OSPF/MPLS Assignment

1

Answer the following questions in a separate answers file. You must type your name at the top

of your answers file (2 points). Note that you do NOT need to implement anything on any real

routers (but you can if you want) – this is just a paper exercise. Please submit your answers in

PDF format with your name in the answer file.

160.2.0.0/16

R1 R2

R3

Fa0/0S0/1S0/0S0/1S0/1

LoopBack0

50.0.0.1

LoopBack0

50.0.0.3 LoopBack0

50.0.0.2

Subnet

S0/0 S0/0

Link 1

4 Mbps

Link 3

4 Mbps

Link 4

4 Mbps 160.2.0.10

LoopBack0

50.0.0.4

R4 Link 2

1 Mbps

S0/0S0/1

1. Consider the network above. R1, R2, R3 and R4 are MPLS Label Switched Routers (LSRs).

They use OSPF as IGP and LDP for label distribution. These LSRs use per-platform label

space, unsolicited downstream label distribution, liberal label retention mode. Each router

has a /32 LoopBack0 address that is used as its LSR ID. The four serial links are /30 subnets.

The interface IP addresses are shown in the address table. Assume OSPF has converged so

all routing tables are up-to-date. Bandwidth speeds, marked on figure, have been configured

on serial interfaces, which determine OSPF link costs. Loopback interface OSPF costs are 1.

a) (7 points) Tell me the contents of the Router R1 Routing Table. Your routing table

should have 5 columns, labeled Subnet ID, Mask, OSPF Cost, Next Hop, Outgoing

Interface and 9 rows, corresponding to the 9 IP subnets in the diagram (including

Loopback0s). Each Next Hop value will either be an IP address or blank.

2. Now the four routers assign Local Labels to each of the 9 subnets in their routing tables as

shown in this Local Label Table. Assume that PHP is not used.

Subnet R1 Local

Label

R2 Local Label

R3 Local Label

R4 Local Label

Subnet R1

Local Label

R2 Local Label

R3 Local Label

R4 Local Label

88.0.0.0/30 100 200 300 400 50.0.0.1/32 104 204 304 404

88.0.0.4/30 101 201 301 401 50.0.0.2/32 105 205 305 405

88.0.0.8/30 102 202 302 402 50.0.0.3/32 106 206 306 406

88.0.0.12/30 103 203 303 403 50.0.0.4/32 107 207 307 407

160.2.0.0/16 108 208 308 408

Router Interface IP Interface IP Interface IP Interface IP

R1 Lo0 50.0.0.1 S0/0 88.0.0.13 S0/1 88.0.0.1

R2 Lo0 50.0.0.2 S0/0 88.0.0.5 S0/1 88.0.0.2

R3 Lo0 50.0.0.3 S0/0 88.0.0.10 S0/1 88.0.0.6 Fa0/0 160.2.0.1

R4 Lo0 50.0.0.4 S0/0 88.0.0.9 S0/1 88.0.0.14

2

a) (7 points) Now the LDP protocol executes so the Label Information Base (LIB) tables

on all routers are up to date. As in the class example on slide #29, show the contents of

the Router R1 LIB. Your LIB table should have 4 columns labeled FEC, Local Label,

Remote Label, Remote LSR ID. Unlike the class example, your LSR IDs must be IP

addresses. Assume that PHP is not used.

b) (7 points) Based on values in its Routing Table and LIB, each router now computes its

Label Forwarding Information Base (LFIB). As in the class example on slide #30, show

the contents of the Router R1 LFIB. Your LFIB table should have 6 columns labeled

Incoming Label, Action, Outgoing Label, FEC, Outgoing Interface, Next Hop.

Unlike in the class example, your Next Hop must be an IP address or blank. Assume that

PHP is not used.

c) (5 points) Now, using LFIB from #2(b), consider a packet sent by R1 to destination

160.2.0.10. This packet will go over Link 1 and Link 3.

i) What MPLS label value, if any, will be in the header of this packet as it passes over

Link 1?

ii) What MPLS label value, if any, will be in the header of this packet as it passes over

Link 3?

3. Now go back and repeat the steps of problem #2 assuming that PHP is used.

a) (6 points) When PHP is active, the assignment of Local Labels will be different than the

Local Label table shown in problem #2, because IMP-NULL values will be in the Local

Label table. Show a feasible Local Label table (5 columns, 9 rows) that assigns local

labels to each of the 9 subnets on R1, R2, R3 and R4 when PHP is used.

b) (6 points) Using your Local Label table from answer #3(a), now go back and repeat the

building of the LIB as in problem #2(a) assuming that PHP is used. Show the contents

of the Router R1 LIB using PHP.

c) (7 points) Now, using the routing table you got in #1(a) and the LIB from #3(b) show

the contents of the Router R1 LFIB, assuming that PHP is used.

d) (5 points) Now, using LFIB from #3(c), consider a packet sent by R1 to destination

160.2.0.10.

i) What MPLS label value, if any, will be in the header of this packet as it passes over

Link 1?

ii) What MPLS label value, if any, will be in the header of this packet as it passes over

Link 3?