Question 19

What is penultimate hop hopping?

Answer 19

The edge LSR advertises a ‘pop’ to neighbor routers to inidcate that he is the border of LSP. Therefore neighbor routers will already remove the label so that the edge LSR can check its FIB, instead of the LFIB

Question 20

How does a Layer 3 MPLS VPN work?

Answer 20

The customer is connected with a edge LSR of the SP with help of VRF. The edge runs MP-BGP with the other edge LSR. Between them an underlying IGP is used.

Question 21

How can MP-BGP handle identical customers network prefixes in L3 MPLS VPN?

Answer 21

It uses a ‘route distinguisher’ (RD) that expands the prefix with a unique value per VRF instance basis.

Question 22

How big is the RD in L3 MPLS VPN?

Answer 22

64-bit that is added to the 32-bit prefix. Together it is called the VPNv4 address

Question 23

What is the MPLS L3 VPN label stack?

Answer 23

A stack of labels required to forward traffic.

Question 24

What are the type of labels for the MPLS L3 VPN label stack?

Answer 24

VPN label -

LDP label - to route between Edge LSR

Question 25

What is the network name for the MPLS domain?

Answer 25

P-network (edge router is called PE)

Question 26

What is the network name for the customer network in MPLS?

Answer 26

C-network

Question 27

Which command is used to associcate an interface with VRF?

Answer 27

ip vrf forwarding vrf-name

Question 28

You have created a VRF instance called RED and associated the required interfaces with it. Which command is used to verify the contect of the VRF routing table?

Answer 28

show ip route VRF RED

Question 29

What type of router is responsible for adding MPLS labels to a packet?

Answer 29

The ignress LSR