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We're back to the definition of "round". If it's an exchange of updates between two peers, then for the first pair of routers, it would be 1. Then add 1 for every additional pair. So if we follow that definition, for the four routers in your first diagram, the answer would be 3.

But it would be technically incorrect.

>I think that the number of updates needed is n-1 where n is the number of routers.

That would be my interpretation of the (incorrect) methodology. :-)

>Would it not take 3 minutes ? 6 routers x 30 sec.

Triggered updates have been in used on Cisco's implementation of RIP for a long time. At least 15 years. The problem is that even if it's not, it comes down to chance. From the time R2 learns routes from R1, how long will it be before R2's 30 second update times expires? It could be 1 second or it could be 30 seconds. So for four routers, the convergence time (once again using the methodology above) could be as short as a couple seconds or as long as 90 seconds. Or somewhere in between.

Bottom line is this: You seem to have a pretty good handle on how Distance Vector Routing Protocols work. Now move on. Don't get bogged down in trying to figure out some screwy interpretation of an exam prep question. Because, for the most part, the real exam questions won't go there.