Best tunnel path

• backup-preferred-color

• fallback-to-best-path

Cisco SD-WAN Manager uses best of worst (BOW) to find a best tunnel when no tunnel meets any of the SLA class requirements.

Assume that the required latency to meet the SLA class requirements is 100 ms and tunnel T1 has 110 ms. Tunnel T2 has 111 ms and tunnel T3 has 112 ms.

Based on the BOW logic, the best tunnel is T1. T2 and T3 are equally the best tunnels, with only a difference of a few ms.

You configure variance in Cisco SD-WAN Manager when configuring an SLA class. Variance accommodates small deviations during best tunnel selection.

For more information, see Configure SLA Class.

Example: Without variance configured

At time t1: T1 has 100 ms, T2 has 101 ms, and T3 has 102 ms

At time t2: T1 has 101 ms, T2 has 100 ms, and T3 has 102 ms

At time t3: T1 has 101 ms, T2 has 112 ms, and T3 has 100 ms

At time t1, the best tunnel changes from T1 to T2, and for time t2, the best tunnel changes from T2 to T3. Because variance is not configured, this leads to data path reprogramming and changes to the data traffic paths.

Assume instead that you configure variance to dampen a small deviation in ms.

For example, you configure variance as 5 ms, which means that the best tunnel SLA = 100 ms. The range is from 100 ms to 105 ms.

Example: With variance configured

BOW(t1) = {T1, T2, T3}

BOW(t2) = {T1, T2, T3}

BOW(t3) = {T1, T2, T3}

With variance configured, there is no data path reprogramming required or changes to data traffic paths.

Example for latency variance

Device# show sdwan policy from-vsmart
from-vsmart sla-class video
latency                       100
jitter                        150
 fallback-best-tunnel         latency

Tunnel T1: Latency: 110 msec, Loss: 0%, Jitter: 200 msec  
Tunnel T2: Latency: 115 msec, Loss: 0%, Jitter: 200 msec  
Tunnel T3: Latency: 120 msec, Loss: 0%, Jitter: 200 msec  

• When latency variance is not configured, tunnel T1 is selected as the best tunnel.

• When a latency variance of 10 ms is configured, tunnels T1, T2, and T3 all qualify as best tunnels.

• The rangeif from 110 ms to 120 ms.

• This range is calculated as best latency (110 ms) + latency variance (10 ms).

• Use the following formula to determine eligible tunnels: (best_latency, best_latency + latency_variance)

Example for jitter variance

Device# show sdwan policy from-vsmart
from-vsmart sla-class video
latency                       100
jitter                        150
 fallback-best-tunnel         jitter

Tunnel T1: Latency: 90 msec, Loss: 0%, Jitter: 160 msec  
Tunnel T2: Latency: 80 msec, Loss: 0%, Jitter: 200 msec  
Tunnel T3: Latency: 70 msec, Loss: 0%, Jitter: 152 msec 

• When jitter variance is not configured, tunnel T3 is selected as the best tunnel.

• When a jitter variance of 10 ms is configured, tunnels T1 and T3 qualify as the best tunnels.

• The range is from 152 ms to 162 ms.

• This range is calculated as best jitter (152 ms) + jitter variance (10 ms).

• Use the following formula to determine eligible tunnels: (best_jitter, best_jitter + jitter_variance)

Example for loss variance

Device# show sdwan policy from-vsmart
from-vsmart sla-class video
latency                       100
jitter                        1
 fallback-best-tunnel         loss

Tunnel T1: Latency: 110 msec, Loss: 2%, Jitter: 200 msec  
Tunnel T2: Latency: 115 msec, Loss: 3%, Jitter: 200 msec  
Tunnel T3: Latency: 120 msec, Loss: 4%, Jitter: 200 msec

1. When loss variance is not configured, tunnel T1 is selected as the best tunnel.

2. When a loss variance of 1% is configured, tunnels T1 and T2 qualify as the best tunnels.

3. The range is from 2% to 3%.

4. This range is calculated as best loss (2%) + loss variance (1%).

5. Use the following formula to determine eligible tunnels: (best_loss, best_loss + loss_variance)