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: .. .. .. 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.
L1, L2 = 10GE L3, L4 = 1GE L5, L6 = 1GE L7, L8 = 1GE L9, L10 = 1GE L11, L12 = 10GE L13, L14 = 10GE
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
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%