Introduction to EIGRP

Lessons Discussion
197

Hello Görgen

Here’s the list of steps again, but with numbers as shown in the lesson:

  1. R4 will advertise the destination network to R3.
  2. R3 will advertise the network to R1 and R2.
  3. R1 will advertise the network to R2.
  4. R2 will advertise the network to R1.
  5. R1 will advertise this network back to R3.
  6. R2 will advertise this network back to R3.

Steps 3 and 4 typically take place at the same time independently from one another. Similarly steps 5 and 6 are also simultaneous and independent. So the list could have been:

  1. R4 will advertise the destination network to R3.
  2. R3 will advertise the network to R1 and R2.
  3. R1 will advertise the network to R2 AND R2 will advertise the network to R1.
  4. R1 will advertise this network back to R3 AND R2 will advertise this network back to R3.

This is probably a more correct listing of the steps taken. Does that make sense?

Concerning your second question, you must keep in mind that the values of AD and FD are calculated from the point of view of R3. Secondly, you have to remember the split horizon rule. A router will not advertise a network to a neighbor from which it received that route. So When R1 sends its AD to R3, it sends this path:

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It will never send the path via R3 because that would violate the split horizon rule.

Similarly, when R2 informs R3 of its AD, it measures the metric of the path via R1. So it is 7+3+4+5 = 19. Does that make sense?

I hope this has been helpful!

Laz