I believe most of you are familiar how OSPF is injecting a default route in a normal area. If not, you can find here all the documentation that you need. Please be familiar with this concept before reading this article.
Now, let’s assume that we have the following topology (click on image to have a more detailed view) :
As you can see we have a BGP peering between PE and CE router, with CE router having and OSPF connection with the Core. CE router is injecting a default route to Core:
router ospf 1
default-information originate always
This configuration is OK, but we can run into the following issue. Imagine that for some reason the BGP peering between PE and CE is broken (e.g. line being down), the CE router will have no clue about this and will still propagate the default route to the Core. In this situation, the Core will still forward all the packets without specific route to CE where it will have no further route to reach the destination, as the CE does not receive any route from the BGP peer. As you can imagine is better to avoid this situation, especially if for some reasons you are not monitoring the connection between PE and CE and you cannot react to change manually the route in case of a failure. We are lucky because some smart engineers have developed a solution to avoid this problem, called Conditional inject of a default route in OSPF.
With this solution, OSPF is monitoring the reachability of the point-to-point IP connection between PE and CE. When OSPF process on CE router notice that the IP connection is not available anymore, it automatically retract the propagation of the default route to the Core. The solution is simple an assume use of an ACL or prefix-list then match this on a route-map and finally use this route-map under “router ospf” process. For step-by-step configuration check below.
First we will create and ACL matching the IP subnet between PE and CE. In this example I’m using a p2p subnet 10.10.10.0 /30:
access-list 1 permit 10.10.10.0 0.0.0.3
Then I will match this into a route-map as follow:
route-map WAN-LINK permit 10
match ip address 1
Finall, we will use this route-map to implement the OSPF conditional injection of default route to Core router (192.168.0.0 /30 is the p2p IP subnet between CE and Core):
router ospf 1
log-adjacency-changes
network 192.168.10.2 0.0.0.0 area 0
default-information originate always route-map WAN-LINK
Now, the OSPF process on CE will inject a default route to the Core as long as the IP subnet between CE and PE is reachable.
IMPORTANT NOTE: This solution might not work if your connection from CE to PE is Ethernet and not Serial like in the example. I will explain why on the next post, when I’ll achieve the same behavior but using EEM together with an Ethernet based connection instead of a Serial one.
Please check below to see a small presentation how this is working on a test environment:
