MPLS (ENARSI)

Question 1

Define MPLS

Answer 1

Multi Protocol Switching Protocol - a routing technique where packets are forwarded on to their destination using labels instead of the IP packet header information. Sometimes referred to as Layer “2.5”.

Question 2

Define LDP

Answer 2

Label Distribution Protocol - This is the protocol that routers in an MPLS network use to exchange information about their labels and routes.

Question 3

Define LSR

Answer 3

Label Switching Router - a router that participates in an MPLS network using Label Distribution Protocol

Question 4

Define LSP

Answer 4

Label Switched Path - a path that network traffic will take across a series of MPLS enabled devices as defined by Labels (LDP)

Question 5

Define LFIB

Answer 5

Label Forwarding Information Base - this is table that contains all of the paths and associated labels for a given MPLS enabled device (LSR)

Question 6

Describe the steps to enable MPLS on the GigabitEthernet 0/1 interface from the command line of Router1 and set the MPLS label range to 100 through 199.

Answer 6

Step 1 - Router1> enable
Step 2 - Router1# configure terminal
Step 3 - Router1(config)# mpls label range 100 199
Step 4 - Router1(config)# mpls ip
Step 5 - Router1(config)# interface gi0/1
Step 6 - Router1(config-if)# mpls ip
Step 7 - Router(config-if)# do write mem

Question 7

What is the full command to verify an MPLS binding for an address of 10.10.10.1 with a 32 bit route?

Answer 7

Router1# show mpls ldp bindings 10.10.10.1 32

Question 8

What is the command to view all the known mpls bindings on a given LSR?

Answer 8

Router1# show mpls ldp bindings

Question 9

What is the command to show the “winning” MPLS labels that will be used for packet forwarding?

Answer 9

Router1# show mpls forwarding-table

Question 10

When using the traceroute command in an MPLS network, what extra piece of information might you see?

Answer 10

The traceroute command will also display the MPLS label for each hop throughout a Label Switched Path.

Question 11

What command can you use to view neighbor relationships on an MPLS router?

Answer 11

Router1# show mpls ldp neighbor

Question 12

Within the context of MPLS Service providers, define CE.

Answer 12

CE is Customer Edge which refers to the customer’s router that will peer with the service provider’s router.

Question 13

Within the context of MPLS Service providers, define PE.

Answer 13

PE is the Provider Edge which refers to the service provider’s router that will peer with a customer’s router.

Question 14

What is the valid range for MPLS label values?

Answer 14

16 through 1048575

Question 15

What does LDP use for the Router ID in an MPLS switched network?

Answer 15

The highest IP on a loopback interface is used for the Router ID.

Question 16

In an MPLS network, describe what happens when LDP does a Label “Swap”.

Answer 16

When the Label Switching Router receives a packet for a destination network that already contains a Label but the destination network is not directly connected to the LSR or on the next hop LDP neighbor. The LSR removes the existing Label and applies a new label for the next hop LDP neighbor.

Question 17

In an MPLS network, describe what happens when LDP does a Label “Pop”.

Answer 17

When the Label Switching Router receives a packet for a destination that is directly connected to the router or the next hop LDP neighbor has the destination. There is no need to add a label because it does not need to be forwarded, so the Label is removed or “Popped”.

Question 18

In an MPLS network, describe what happens when LDP does a Label “Push” or “Impose”.

Answer 18

In any scenario when LDP assigns a Label to a destination, this is called a “Push” or “Impose”. This could be the first Label Switching Router in a path where no Label has been applied yet, or in the middle of the path when the local label needs to be replaced with a destination label for its LDP neighbor.

Question 19

When referring to LDP, what is meant by an “implicit-null”?

Answer 19

When using the ‘show mpls ldp bindings’ command, if a destination network shows a label of “impl-null” that means the network is directly connected to the LSR and does not require a Label.

Question 20

In an MPLS network, define a RD.

Answer 20

Route Distinguisher - a mechanism used in an MPLS network to differentiate between virtual private network (VPN) routes from different customers or different VRFs.

Question 21

Describe the characteristics of an MPLS Layer 3 VPN

Answer 21

A collection of routes that are dedicated for one customer on a service provider network using a VRF. The VRF routes are advertised across the Service Provider’s network from one PE to another PE using RDs. In this scenario, the Service Provider network is almost always running iBGP as the routing protocol.

Question 22

When creating a route distinguisher, describe a couple of common methods used to assign an RD used by network admins.

Answer 22

1.) Use the Autonomous System (AS) number followed by the site ID, for example 61234:99.

2.) Use the IP address followed by the site ID, for example 1012341:99

Question 23

What is meant by a VPNv4 address in a MPLS network.

Answer 23

This is a 96-bit address composed of the Route Distinguisher as the first 64 bits and the IPv4 address as the last 32 bits.

Question 24

Why are RDs and Route Targets notated as “##:##” ?

Answer 24

Because this follows the same format as BGP extended communities which is essentially what the VRFs are using to advertise VRF routes across an MPLS Service Provider network. BGP extended communities are one of many extended properties of the BGP routing protocol.

Question 25

When creating an MPLS VPN over a network running iBGP, describe the steps to setup a VRF named customer1 with a Route Distinguisher of 123:45 then export a route target of 123:11 and import 123:22.

Answer 25

Router-PE1# config t
Router-PE1(config)# vrf definition customer1
Router-PE1(config-vrf)# rd 123:45
Router-PE1(config-vrf)# address-family ipv4
Router-PE1(config-vrf)# route-target export 123:11
Router-PE1(config-vrf)# route-target import 123:22

Question 26

Explain what is happening on the PE router when a VRF is doing an “export”.

Answer 26

The VRF is sending out it’s routes to iBGP. In other words, it is exporting the VRF routes to BGP running in the provider network.

Question 27

What two commands might you use on an MPLS service provider router to show the VPNv4 routes?

Answer 27

Router-PE1# show ip bgp vpnv4 all

Router-PE1# show bgp vpnv4 unicast all

Question 28

What command would you use to see the value of a route distinguisher for a given network?

Answer 28

Router-PE1# show ip vrf

Question 29

What command would you use to see the Labels for a given destination network/ip address?

Answer 29

Router-PE# show mpls forwarding-table