Note

Of course, the break-even scalability point isn't the only reason to consider a particular type of protection. You also need to consider things such as where your failures occur, how often they happen, how much data you have to protect, and so forth.

Given the formula and assumptions here, the break-even point for link protection is at four nodes, and node protection is at ten nodes.

It should be obvious that path protection is always just at the break-even point, because Fn = Ln. This assumes that multiple LSPs are not protecting a single LSP.

Table 9-16 shows different numbers of nodes, the total number of LSPs used in the network (assuming that all links in the TE portion of the network are to be protected), and the number of protection LSPs used. In this table, the average degree of connectivity (D) is 3.

Table 9-16. Link, Node, and Path Protection Scalability

Number of

Number of

Number of

Number of

Primary

LSPs Used in

LSPs Used in

LSPs Used in

Number of

LSPs(Rn *

Link

Node

Path

Nodes (Rn)

(Rn - 1))

Protection

Protection

Protection

1

0

3

9

0

10

90

30

90

90

20

380

60

180

380

30

870

90

270

870

40

1560

120

360

1560

50

2450

150

450

2450

60

3540

180

540

3540

70

4830

210

630

4830

80

6320

240

720

6320

90

8010

270

810

8010

100

9900

300

900

9900

110

11,990

330

990

11,990

120

14,280

360

1080

14,280

130

16,770

390

1170

16,770

140

19,460

420

1260

19,460

150

22,350

450

1350

22,350

The number of LSPs used in node protection is the formula Rn * D * (D - 1) plus the number of LSPs in link protection (Rn * D). So the entire formula is

which reduces to Rn * D2.

Why do you need both link and node protection? Because if you have LSPs that terminate on a directly connected neighbor, you can't node-protect that neighbor; you need to link-protect it. You might not always need to link-protect every node (not all nodes are TE tunnel tails), but the number of additional LSPs is so small that it's not worth calculating and trying to factor out here.

Figure 9-11 shows a graph of link, node, and path scalability. Figure 9-12 shows a graph of only the link and node scalability numbers, because it's easier to see the difference between these two if path protection isn't in the way. It's important to note, however, that the graphs in both figures use the exact same data. The reason it's hard to see link and node protection numbers on the path protection graph is because of how poorly path protection scales to large numbers.

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