Rick Gallahers MPLS Training Guide: Building Multi Protocol Label Switching Networks

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If you are the “hands-on” type and you want to see MPLS packets on a protocol analyzer, you need the two items of software (Ethereal and the MPLS-basic-cap sample) mentioned in the previous hands-on exercise.

  1. From your desktop, go to Start | Programs and click Ethereal.

  2. Once Ethereal opens, open the file called MPLS1.cap.

  3. Wait for the file to open. It will take a few minutes.

The file should look like Figure 1.17. Now let’s review the file content in the following steps.

Figure 1.17: Open MPLS_basic File

1. 

Look at Frame 9, as shown in Figure 1.17. Note the values found at offsets 14 to 21. Record them in hex here:

_____ _____ _____ _____ _____ _____ _____ _____ 14 15 16 17 18 19 20 21

2. 

Using the following chart, translate the hex number into binary for Label 1 found at offsets 14–17.

128 64 32 16 8 4 2 1. 128 64 32 16 8 4 2 1. 128 64 32 16 8 4 2 1. 128 64 32 16 8 4 2 1.

3. 

What are the values of each of the following for Label 1?

  1. The label __________

  2. The experimental bits __________

  3. The stack bit __________

  4. The TTL value __________

4. 

Using the following chart, translate the hex number into binary for Label 2 found at offsets 18–21.

128 64 32 16 8 4 2 1. 128 64 32 16 8 4 2 1. 128 64 32 16 8 4 2 1. 128 64 32 16 8 4 2 1.

5. 

What are the values of each of the following for Label 2?

  1. The label __________

  2. The experimental bits __________

  3. The stack bit __________

  4. The TTL value __________

6. 

Is the stack bit set for Label 1 (offset 14–17)? __________

7. 

Is the stack bit set for Label 2 (offset 18–21)? __________

8. 

Explain why the stack bit may be set differently. __________

Answers

1. 

00   01   20   ff   00   01   01   ff

14 15 16 17 18 19 20 21

2. 

3. 

  1. 18  

  2. 0  

  3. 0  

  4. 255

4. 

Label

E

S

TTL

5. 

  1. 16  

  2. 0  

  3. 1  

  4. 255

6. 

OFF

7. 

ON

8. 

The stack bit is turned on to indicate that this is the last header in the stack (or the header closest to the IP header).

Checkpoint 

Match the lettered item with its appropriate numbered description.

1.

____ is the path

A.

LER

2.

____ pushes, pops labels

B.

FEC

3.

____ swaps labels

C.

LSP

4.

____ traffic class

D.

LSR

Answers: 1. C; 2. A; 3. D; 4. B.

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