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RIP Number of update rounds until convergence

Posted on 2011-09-06
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Last Modified: 2012-06-21
This question is pretty straightforward.
Given X number of routers all using RIP, how many rounds of updates are needed until all routers have a converged routing table ?


Is there a formula for this ?


Also I am interested in how to calculate the time required until RIP has converged.
I know that this is a distance vector routing protocol and most of the info on the net tells of slow convergence time but no info on how to calculate this.

Thanks.
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Question by:_3mp3ror_
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6 Comments
 
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Expert Comment

by:Don Johnston
ID: 36489200
>Is there a formula for this ?

I can't see how you could get a one size fits all formula since it would depend on the topology. Also, the definition of "rounds of updates", could also affect the answer since a RIP update can only hold 25 entries.
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by:_3mp3ror_
ID: 36489732
As I understand it the time a network takes to converge is directly proportional to the size of the network.
Yes topology is important but I am only interested in a basic flat topology since RIP does not scale well and as you said only 25 entries are allowed for RIP

Here's an example:
R1---R2---R3---R4---R5---R6
You have 6 routers.
I want to know:
How much time does it take for rip to converge ?
How many rounds of updates are needed ?
And I want to know how to calculate this for 3 routers for instance if i need to.
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Expert Comment

by:Don Johnston
ID: 36489786
>How much time does it take for rip to converge ?

Depends on the implementation of RIP. Could be 2:30 if triggered updates aren't used. Could be 5 seconds if they are.  Or anywhere in between.

>How many rounds of updates are needed ?

What's your definition of "round"?

>And I want to know how to calculate this for 3 routers for instance if i need to.

Whenever a router has a link with another router, there will be an exchange of routing updates. So when the link between R1 and R2 is active, there will be a pair of updates (one in each direction). So that's two updates.

When the R2 - R3 link comes up, there will be an exchange there PLUS an update from R2 to R1. So now you're up to 5 updates overall.

When the R3 - R4 link comes up, there will be an exchange there PLUS an update from R3 to R2 and  R2 to R1. So now you're up to 9 updates overall. And so on.
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Author Comment

by:_3mp3ror_
ID: 36490140

There was a question in one of the exams I took and said something along the line of
"If all routers are running RIP, how many rounds of updates will occur before all routers know all networks?"
This is a packet tracer topology of the question.

r-convergence
Now I looked for the answer and it's 3.
But I never understood WHY it's 3.

Now you say it's 9 :)

I think that the number of updates needed is n-1 where n is the number of routers.
So if I have 4 routers in this flat topology there would be 4-1 rounds of updates. If you have 8 it's 7 rounds of update untill all routers know about all networks.

Now this is a simple example but this is a more complex example
convergence
Here it would take 5 updates to have a fully converged network.
The routing information is propagated faster when there are more interfaces.


Also you said it would take 2:30 (I don't think triggered updates are used by default on cisco routers) but there are 6 routers.
Would it not take 3 minutes ? 6 routers x 30 sec.
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Don Johnston earned 250 total points
ID: 36490565
>But I never understood WHY it's 3.  Now you say it's 9 :)

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.
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Author Closing Comment

by:_3mp3ror_
ID: 36491813
Thanks for the info.
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