Question

How do low delay and low loss work in an SLA profile?

Answer:

The low delay and low loss behavior in an SLA profile is different in Releases 16.1R1 and 16.1R2.

Release 16.1R1

When you include multiple best-path criteria, such as low loss, low delay, and low delay variations, in an SLA profile, the criteria are applied in a fixed order to select the list of eligible paths. First, the path with the least delay is selected. If multiple paths have the same delay, the path with the least loss is selected. If multiple paths have the same loss, the path with the least delay variation is selected.

When an SLA profile contains both best path and SLA threshold criteria, path selection is a two-step process. First, paths are sorted into priority buckets according to whether they meet the SLA threshold criteria. Second, the best-path criteria are applied to the paths in the highest priority bucket to determine the path with the best performance. Then the Versa software rotates among all the paths that are within 10 percent of the best path's performance, because these paths are similar to the best path.

Example:

Consider an SLA profile with a latency threshold of 80 milliseconds and three circuits (paths) (L1, L2, and L3) that are all configured as priority 1. If only L1 and L2 meet the latency threshold, these two circuits are considered as priority 1, and L3 has an SLA-violated priority. Now, the best-path criteria are applied to L1 and L2 to select the path with the least latency and loss, depending on how you have configured low latency and low loss.

Release 16.1R2

When you include one or more of the low delay, low loss, and low delay variations in an SLA profile, the Versa software computes a performance score based on the options you include. For instance, if you select low delay, the score computation considers only the  low delay. If you select low delay and low loss , the score computation considers both the delay and the loss. Instead of using only the one path with the best score, the Versa software rotates among all the paths that are within 10 percent of the best score, because these paths are similar to the best path.

Example:

Consider an SLA profile with a latency threshold of 80 milliseconds and four circuits (paths) (L1, L2, L3, and L4) that are all configured at priority 1. If only L1, L2, and L3 meet the latency threshold, these three circuits are considered as priority 1, and L4 has an SLA-violated priority. Now, the Versa software computes the performance score for L1, L2, and L3. If the scores are L1 = 10000, L2 = 20000, L3 = 19050, L2 and L3 are used (L2 because it has the best score, and L3 because it is within 10 percent of the best score). L1 is not used at all.

To display the computed performance score, use the following vty command:

vsm-vcsn0> show sd-wan forwarding-profile detail Customer2/fp-ll 102 12

  Name                    : Customer2/fp-ll

  FP prof id/gen        : 15/93063

  SLA profile Name      : sla-ll

  SLA profile id        : 2

  SLA latency           : 0

  SLA loss percent      : 10000

  SLA fwd loss percent  : 10000

  SLA rev loss percent  : 10000

  SLA delay variation   : 0

  SLA Circuit TX usage  : 0

  SLA Circuit RX usage  : 0

  SLA Low Latency       : True

  SLA Low Loss          : True

  SLA Low Forward Loss  : False

  SLA Low Reverse Loss  : False

  SLA Low DelayVariation: False

  SLA MOS score         : False [0]

  Connection sel method : Weighted round robin

  SLA violate action    : Forward

  Evaluate continuously : False

  Gradual migration     : False

  Symmetric forwarding  : True

  Verify reverse route  : True

  Autosteering : False

  Replication(Factor)   : False(2)

  Reorder               : True

  FEC sender            : Disable

  FEC receiver recovery : Enable

  Preserve order      : Enable

  Max packet length   : 1400

  number of windows   : 3

  FEC Parity cache sz : 16

  TURN redirection      : False

  Load-balance          : Per-flow

  Recompute time        : 10 seconds

  Path Reevaluate Interval : 60 seconds

  Encryption capability : Both

  Active nexthops

  Branch 102: Branch2, Inline Loss: Yes

  Forwarding Class: fc_be  Current Priority Index: 10

  Priority[10][Default] BL 0x200

  VL [b1-w2::h1-w2] [0x22] Flaps: 0, In current state for 00:00:08

  D: 0, FDV: 0 RDV: 0 FLP: 0.00 RLP: 0.00 PLP: 0.00 (0.00) score: 183750

  Priority[14][Unused] BL 0x1

  VL [b1-w1::h1-w1] [0x11] Flaps: 0, In current state for 00:00:08

  D: 80, FDV: 0 RDV: 0 FLP: 0.00 RLP: 0.00 PLP: 0.00 (0.00) score: 2296