EIGRP - Understanding Advertised Distance (AD)

When an EIGRP router sends its distance to a particular network destination to its neighbor, from the point of view of its neighbor, this is considered the advertised distance (AD). The transmission of the AD is subject to the split horizon rule. For this reason, the AD will not be sent to the router from which it was originally learned.

However, this does not preclude an EIGRP router from sharing with a particular neighbor the AD to the intended destination that was learned from another EIGRP router.

For example, take a look at this EIGRP topology:

R1 has learned about the Destination network from two sources: from R3 as well as from R2. R1 will not send the AD to the destination that was learned from R3 (which would be 3+4+5=12), even though this is the shortest route. But it will send the AD to the route that it learned via R2 as shown in the diagram.

Note that even though R3 was still in the path of the AD that was advertised, it does not consider this a violation of the split horizon rule. The split horizon rule takes into account only the next hop IP and not the IP address of subsequent routers in the path.

Keep in mind that the AD is cumulative.

Links

https://networklessons.com/eigrp/troubleshooting-eigrp-route-advertisement https://networklessons.com/eigrp/introduction-to-eigrp