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.

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:

..

..

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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?

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%

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14% > 10%

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