Within the cisco infrastructure, there is a special command to generate a default route in the entire RIP domain, not only with RIP protocol this feature exist, it does exist also with other routing protocol like ospf, but the behavior is different.
With RIP protocol, using the default information originate under the routing process will generate automatically a default route even if the route is not present in the routing table. Unlike ospf where the route has to exist in the routing before being propagated using the default information originate, RIP does not take into account that condition, it automatically generate the default if this exist or not in its routing table.
Below is the syntax of the command.
To generate a default route into Routing Information Protocol (RIP), use the default-information originate command in router configuration mode. To disable this feature, use the no form of this command.
default-information originate [route-map
no default-information originate
(Optional) Routing process will generate the default route if the route map is satisfied.
Let’s build a simple scenario to test this feature
First step: Checking our basic connectivity between routers.
R2 routing table:
R2 is running RIP version 2 with R1, also R2 has two links to the internet, one through R3 and another one through R4.
R1 routing table:
NB: we don’t have access to R3 and R4 routing table, those routers belong to the service provider.
Task1: On R2 using ip sla, enable a conditional static default route pointing to R3 and R4, this route should only be present in the routing of R2 if R3 interface or R4 interface is reachable.
Default static routes created:
Routing table of R2 having static routes:
At this point R1 does not have a default route.
Task2: configure R2 so that it generate a default route which should be propagated with the rip domain.
On R1, we now have the default route learned via rip:
At this stage, the default route is injected within the domain without being relied on a particular condition, which means if we disable static routing on R2, we will still have default route installed on the R1 routing table.
Let’s do that:
However, R1 still have the default route in its routing table and subsequently, traffic sending to R2 using the default route will be black hole on R2. This is something that we can prevent using the Conditional default Origination.
R1 routing table
Task 3: Using the conditional default origination, ensure that default route is propagated within the domain only if upstream routers interfaces (R3 and R4) are reachable.
We have already created our ip sla and tracker, let’s re-enable our default routes on R2 and create a route-map to satisfy our condition when enable default-information originate under the RIP configuration mode. The route-map we are going to create will be applied with a standard access list matching the default route.
Now the existence of the default route within the RIP domain rely on the presence of default static route on R2’s routing table.
If unfortunately those two static routes were to disappear in the routing table of R2, this one would cease to announce default route to R1 or to the entire domain.
Let’s examine by disabling one by one R3 and R4 interface.
R2 routing table:
R2 has two paths to the default route 0.0.0.0/0
Let’s disable R3 interface and observe the routing table of R2
The first line indicates that R3 interface went down and consequently R2 has now only one path through the internet. However R1 still have the default route through R2.
Now let’s disable the last interface connecting R2 to the internet and observe the result on R2 and R1 routing tables.
R1 has lost the default route entry in its routing table due to the fact that R2 has stopped announce this route because of the conditional default origination that has taken effect. When at least one of the upstream links will be back, R2 would start announce the default route.
That are all for the Conditional default Origination.
I hope these have been informative.
Blaise Annesti NGADJUI