Tshoot OSPF Default-Information Originate always ( track sla failure solution)

This exercise was first post by Costi Serban (http://www.costiser.ro/2014/05/10/quiz-24/). I just found my own solution to the problem that he proposed

My solution includes tracking a sla failure instead of the traditional "ip sla reachability" (up)

 

Startup file (VIRL)

 Final configuration (VIRL)

THE PROBLEM

CORE-A must always have internet Access even if ISP-1 goes down. This is not happening with the actual configuration. Please do the tshoot to find the answer and propose a solution

THE SOLUTION

BR-B is getting a default route from ISP-1 (EBGP), but from BR-C too (OSPF). So which one it will choose?. it chooses the route with the lower administrative distance in this case the one from ISP-1. the same applies for BR-C

BGP administrative distance = 20, OSPF administrative distance = 110

BR-B#show ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "bgp 65001", distance 20, metric 0, candidate default path
  Tag 100, type external
  Last update from 1.1.1.1 00:38:22 ago
  Routing Descriptor Blocks:
  * 1.1.1.1, from 1.1.1.1, 00:38:22 ago
      Route metric is 0, traffic share count is 1
      AS Hops 1
      Route tag 100
      MPLS label: none


BR-B is advertising a default route into ospf using "default-information originate always". the same happens in BR-C

BR-B#show run | section ospf
router ospf 1
 network 192.168.0.0 0.0.255.255 area 0
 default-information originate always



CORE-A gets the advertisement from BR-C and install a default route in its routing table

CORE-A#show ip route 0.0.0.0
Routing entry for 0.0.0.0/0, supernet
  Known via "ospf 1", distance 110, metric 1, candidate default path
  Tag 1, type extern 2, forward metric 2
  Last update from 192.168.15.1 on GigabitEthernet0/1, 02:11:50 ago
  Routing Descriptor Blocks:
  * 192.168.15.1, from 192.168.0.5, 02:11:50 ago, via GigabitEthernet0/1
      Route metric is 1, traffic share count is 1
      Route tag 1


The "always" means BR-B will continue telling everyone "hey guys use me as the default route" even if that's is true or not. To check this will shutdown interface in ISP-1 which faces BR-B and let's check BR-B's routing table again

BR-B#show ip route 0.0.0.0

There is no default route in the routing table, so now BR-B will receive the default route from BR-C (OSPF) right...no??. Well .., no, when you use "default-information originate always" you are telling everyone " hey guys i accept default routes from no one cause i got my own "
 so CORE-A will never reach internet when BR-B lose conectivity with ISP-1


SOLUTIONS

1) The most easy solution is to remove the "always" from "default-information originate "BR-B

BR-B#show run | section ospf
 network 192.168.0.0 0.0.255.255 area 0
 default-information originate

This will let BR-B receive the default Gateway from BR-C and CORE-A will no lose internet connectivity because now it will go out through ISP-2


2) The solution that i propose is not the most neat but it will work too

If the conection between BR-B and ISP-1 fails, i will install a default route in BR-B so all the internet goes thorugh 192.168.12.2

I'll use SLA/TRACK to achieve this. Generally we add a static route when the track response is OK, but this time we'll add the static route when the track response is FAIL

Let's create the sla which will make ping to ISP-1 interface

BR-B#show run | section sla
ip sla 1
 icmp-echo 1.1.1.1
 frequency 5
ip sla schedule 1 life forever start-time now


And the track which will check the sla

BR-B#show run | section track 1
track 1 ip sla 1 reachability


Here comes the magic. Now we'll créate another track object which denies track 1

BR-B#show run | section track 2
track 2 list boolean and
 object 1 not

Now we'll add a route static to BR-B routing table only if connectivity between BR-B and ISP-1 fails

ip route 0.0.0.0 0.0.0.0 192.168.12.2 track 2

That's it, make some tests with traceroute from CORE-A to 34.34.34.3 when interface in ISP-1 is down or up, and check the routing table everytime

I will upload the startup and solution files...They are in VIRL format but you can copy the configuration sections and paste them in GNS3

Please give me your feedback