EIGRP advertising only routes which are installed in self routing table

[Deepak Kumar]

Hi,

I am reading a CCIE book and found a topic on Distance Vector Routing Protocol (as RIP and Eigrp) migration and getting stuck to understand the real logic behind the seen

"A learned route will be advertised further only if the router has placed that very learned route into the routing table as well. In other words, a learned route is advertised through the same routing protocol only if the router is using that route itself."

Why not advertising on behacb of database?

Please explain.

Regards,

Deepak Kumar

[pieterh]

i think of these arguments.

if a route learned from another router is not reachable from this router, it's no good to advertise it!

if learned  routes are filtered, so not all routes are placed in the routing table, it's also no good to advertise those learned routes.

they are "learned" but also "forgotten".

[fbabashahi]

Hi , if i get your question right , that is because of the differences between routing protocols in link state routing protocol (like OSPF) the whole database is advertised but distance vector is not the same as link state routing protocol

[Richard Burts]

I understand that logic to be like this: First think about what it means when an EIGRP router advertises a route to its neighbors. It is indicating that this is the best way that I know to get to that destination.  if a router learns a route from an EIGRP neighbor but for some reason does not place that route into the routing table (and if the router has not placed that route into the routing table learned from a source with better AD) then it has no better way to get to that destination than using the default route. If the router has nothing better than the default route then why advertise that route to its neighbors.

Another way to think about it is that if the router does not trust the advertised route enough to use it then it would be misleading to advertise that route to other neighbors. 

HTH

Rick

[Deepak Kumar]

Hi @Richard Burts

Thanks for your explanation. But may I understand that this a default behaviour of Distance vector routing protocol? Why I saying that RIP theory is also saying same that RIP will send Routing update on every 30 seconds, not a database update.

Regards,

Deepak Kumar

[Richard Burts]

@Deepak Kumar I am not clear what you are asking here. In your original post you tell us that the book you are reading indicates that this behavior is a characteristic of distance vector routing protocols. I do not remember it being described that way in many of the explanations I have read but I have not problem believing that it may be a characteristic of distance vector protocols. For any routing protocol that does advertise routes I believe that this is a very appropriate behavior. 

This behavior (advertise only if it is found in the active IP routing table) would not apply to protocols like OSPF which advertises LSAs (from which it calculates routes) and does not advertise routes.

Or are you trying to say something about the databases maintained by EIGRP and RIP?

HTH

Rick

[Deepak Kumar]

Hi,

Thanks for your reply. I got my answer after reading some blogs on the same. I got that Rip database and Eigrp topology table learning sorce is neighbour router's routing table. 

Is it correct?

Regards,

Deepak Kumar

[Richard Burts]

@Deepak Kumar yes it is correct that distance vector routing protocols like rip and eigrp start the decision about what to route with the content of their database, and then apply some criteria to determine which database entries should be advertised and which entries (if any) should not be advertised. 

HTH

Rick