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Understanding the algorithm for "least-fill" MPLS LSP optimization

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Article ID: KB17214 KB Last Updated: 21 Oct 2020Version: 1.0
Summary:

When using "least-fill" option, an LSP will be re-routed if the aggregate bandwidth on the new candidate path is reduced by at least 10% over the current path.

Symptoms:

In the Junos manual section "Optimizing Signaled LSPs", the criteria to select a path for optimization is as follow:

After reoptimization is run, the result is accepted only if it meets the following criteria:
   ..
   ..
   ..
7.  If you choose least-fill as a load-balancing algorithm and if the new path reduces congestion by at least 10 percent aggregated over all links it traversed, it is accepted. For random or most-fill algorithms, this rule does not apply.

How do we calculate the reduction in congestion with "least-fill" option?

 

Solution:
The following example will help explain how the algorithm works.

Using a 7-hop example, such as:


  +---+ L1 +-+-+ L3 +---+ L5  +---+ L7 +---+ L9 +---+ L11 +---+ L13 +---+
  |   |----|   |----|   |-----|   |----|   |----|   |-----|   |-----|   |
  | A |    | B |    | C |     | D |    | E |    | F |     | G |     | H |
  |   |----|   |----|   |-----|   |----|   |----|   |-----|   |-----|   |
  +---+ L2 +---+ L4 +-+-+ L6  +---+ L8 +---+L10 +---+ L12 +---+ L14 +---+

Where:
  • All the odd Links are one path and all the even links are the other path.
  • Also, every link (even and odd) between 2 routers is the same bandwidth.
The LSP from A to H would take either the 'odd' link path or the 'even' link path.
L1, L2 = 10GE
L3, L4 = 1GE
L5, L6 = 1GE
L7, L8 = 1GE
L9, L10 = 1GE
L11, L12 = 10GE
L13, L14 = 10GE

It is more likely that the 1GE links are the ones with the least amount of available bandwidth. For this example:
Apr 9 20:29:45.610873 CSPF reoptimize new avail bw 41% 56% 66% 71%
Apr 9 20:29:45.610886 CSPF reoptimize old avail bw 37% 52% 61% 70%
Apr 9 20:29:45.610896 CSPF reoptimize: equal metric reduce congestion >= 10
Apr 9 20:29:45.610911 mpls lsp er1.iah1.us-to-mpr1.lga7.us primary CSPF: Reroute due to re-optimization


In this example the 'old' path are the 'odd' links and the 'new' path are the 'even' links. In the above case, we would have:

New path vs Old path
------------------------
L4  - L3 = 41% - 37% = 4%
L6  - L5 = 56% - 52% = 4%
L8  - L7 = 66% - 61% = 5%
L10 - L9 = 71% - 70% = 1%
                  -------
                 14% > 10%

14% is greater than 10% so according to the least-fill algorithm, the LSP should move to the other path.

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