Today we are going to configure an OSPF Virtual Link.
What is it?
Before we answer what it is, take a step back and look at one of the basic requirements of OSPF. That is, all areas must be connected to the backbone area (area 0). A virtual link gives us a way around that requirement.
If for some reason, you are stuck with a topology where you must connect a non-zero area via another non-zero area you can create a virtual link between the ABR in non-connected area to a router in area 0. That will hopefully make a bit more sense in a few minutes.
This is a pretty common scenario. You are running and IGP (in this case, OSPF) on your local network and you connect to another company or ISP via an eBGP link.
Your goal is to leak some of the external routes received via BGP into your IGP, and in turn leak some of your internal routes out via BGP. The constraint is you don’t want everything. Only the routes you decide.
Here is a step by step example of how to set up a very simple MPLS-VPN. Like last time I am doing this entirely in GNS3 using 2691s running 12.4(25d).
Here is a step by step configuration example to configure a specific path using MPLS-TE tunnels.
This was originally going to be a post about MPLS Fast Reroute. However two things happened, one the post became excessively long and two, I realised none of my simulated routers support MPLS FRR. Also, the Cisco Feature Navigator is lying to me again (FRR is supported on a 6500, FN claims this isn’t so.)