1. Which routing protocol is used in MPLS Layer 3 VPNs to form peerings between PE routers?
- MP-BGP
- OSPFv3
- ISIS
- EIGRP
Explanation: PE routers advertise routes learned from customer routers to other PE routers over MP-BGP peerings.
2. What is used by provider routers to forward packets through the MPLS domain?
- labels
- destination IP address
- DLCI
- destination MẠC address
Explanation: Provider routers use labels attached to IP packets to forward traffic through the MPLS domain.
3. Refer to the exhibit. After creating three VRF instances on router R1 the network administrator verifies they are created with the show ip vrf command and notices that the interface column is still empty. What action does the administrator need to take to correct this issue?
- Enable the interfaces.
- Apply IP addresses to the interfaces.
- Configure an IGP.
- Assign the interfaces to the VRFs.
Explanation: Once VRFs are created on a router, an interface must be assigned to the VRFs by using the ip vrf forwarding command.
4. What must be assigned to each VRF instance to separate and isolate traffic?
- interfaces
- VLANs
- routing protocols
- subnets
Explanation: Separating and isolating traffic using VRF requires that interfaces be assigned to each VRF instance. If a single physical interface needs to support multiple VRF instances, the physical interface needs to be broken into subinterfaces with each subinterface supporting a unique VRF instance.
5. Which protocol is used in an MPLS network by PE and P routers to advertise labels?
- LDP
- CDP
- LLDP
- ARP
Explanation: When packets arrive at the MPLS ingress edge, the PE routers use Label Distribution Protocol (LDP) to add a label to the packets and forward them to the intermediate LSR, P routers. The P routers forward the packets through the MPLS network to the egress PE routers.
6. What term is used to describe the path a packet takes through routers in an MPLS domain?
- the label-switched path
- the autonomous system path
- the link-state path
- the packet switch path
Explanation: The cumulative labeled path or sequence of routers that a labeled packet takes through the MPLS domain is known as the label-switched path (LSP).
7. Which router plane on an LSR is responsible for making forwarding decisions on labeled packets?
- data plane
- control plane
- service plane
- management plane
Explanation: When labeled packets arrive at an LSR, the router uses the LFIB in the data plane to make forwarding decisions.
8. What information is located in the first 20 bits of the MPLS label?
- the label number
- the time to live
- the quality of service marking
- the label path
Explanation: The MPLS label has four fields. A 20-bit label field containing the label number, a 3-bit EXP field containing QoS markings, a 1-bit S field that indicates if this is the last label in the stack, and an 8-bit time-to-live field.
9. Which router adds LDP labels to packets in an MPLS Layer 3 VPN network?
- ingress PE
- egress PE
- CE
- P
Explanation: When the IP packet arrives at the ingress PE router, the router attaches both the VPN and the LDP label.
10. Where is the MPLS label added inside a packet?
- between the Layer 2 and Layer 3 headers
- between the Layer 3 and Layer 4 headers
- inside the Layer 3 header
- inside the Layer 2 header
Explanation: MPLS requires a label to be added to the packet. The label is added as a shim header between the Layer 2 frame header and the Layer 3 packet header.
11. Which MPLS-enabled router database is responsible for making forwarding decisions when unlabeled packets arrive on an interface?
- IP forwarding table (FIB)
- label forwarding table (LFIB)
- IP routing table (RIB)
- label information base (LIB)
Explanation: The data plane of an LSR consists of an IP forwarding table (FIB) and a label forwarding table (LFIB). The FIB makes forwarding decisions for unlabeled packets. The LFIB makes forwarding decision for labeled packets.
12. Refer to the exhibit. What label will router R1 use to forward packets to network 10.0.0.0/24?
- 11
- 19
- 65
- 87
Explanation: Router R1 has learned from R2 that network 10.0.0.0/24 is reachable via R2 with a label of 87. R1 will attach a label of 87 on packets with a destination of 10.0.0.0/24 and forward them to R2.
“Do I Know This Already?” Quiz Answers:
1. What does VRF allow you to do?
- Divide a single physical router into multiple virtual routers.
- Run Spanning Tree Protocol on a router.
- Use BGP on a router that does not support BGP.
- Use a server as a virtual router.
2. Which command is used to associate an interface with VRF?
- ip vrf vrf-name
- vrf vrf-name
- ip vrf forwarding vrf-name
- vrf forwarding vrf-name
3. You have created a VRF instance called RED and associated the required interfaces with it. Which command is used to verify the contents of the VRF routing table?
- show ip cef
- show ip vrf
- show ip route vrf
- show ip route vrf RED
4. How are packets forwarded in an MPLS domain?
- Using the destination IP address of the packet
- Using the source IP address of the packet
- Using a number that has been specified in a label
- Using the MAC address of the frame
5. What type of router is responsible for adding MPLS labels to a packet?
- Ingress edge LSR
- Egress edge LSR
- Intermediate LSR
- P router
6. Which protocol do routers use to exchange labels?
- LLDP
- STP
- LDP
- CDP
7. To improve MPLS performance, how can labels be removed on the second-to-last LSR in the LSP instead of waiting until the last LSR to remove the label?
- Use LLDP.
- Use LDP.
- Use PHP.
- Use HTTP.
8. What types of labels are used for MPLS Layer 3 VPNs? (Choose two.)
- LDP label
- VPN label
- 802.1q label
- MPLS label
9. Which dynamic routing protocol is used to form peerings between PE routers in MPLS Layer 3 VPNs?
- OSPF
- EIGRP
- IS-IS
- MP-BGP
10. How are customer routes isolated on PE routers in an MPLS Layer 3 VPN?
- By using VRF
- By using VDCs
- By using MP-BGP
- By using LDP