Answer to the #CCNA EIGRP Question

 In 200-301 V1 BBB Not in new books, Q&A

Today’s #CCNA post shows the answer to the most recent EIGRP question –  dig in!

The Answer

(Wendell’s note to self: internal question number 211.)

Answer(s): C

The Explanation

By default, EIGRP calculates metrics based on:

  • The cumulative interface delay…
  • The minimum interface bandwidth…
  • Of all outgoing links in the end-to-end route.

Analyzing these basics, you can rule out a couple of the answers as incorrect, based on the following logic:

  • Router A’s route to subnet 10.1.1.0/24 uses the following outgoing interfaces: A’s S0/0/1, B’s F0/0, C’s S0/0/0, and D’s G0/0.
  • Changes to the bandwidth or delay on any interfaces not listed in the previous bullet cannot possibly change A’s metric over this route.
  • Note that the interface settings do not have to be the same on both ends of a link, although in practice, most engineers choose to keep them consistent because that makes more sense.

Two answers can be ruled out per the above list. One answer mentions a change to the delay on router D’s S0/0/1, an inbound interface for this route. Another answer changes router B’s S0/0/0 bandwidth, again an inbound interface, so it cannot possibly be a correct answer.

Then, you have to ask if any of the answers changes the delay of an outgoing interface. Any change to the delay changes the calculated metric, because the delay is cumulative in the EIGRP metric formula. That is, a change to any interface delay, for the interfaces in the end-to-end route, will have some impact on the calculated metric. Of the remaining possible answers, one refers to changing the delay:

  • Set router A’s S0/0/1 delay to 189.

If router A’s delay setting on S0/0/1 is not already set with to 189 (meaning 189 tens-of-microseconds, or 1890 microseconds), then that answer changes the delay. As it turns out, with all default settings, a serial interface will have a delay of 2,000 (tens-of-microseconds), meaning 20,000 microseconds. This answer seems a safe bet to be a correct answer, because a delay of 189 is not the default setting, although I could have mentioned a little more detail about assumptions about defaults.

Finally, for bandwidth, you have to ask yourself whether any bandwidth changes reduce the minimum bandwidth in the end-to-end route (outgoing interfaces only). The EIGRP metric calculation uses only the minimum bandwidth, so any changes to faster bandwidth has no effect. Only one answer that lists a bandwidth change on an outgoing interface:

  • The bandwidth 15000 command on B’s F0/0 interface

On interfaces in the Ethernet family, IOS defaults to use a bandwidth that matches the current speed. Assuming 100 Mbps on this interface, the bandwidth setting would be 100,000, because the bandwidth command’s unit is Kilobits/second. The bandwidth 15000 command lowers the interface bandwidth. However, it does not become the lowest bandwidth in the end-to-end route; that lowest bandwidth remains as router C’s S0/0/0 at 768 Kbps. So this answer does not change the metric for this route.

More Practice Questions:

This question is like those you get if when you buy the ICND2 200-101 Official Cert Guide. This blog also lists various practice questions as well. For more questions on a large variety of topics:

#CCNA Fast Start: an EIGRP Question
#CCNA Fast Start: an IOS Licensing Question
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