What is the different from inject a default route from ASBR ( default-information originate) into ospf domain compared with manually configure ( ip route 0.0.0.0 0.0.0.0 next hop ) to all ospf router ? Does the purpose is the same ? Just default-information originate is generate the default route and propagate to ospf domain automatically and ip route 0.0.0.0. 0.0.0.0 is manually.
Though the goal is the same the former choice, injecting a default route via OSPF, is an efficient way of doing it.
I shall try to explain this with a small example.
In this scenario, R3, by virtue of it's connection to the Internet, is advertising the default route via OSPF or another IGP to the rest of the domain and everyone routes through R3 to get to the Internet. Let's say, the connection to the Internet from R3 is down then R3 would stop advertising the default route. A packet destined to the Internet arriving on R1 will be dropped right there as R1 wouldn't have a default route that it could use to forward the traffic.
On the other hand, if you were using static routes on all 3 routers then when the Internet link from R3 is down both R1 & R2 would keep sending packets to R3 and R3 would eventually drop it. As you can see, this setup causes packets to traverse links and hops unnecessarily only to get dropped at a remote destination.
Thanks Sundar, I know what you mean, your explain is based on default-information originate (no always). But how about if use "always" which always advertise default route without proper default route configured in ASBR
If the ASBR doesn't have a default route, you can add the keyword always to the default-information originate command (default-information originate always).
This command will advertise a default route into the OSPF domain, regardless of whether it has a route to 0.0.0.0. Another benefit of adding always keyword is that it can add stability to the internetwork. For example, if the ASBR is learning a default route from another routing domain such as RIP and this route is flapping, then without the always keyword, each time the route flaps, the ASBR will send a new Type 5 LSA into the OSPF domain causing some instability inside the OSPF domain. With the always keyword, the ASBR will advertise the default inside the OSPF domain always, and thus the flapping of the default route from the RIP domain will not cause any instability inside the OSPF domain.
Benefit can be concluded as R1 will be dropped right there as R1 wouldn't have a default route that it could use to forward the traffic.
But if according to below example, then this benefit can not be apply, right ?
If "default-information originate always" is configured in ASBR. Even though ASBR has no default route to outside, R1 still will try to access internet which packet from R1 will travel to R2 and R3 and loop on it.
Your are partly correct. The packet would travel to R3 though R3 wouldn't know how to route it and drop the packets but the packets wouldn't loop. As you can see, the packet would end up unncessarily using the bandwidth on those links and R2 & R3's CPU cycle when it could have been easily avoided. You have to be prudent about using the 'always' keyword and one of the situations that you would use it is what was mentioned in the link that I had posted earlier.
This is actually a pretty cool feature, i didn't even know it existed until I was looking for a solution to advertise a subnet (prefix in BGP talk), only if a certain condition existed. This is exactly what conditional advertisements does
j ai une question j ai achete un routeur cisco 887VA-k9 , je le configuré avec la configuration ci- dessous
si je le lier avec mon pc portable sur l un de ses ports directement ça marche toute est bien ( la connexion internet + m...
Attached policy provides CLI access to the Cisco 4G router over text messaging. Two files are in the attached .tar file:
2. PDF with instructions on how to load and use the .tcl file.