Bandwidth cost, WAN latency, and lack of bandwidth availability all contribute to the complexities of running an efficient and cost-effective network that meets the unique, application-heavy workloads of today’s enterprise organizations. But as the volume of content and applications traveling across the network grows exponentially, organizations must optimize their WAN investments.
Cisco Performance Routing (PfR) is the IWAN intelligent path control component that can help administrators to accomplish the following:
Augment the WAN with additional bandwidth to including lower-cost connectivity options such as the Internet
Realize the cost benefits of provider flexibility and the ability to choose different transport technologies (such as MPLS L3VPN, VPLS, or the Internet)
Offload the corporate WAN with highly secure direct Internet access
Improve application performance and availability based upon an application’s performance requirements
Protect critical applications from fluctuating WAN performance
Performance Routing (PfR)
Cisco Performance Routing (PfR) improves application delivery and WAN efficiency. PfR dynamically controls data packet forwarding decisions by looking at application type, performance, policies, and path status. PfR protects business applications from fluctuating WAN performance while intelligently load-balancing traffic over the best-performing path based on the application policy.
Simplified Routing over a Transport-Independent Design
One of the critical IWAN components and also a key design decision was to architect the next-generation WAN around a transport-independent design (TID). The choice of DMVPN was extensively explained in Chapter 2, “Transport Independence.” This overlay approach allows the use of a single routing protocol over the WAN and greatly simplifies the routing decision process and Performance Routing in multiple ways, two of the main ones being
Simplified reachability information
Single routing domain
The first benefit of this overlay approach is simplified reachability information.
The traditional routing protocols were designed to solve the endpoint reachability problem in a hop-by-hop destination-only forwarding environment of unknown topology. The routing protocols choose only the best path based on statically assigned cost. There are a few exceptions where the network path used can be somewhat engineered. Some routing protocols can select a path that is not the shortest one (BGP, MPLS traffic engineering [TE]).
Designing deterministic routing behavior is difficult with multiple transport providers but is much simpler thanks to DMVPN. The DMVPN network topology is flat, and it is consistent because it is an overlay network that masks the network complexity underneath. This approach simplifies the logical view of the network and minimizes fundamental topology changes. Logically, only reachability to the next hop across the WAN can change.
An overlay network’s routing information is very simple: a set of destination prefixes, and a set of potential transport next hops for each destination. As a result, PfR just needs a mapping service that stores and serves all resolved forwarding states for connectivity per overlay network. Each forwarding state contains destination prefix, next hop (overlay IP address), and corresponding transport address.
The second benefit of using overlay networks is the single routing domain design. In traditional hybrid designs, it is common to have two (or more) routing domains:
One routing domain for the primary path over MPLS—EBGP, static, or default routes
One routing domain on the secondary path over the Internet—EIGRP, IBGP, or floating static routes
The complexity increases when routes are exchanged between the multiple routing domains, which can lead to suboptimal routing or routing loops. Using DMVPN for all WAN transports allows the use of a single routing protocol for all paths regardless of the of transport choice. Whether the topology is dual hybrid (MPLS plus Internet) or dual Internet (two Internet paths), the routing configuration remains exactly the same, meaning that if there is a change in how your provider chooses to deliver connectivity, or you wish to add or change a provider underneath the DMVPN, the investment in your WAN routing architecture is secure.
EIGRP and IBGP are the best routing protocol options today with DMVPN.
After routing connectivity is established, PfR enters the picture and provides the advanced path control in IWAN. PfR is not a replacement for the routing protocol and never will be. As an adjunct, PfR uses the next-hop information from the routing protocol and overrides it based on real-time performance and link utilization ratio. This next-hop information per destination prefix is critical for PfR to work correctly and is a critical element in the routing design. Having a single routing domain and a very basic mapping service requirement has greatly simplified PfR interaction with the routing protocol.
“Classic” Path Control Used in Routing Protocols
Path control, commonly referred to as “traffic engineering,” is the process of choosing the network path on which traffic is sent. The simplest form is trivial: send all traffic down the primary path unless the path goes down; in that case, send everything through the backup path.
Figure 7-1 illustrates the concept where R31 (branch) sends traffic to R11 (headquarters). When R31’s link to the MPLS provider fails, traffic is sent through the Internet.
Figure 7-1 Traffic Flow over Primary and Backup Links
This approach has two main drawbacks:
Traffic is forwarded over a single path regardless of the application type, performance, or bandwidth issues.
The backup path is used only when the primary link goes down and not when there is performance degradation or brownouts over the primary path because the routing protocol peers are usually still up and running and do not detect such performance issues.
Path Control with Policy-Based Routing
The next level of path control lets the administrator specify categories of traffic to send on a specific path as long as that path remains up. One of the most common options is the use of policy-based routing (PBR), routing based on DSCP values:
DSCP values that are mapped to critical business applications and voice/video types of applications are assigned a next hop that is over the preferred path.
