The likelihood of paths with links sharing SRLG values with a primary path being used by a bypass or detour LSP can be configured if a penalty weight is specified for the link. The higher the penalty weight, the less desirable it is to use the link with a specific SRLG.
Figure: SRLG penalty weight operation illustrates the operation of SRLG penalty weights.
The primary LSP path includes a link between A and D with SRLG (1) and (2). The bypass around this link through nodes B and C includes links (a) and (d), which are members of SRLG (1), and links (b) and (c), which are members of SRLG 2. If the link metrics are equal, then this gives four ECMP paths from A to D via B and C:
(a), (d), (e)
(a), (c), (e)
(b), (c), (e)
(b), (d), (e)
Two of these paths include undesirable (from a reliability perspective) link (c). SRLG penalty weights or costs can be used to provide a tiebreaker between these paths so that the path including (c) is less likely to be chosen. For example, if the penalty associated with SRLG (1) is 5, and the penalty associated with SRLG (2) is 10, and the penalty associated with SRLG (3) is 1, then the cumulative penalty of each of the paths above is calculated by summing the penalty weights for each SRLG that a path has in common with the primary path:
(a), (d), (e) = 10
(a), (c), (e) = 15
(b), (c), (e) = 20
(b), (d), (e) = 15
Therefore path (a), (d), (e) is chosen because it has the lowest cumulative penalty.
Penalties are applied by summing the values for SRLGs in common with the protected part of the primary path.
A user can define a penalty weight value associate with an SRLG group using the penalty-weight parameter of the srlg-group command under the configure>router-if-attribute context. If an SRLG penalty weight is configured, then CSPF includes the SRLG penalty weight in the computation of an FRR detour or bypass for protecting the primary LSP path at a PLR node. Links with a higher SRLG penalty should be more likely to be pruned than links with a lower SRLG penalty.
Note that the configured penalty weight is not advertised in the IGP.
An SRLG penalty weight is applicable whenever an SRLG group is applied to an interface, including in the static SRLG database. However, penalty weights are used in bypass and detour path computation only when the srlg-frr (loose) flag is enabled.