Table 915 Both minfill and maxfill Fail

LSPs Placed in This Order

0

26

25

24

23

22

21

min-fill

{66,75}

{40,75}

{15,75}

{15,50}

{15,27}

{15,6}

{F}

max-fill

{66,75}

{66,49}

{41,49}

{41,24}

{18,24}

{18,2}

{F}

As you can see, min-fill and max-fill can't guarantee a solution to this packing problem. To work around this problem, you can use a few techniques:

• Use tunnel priority to give larger trunks better priority. The idea here is to divide your tunnel bandwidths into four or five bands, grouped by size. Maybe 0 to 50-Mbps tunnels go in one band, 51 to 150-Mbps in another, 151 to 500-Mbps in another, and 501-Mbps+ in another.

Give the larger tunnels a better (that is, lower) setup and holding priority with the command tunnel mpls traffic-eng priority. This means that bandwidth-hungry LSPs will push smaller LSPs out of the way, and these smaller LSPs will then coalesce around the larger ones. See Chapter 3, "Information Distribution," for information on the mechanics of tunnel priority.

• The packing problem gets worse as your LSPs get larger. The larger your LSPs are relative to the size of your links, the more likely you are to waste bandwidth. If you have a 100 Mb link and four LSPs of 30 Mb, you can place only three of them across that link, and you have 10 Mb of unused bandwidth. If you have two 60 Mb LSPs instead, you can place only one of them, leaving 40 Mb wasted. Consider making it a rule not to have any LSPs that are more than a fixed percentage of the minimum link size they might cross. If this rule means that you never have any LSPs of more than 300 Mbps in your network, for example, and you need 500 Mbps of LSP bandwidth from one router to another, build two LSPs. It's up to you whether you build them both as 250 Mbps, or one as 300 Mbps and one as 200 Mbps. As long as you fit within CEF's load-balancing rules (see Chapter 5), you're OK.

• Use an offline tool to find the paths your LSPs should take. This is the most efficient solution in terms of network utilization. It's also the most complicated. Not everybody buys into this approach, because it involves an external box that builds your network for you. But even those who don't use external path placement tools agree that external placement is more efficient than having the headend place each LSP.

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