Wednesday, August 31, 2016

CCNA Question of the Week 2

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In the following image, you'll see a network topology.  In this topology, the routers are running the RIP routing protocol.  As is traditional with these questions, I'm going to strip out all the irrelevant information.  We're not going to see any router configuration, IP addressing, or anything else that would distract from the question at hand.  The key thing we're going to focus on here is that there are 20 routers.  First, we'll start with a couple assumptions:
  • The RIP routing protocol is configured correctly on every router.  
  • Nothing in the routers configurations differ except for IP addresses and networks in the routing protocol.
  • The IP addressing scheme is correctly subnetted, and the routers are addressed correctly on every interface.
So this week's question is, can the RIP protocol function correctly in this topology?  And for a couple follow up questions:  Why or why not?  Does it make a difference if we're running RIPv1 or RIPv2?

The first thing that probably comes to mind is that the RIP protocol has a maximum hop count.  Most CCNA students go here first when something of this nature comes up.   Now let's consider the difference between hop count, and the number of routers in the topology.  The hop count refers to the number of hops between two routers.  It says nothing about the number of routers in the topology.  

So Let's look at the two routers that are furthest apart, R1 and R20.  In this particular topology, there is no path from R1 to R20 that is more than 7 hops.  And if there a path that exceeded the maximum hop count, it would be ignored by the routing protocol, not having any effect on a different route that didn't exceed 15 hops.  

So the answer to the question is yes, this is a valid RIP topology.  Now two routers exceed 15 hops apart, so there is no part of the topology that is unreachable by any other portion of the topology.  PC1 can reach PC2.

And for the final follow up question, it doesn't matter if we're running RIPv1 or RIPv2.  Neither version of RIP will balk at a hop count of 7.


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