While they are many ways to optimize routing by aggregating many technologies together as using BGP with MPLS, it is often easier to simply fall back to OER to influence traffic routing based on Netflow events/IP-LSA, packet loss, response time, load balancing policies, line jitters and thus dynamically adapt route metrics, inject or remove routes using IGP/BGP.
OER is therefore able to automatically detect degradation/congestion on specific links and influence routing over alternate paths, based upon its configuration.
Having said that, in every deployed topology, there must be one “Master Controller OER”, either itself a border router or connecting to the border routers, each interface (inbound/outbound) of the border routers should be registered and under OER’s control. the MC router would then upload routing policies to the Border Routers, themselves connecting to the Wan links.
Now before we go on, it is important to keep in mind that OER only monitors outbound interfaces and will only control outward traffic. It will not affect Inter-domain routing nor asymmetrical routing.
There are 5 phases to remember when dealing with OER
1. Profile Phase
OER is running in “learning mode” during which it monitors the traffic flowing through the outbound interfaces ‘under its control’, evaluating traffic classes with a performance problem such high delays etc…
2. Measure Phase
Generated Performance Metrics of the Profiled traffic classes are then reported from the Border Routers to the Master Controller. Those Metrics can either be generated while OER evalues the traffic flowing through the data path or actively simulate the traffic to evaluate performance. Those 2 methodology are respectively refered as Passive and Active Monitoring.
3. Apply Policy Phase
This phase is actually quite miss-leading, as OER is simply evaluating traffic performance against a set of threasolds manually configured for each traffic class. For example, how many % of packet loss is acceptable for a specific traffic class or on a specific link etc.. The MC therefore will evaluate which flow is OOP (Out Of Policy). They are therefore 2 type of defined Policy, Traffic class Policies (Application, Prefixes) and Link Policies (Inbound/Outbound link).
4. Control Phase
Once the flow has been identified to be OOP, the MC will dynamically adjust the data path by injecting routes or modifying the routes using IGP or BGP and redirect the traffic from one exit to the other.
5. Verify Phase
Once the flow has been redirected, it is again evaluated as in “Policy Phase” and if determined to be in OOP, the changes are reverted back and the “Measure Phase” is triggered for the flow to be re-evaluated.