Introduction to OER (Optimized Edge Routing)

What is OER? In short, OER adds intelligence to the network by looking at the current state of the network and injecting routing information to choose the most optimal path. It’s pretty cool if you think about it. All our routing protocols basically select a path based on static information and they don’t care about the current state of the network. For example,  OSPF will prefer a gigabit link over a FastEthernet link, even if the Gigabit link is 100% congested and the FastEthernet link is doing absolutely nothing.

OER can look at the current state of the interfaces and select the best exit path, it can do this by looking at the delay, response time (IP SLA), utilization of the link but also the MOS (Mean Opinion Score) so that it can select the best path for VoIP traffic. Optimized edge routing and Performance routing got me hooked because I believe this is what the future of networking will look like. To demonstrate why OER could be useful in your networks I want to show you a couple of scenarios:

Above you see a customer router that is connected to two ISPs. ISP A offers a 10 Mbit link and is used as the primary link. ISP B offers a 5 Mbit link and this one is used as the backup link. Our ISPS don’t want to run BGP with us so we can use a routing protocol or static routes for connectivity. The customer is using two static routes for connectivity:

• 0.0.0.0 /0 with the default administrative distance of 1 for ISP A.
• 0.0.0.0 /0 with an administrative distance of 5 for ISP B.

The default route for ISP B is called a “floating static route” because it will only show up in the routing table if the default route to ISP A is removed from the routing table. This configuration will work but the downside is that we are only using the link to ISP A. In fact we have 15 Mbit (10 Mbit + 5 Mbit from both ISPs) but we can only use 10Mbit at the moment. What if you wanted to use both links? Maybe you are thinking about changing the default route so the administrative distance is equal so that we can do 50/50 load balancing.

This might sound like a good idea but technically it’s not possible. On Cisco routers nowadays we use CEF (Cisco Express Forwarding) and despite what most people believe it doesn’t do load balancing but load sharing. The difference is that load balancing means that the “load” is shared equally on both links…load sharing means we use both links but it’s not balanced. When we have two equal entries, CEF will load share based on the source and destination IP address of a flow. For example, let’s say we have two default routes with the same administrative distance in the topology above and we have the following sessions:

• A computer with source IP address 1.1.1.1 connecting to a webserver with destination IP address 4.5.6.7, we’ll call this “flow 1” and It’s consuming about 500kb/sec.
• A server with source IP address 2.2.2.2 connecting to a remote backup server with destination IP address 8.9.10.11, we’ll call this “flow 2” and it’s consuming about 4Mbit / sec.

CEF will put flow 1 on the link to ISP A and flow 2 on the link to ISP B. We did “load sharing” but it’s not balanced at all. The link to ISP A now has about 5% utilization (500 kb/sec out of 10Mbit) while the link to ISP B has about 80% utilization (4 Mbit out of 5Mbit).

CEF really doesn’t look at load balancing…it would be difficult to configure true load balancing in the scenario above right? Using OER this is no problem at all!

Let me show you another scenario:

Above we have the same network but we are using VoIP. The link to ISP A is 50 Mbit and the link towards ISP B is 2 Mbit. Since the link to ISP A has a higher bandwidth we are using this as the primary link. Unfortunately the link has a very high delay at the moment (500 ms) so it would be better to use the backup link to ISP B at this moment since it only has a delay of 20 ms. If you wanted to you could configure something like this using policy based routing in combination with IP SLA but it’s not a very good solution. OER can check the delay of our interfaces and automatically reroute VoIP traffic over another interface when it meets certain criteria.

Let’s make it even more complex…

What about the topology above? We have two routers and there are three links to different ISPs. We have a 50, 20 and 10 Mbit link for a total WAN bandwidth of 80 Mbit. Is there any way to load balance traffic on these links? It’s impossible because R1 and R2 have no idea about each others interfaces. They don’t know each others utilization, delay or anything else. Now let’s talk a bit about OER and how it adds more intelligence to the network.

Above we see a topology and on the left side is a router called “MC” (Master Controller). On the right side, our two routers are called “BR” (Border Router). The way it works is that the border routers will report information about their interfaces and traffic flows to the master controller. The master controller will make decisions for the network and instruct the border routers to change their routing. In the network above we have 3 exit paths…ISP A,B, and C. It’s possible that we use the link through ISP A for all our exit traffic but as soon as something happens that we switch over to ISP B or C.

Perhaps the delay through ISP A is too high for VoIP traffic, if something like this happens the master controller can instruct the border router on top to switch over to ISP B but it’s also possible to change the local preference of BGP and to send traffic towards ISP C. OER has a number of methods to influence routing including static routes, route redistribution/tagging and changing the BGP local preference.

Whenever something happens to OER (perhaps the MC crashes) nothing bad will happen, we will switch back to “normal” routing. OER is not a replacement for routing protocols. You need to have all your routing in place before you can configure OER…it just adds more intelligence to the network.

OER has been renamed to Performance Routing (PfR) but is still 90% the same.

I hope this introduction has given you an impression of OER and the advantages that it can bring. In the next lesson, I will show you how to configure a basic OER setup. If you have any questions feel free to ask!