This section draws attention to a specific feature that is not described in detail in this book—Optimized Edge Routing (OER), another feature in Cisco IOS. OER is not covered in Part II because this feature is really at the edge, between performance monitoring and routing optimization. OER was renamed to Performance Routing (PFR).
Enterprise networks use multiple ISP or WAN connections for reliability and/or load distribution. Existing reliability mechanisms depend on link state or route removal on the border router to select a better exit link for a prefix or set of prefixes. Multiple connections protect an enterprise network from catastrophic failures but do not protect the network from "brownouts" or soft failures caused by network congestion. Existing mechanisms can respond to catastrophic failures at the first indication of a problem. However, brownouts can go undetected and often require the network operator to take action to resolve the problem.
When a packet is transmitted between external networks (nationally or globally), the packet spends the vast majority of its life on the network's WAN segments. Optimizing WAN route selection in the enterprise network can give the end user the greatest performance improvement—even better than LAN speed improvements in the local network.
The OER feature helps enable intelligent path selection at the WAN edge, based on performance-sensitive routing metrics such as response time, packet loss, path availability, traffic load distribution, and cost minimization. If a prefix's performance falls below default or user-defined policy parameters, routing is altered locally in the enterprise network to optimize performance and to route around failure conditions that occur outside the enterprise network. OER is best deployed with multihomed connections to the Internet or WAN intranet connections with two or more possible routes to the destination.
In the enterprise network shown in Figure 13-1, the OER feature, which is located on the border routers, inspects the metrics of the Top-N flows that are exchanged with the Internet. NetFlow provides the flow information, and the IP SLA does proactive measuring of the different paths through the network. Figure 13-10, more OER-oriented, displays the different OER components.
The OER architecture has two main functions:
The border controller— An enterprise edge router with one or more exit links to an ISP or other participating network. At the border router, all policy decisions and changes to routing in the network are enforced. The border router participates in prefix monitoring and route optimization by reporting prefix and exit link measurements to the master controller and then enforcing policy changes received from the master controller. The border router enforces policy changes by injecting a preferred route to alter routing in the network. The border router is deployed on the edge of the network, so it must be in the forwarding path. A border router process can be enabled on the same router as a master controller process.
The master controller— A single router that coordinates all OER functions within an OER managed network. A Cisco router can be configured to run a standalone master controller process. It also can be configured to perform other functions, such as routing or running a border router process. The master controller maintains communication and authenticates the sessions with the border routers. The master controller monitors outbound traffic flows using active or passive monitoring and then applies default or user-defined policies to alter routing to optimize prefixes and exit links. OER administration and control are centralized on the master controller, which makes all policy decisions and controls the border routers. The master controller does not need to be in the traffic forwarding path, but it must be reachable by the border routers.
The typical broadband scenario is when one or two border routers are available, and the OER master controller functionality coexists with the border functionality on the same router. By default, the Border Gateway Protocol (BGP) chooses the best path based on the fewest autonomous system path hops. However, the border routers shown in Figure 13-10 test the two paths to the remote office. Based on the comparison of the metrics between the different paths, the BGP parameters are automatically adjusted for the traffic to take the route with the best metrics.
Note that Figure 3-10 shows the three different possible OER configurations:
Headquarters— Three different routers, composed of a master controller and two border controllers
Remote office— Two different routers, one combining the functions of the master and border controllers
Telecommuter— A single router with two WAN interfaces, combining the functions of the master and border controllers
The website http://www.cisco.com/go/oer is a good starting point for more details.