SLA Profiles and SD-WAN Policies Overview
Contrail Service Orchestration (CSO) enables you to create service-level agreement (SLA) profiles and map them to software-defined WAN (SD-WAN) policies for traffic management.
SLA profiles are created for applications or groups of applications for all tenants. An SLA profile consists of a set of configurable constraints that can be defined in the unified portal for both the Administration and Customer Portals. Table 1 lists the categories of configurable constraints that are defined in an SLA profile.
Table 1: SLA Profile Categories
Path preference and priority
Paths are the WAN links to be used for the SLA profile. You can select an MPLS or Internet link as the preferred path. For SLA profiles you must select a path preference or configure at least one SLA parameter. MPLS is more latency-sensitive than Internet.
You can define priority or precedence for the SLA profile. A value of one (1) indicates highest priority. SLA profiles with higher priorities are given precedence over SLA profiles with lower priorities. Priority is used when SLA requirements are not met on a WAN link and the site switches WAN links to meet the SLA requirements.
You can also define one or more than one of the following SLA parameters:
SLA parameters have precedence over path preference. Even if one SLA parameter is defined, then it is given a higher priority and will override the path preference. SD-WAN policies mapped to an SLA profile with defined SLA parameters are called dynamic policies. Dynamic policies applied to sites enable the site to override the path preference and switch WAN links when the preferred WAN link is not meeting SLA requirements as defined in the SLA parameters.
Class of service
Class of service (CoS) provides different levels of service assurances to various forms of traffic. CoS enables you to divide traffic into classes and offer an assured service level for each class. The classes of service listed in increasing order of priority and sensitivity to latency are best effort, voice, interactive video, streaming audio or video, control, and business essential. The default CoS is voice.
Rate limiters are defined for traffic shaping and efficient bandwidth utilization. You can define the following rate limiters:
You must define at least one of the SLA parameters or path preference. You cannot leave both path preference and SLA parameters fields blank at the same time.
SD-WAN policy intents help in optimum utilization of the WAN links and efficient load distribution of traffic. SD-WAN policy intents are applied to source endpoints (such as sites and departments) and destination endpoints (applications or application groups) and can be defined for site-to-site traffic (by using SLA profiles) or for breakout traffic (by using breakout profiles).
Applications are classified into the following categories:
Cacheable applications, which refer to applications or application groups that are stored in the application cache when they are recognized by the device. After they are stored in the application cache, subsequent sessions are routed directly through the correct WAN link.
Non-cacheable applications, which refer to applications or application groups that are not stored in the application cache and all sessions are first routed through the default path, and then routed to the correct WAN link based on the SD-WAN policy.
Policy intents consist of the following parameters:
Source—A source endpoint that you can choose from a list of sites, site groups, and departments or a combination of all of these. The SD-WAN policy intent is applied to the selected source endpoint.
Destination—A destination endpoint that you can choose from a list of applications and predefined or custom application groups. You can select a maximum of 32 applications or application groups as destination endpoints. The SD-WAN policy intent is applied to the selected destination endpoint.
SLA profile or breakout profile—Depending on whether you want to apply the policy intent to site-to-site traffic or breakout traffic, you can associate an SLA profile or a breakout profile with the policy intent.
Intent name—A unique name for the SD-WAN policy intent.
SD-WAN supports advanced policy-based routing (APBR). APBR enables you to dynamically define the routing behavior of the SD-WAN network based on applications. Dynamic application-based routing makes it possible to define policies and to switch WAN links on the fly based on the application's defined SLA parameters. The APBR mechanism classifies sessions based on applications and application signatures and uses policy intents to identify the best possible route for the application. When the best possible route does not meet the application's defined SLA requirements, the SD-WAN network finds the next best possible route to meet SLA requirements.
For example, consider an application in a site. If you want the application group to use custom throughput, latency, or jitter, you can create an SLA profile with these custom values. You can then create an intent and configure the intent with the application and apply the custom SLA profile. When the intent is deployed, CSO determines the best suited WAN link to route traffic based in the application. If the WAN link fails to meet SLA requirements in runtime, the SD-WAN network switches WAN links to the next best suited path.
On the basis of the configured SLA profile constraints, you can categorize SD-WAN policies into two types:
Static policy—If only the path preference is defined and none of the SLA parameters are defined in the SLA profile, then the policy is called a static policy. In static policies, if the defined WAN link under path preference is unable to meet the SLA requirements, link switching cannot occur and SLA performance deteriorates.
Dynamic policy—If one or more SLA parameters in the SLA profile are defined, then the policy is called a dynamic policy.
In dynamic policies, because SLA parameters override the path preference, the SD-WAN network chooses the best possible WAN link for traffic management. When an intent is deployed on a site, if the WAN link chosen by the SD-WAN network does not meet the SLA requirements and the network performance deteriorates, then the site switches WAN links to meet the SLA requirements. The link switching is recorded as an SD-WAN event and displayed in the SD-WAN Events page in the customer portal and the Tenant_name SLA Performance pages in the administration and customer portals. Link switching occurs only when the SD-WAN policy is dynamic because SLA parameters override the path preference and the site is able to switch WAN links.