DSCP values that are mapped to best-effort applications or applications that do not suffer from performance degradation are assigned a next hop over the secondary path.
However, this approach is not intelligent and does not take into account the dynamic behavior of the network. Routing protocols have keepalive timers that can determine if the next hop is available, but they cannot determine when the path selected suffers from degraded performance, and the system cannot compensate.
Figure 7-2 illustrates the situation where R31 (branch) sends traffic to R11 (headquarters). When R31’s path across the MPLS provider experiences performance issues, traffic continues to be sent through the MPLS backbone. PBR alone is unaware of any performance problems. An additional mechanism is needed to detect events like these, such as the use of IP SLA probes.
Figure 7-2 PBR’s Inability to Detect Problematic Links
Intelligent Path Control—Performance Routing
Classic routing protocols or path control with PBR cannot detect performance issues and fall back affected traffic to an alternative path. Intelligent path control solves this problem by monitoring actual application performance on the path that the applications are traversing, and by directing traffic to the appropriate path based on these real-time performance measurements.
When the current path experiences performance degradation, Cisco intelligent path control moves the affected flows according to user-defined policies.
Figure 7-3 illustrates the situation where R31 sends traffic to R11. When R31’s path across the MPLS provider experiences performance issues, only affected traffic is sent to the Internet path. The choice of traffic to fall back is based on defined policies. For example, voice or business application flows are forwarded over the secondary path, whereas best-effort traffic remains on the MPLS path.
Figure 7-3 Traffic Flow over Multiple Links with Cisco Intelligent Path Control
Advanced path control should include the following:
Detection of issues such as delay, loss, jitter, and defined path preference before the associated application is impacted.
Passive performance measurement based on real user traffic when available and passively monitored on existing WAN edge routers. This helps support SLAs to protect critical traffic.
Efficient load distribution across the WAN links for medium-priority and best-effort traffic.
Effective reaction to any network outages before they can affect users or other aspects of the network. These include blackouts that cause a complete loss of connectivity as well as brownouts that are network slowdowns caused by path degradation along the route to the destination. Although blackouts can be detected easily, brownouts are much more challenging to track and are usually responsible for bad user experience.
Application-based policies that are designed to support the specific performance needs of applications (for example, point of sale, enterprise resource planning [ERP], and so on).
Low WAN overhead to ensure that control traffic is not contributing to overall traffic issues.
Easy management options, including a single point of administration and the ability to scale without a stacked deployment.
Cisco Performance Routing (PfR), part of Cisco IOS software, provides intelligent path control in IWAN and complements traditional routing technologies by using the intelligence of a Cisco IOS infrastructure to improve application performance and availability.
As explained before, PfR is not a replacement for the routing protocols but instead runs alongside of them to glean the next hop per destination prefix. PfR has APIs with NHRP, BGP, EIGRP, and the routing table to request information. It can monitor and then modify the path selected for each application based on advanced criteria, such as reachability, delay, loss, and jitter. PfR intelligently load-balances the remainder of the traffic among available paths based on the tunnel bandwidth utilization ratio.
Cisco PfR has evolved and improved over several releases with a focus on simplicity, ease of deployment, and scalability. Table 7-1 provides a list of features that have evolved with each version of PfR.
Table 7-1 Evolution of PfR Versions and Features
PfR/Optimized Edge Routing (OER)
Provisioning per site per policy
Thousands of lines of configuration
App path selection
Scale 500 sites
Tens of lines of configuration
Application Visibility Control (AVC) infrastructure
Scale 2000 sites
Hub configuration only
Multiple data centers
Multiple next hops per DMVPN network
Introduction to PfRv3
Performance Routing Version 3 (PfRv3) is the latest generation of the original PfR created more than ten years ago. PfRv3 focuses on ease of use and scalability to make it easy to transition to an intelligent network with PfR. It uses one-touch provisioning with multisite coordination to simplify its configuration and deployment from previous versions of PfR. PfRv3 is a DSCP- and application-based policy-driven framework that provides multisite path control optimization and is bandwidth aware for WAN- and cloud-based applications. PfRv3 is tightly integrated with existing AVC components such as Performance Monitor, QoS, and NBAR2.
PfR is composed of devices performing several roles, which are master controller (MC) and border router (BR). The MC serves as the control plane of PfR, and the BR is the forwarding plane which selects the path based on MC decisions.
Figure 7-4 illustrates the mechanics of PfRv3. Traffic policies are defined based on DSCP values or application names. Policies can state requirements and preferences for applications and path selection. A sample policy can state that voice traffic uses preferred path MPLS unless delay is above 200 ms. PfR learns the traffic, then starts measuring the bandwidth and performance characteristics. Then the MC makes a decision by comparing the real-time metrics with the policies and instructs the BRs to use the appropriate path.
Figure 7-4 Mechanics of PfRv3