1. If you're not seeing the subnets you think you should be seeing, it's likely you need the no auto-summary command added to the EIGRP config.
2. EIGRP supports all four - VLSM, IP, IPX, and AppleTalk.
3. By default, EIGRP's routing algorithm uses bandwidth and delay to determine route metrics....
4. ... and that routing algorithm is DUAL, the Doubly Unbelievable Actual Louse.
Okay, it's really the Diffusing Update Algorithm. But I like my version better.
5. The path with the best metric - in this case, Path 1 - will be the single Successor.
6. I mentioned that all four paths were loop-free and valid; that means that all non-Successor paths will be Feasible Successors.
7. By default, only the Successor routes will appear in the routing table. Both the Successor and Feasible Successor routes will appear in the EIGRP topology table, though.
8. Notice I said "by default". We can use the variance command to bring Feasible Successors into the routing table.
In this case, we want to bring Paths 2 and 3 into the routing table without doing so for Path 4.
We'll use these metrics to do so...
Path 1: 1000
Path 2: 2345
Path 3: 6234
Path 4: 7100
... and we'll use the variance command.
The variance command is one of those commands that sounds complicated when you hear or read the theory, and then when you do it, you say "Hey, that was easy!"
That's exactly what you'll be saying in just a minute.
The variance command is just a multiplier. When you configure it, the router multiplies the metric of the Successor and then puts all Feasible Successors with a metric smaller than the result of the multiplication into the routing table.
I told you it sounded complicated. Any paragraph with three variations of the word "multiply" can't be good!
Let's practice with this example and you'll see it's quite simple.
We can take a look at that metric of 1000 and quickly figure that we need a variance of 6 to bring paths 2 and 3 into the routing table.
The result of entering variance 6 into the EIGRP config:
DUAL looks at metric of our Successor, which is 1000.
Result of 6 x 1000: 6000
All Feasible Successors with metric of 6000 or less will now be put into the routing table. That's Path 2... and no others.
Whoops! Looks like we need a variance of 7 instead. Let's do that...
DUAL looks at metric of our Successor, which is 1000
Result of 7 x 1000: 7000
Now both Paths 2 and 3 will be put into the routing table, while Path 4 will not.
The great thing about the variance command is that you don't need a laptop or practice exam to practice it - just a piece of paper and a pencil.
Or a pen, if you're as hardheaded / confident as I am.
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