Generic Routing Encapsulation(GRE) tunnels are a technology that was developed by Cisco but, is now an open standard through RFC 2784. GRE tunnels can provide use with a lot of versatility when needing to move different protocols between networks. Though we have now moved into an era that is predominantly TCP/IP, when GRE was developed it was at a time of multiple network technologies, IPX, Appletalk, Token Ring, etc, flourished. GRE provided the ability to encapsulate these technologies and let them traverse an Internet Protocol network.

In my previous post I worked through how to use EEM when we had the constraint, of having to keep the Backup circuit interface shutdown when not in use. Without this constraint we can implement redundancy with just Administrative Distance.

Administratvie Distance

This is a metric that is used by vendors to rate the reliability of a route. Vendors do use different metrics. I’ll be using Cisco devices in this example. The lower the AD(Administrative Distance) value the more reliable a route is deemed.

The Embedded Event Manager is a brilliant tool that can be found in the Cisco IOS. Before the development of IOS-EX, devices were limited to TCL and EEM to implement scripting locally on the device. Though we now get python available to us, TCL and EEM are still very useful.

I’m just going to roll through a simple example of using EEM. We have a Primary and a Backup Circuit in our environment, but prefer to keep the interface of the Backup circuit shutdown when the Primary is active. So if we lose our Primary Connection, the interface connected to the Backup circuit needs to be brought up. This can be scripted with EEM, we’ll also put recovery in there. So when the Primary circuit comes back we shut down our interface to the Backup circuit.