# quick EIGRP question

Member Posts: 800
So everyone knows that to be a feasible successor a routers Advertised Distance must be less than the successors feasible distance right?

so what if a router has an AD that is exactly the same as the successors FD. Does that make it a feasible successor or does it have to be less?

thanks
CCIE# 38186
showroute.net

• Mod Posts: 11,962 Mod
I think you answered your own question. It must be less.
An expert is a man who has made all the mistakes which can be made.
• Member Posts: 953
rakem,
rakem wrote:
So everyone knows that to be a feasible successor a routers Advertised Distance must be less than the successors feasible distance right?

I'm someone and I didn't know "...that to be a feasible successor a routers Advertised Distance must be less than the successors feasible distance..." Are you talking about EIGRP's calculated metric?
rakem wrote:
so what if a router has an AD that is exactly the same as the successors FD. Does that make it a feasible successor or does it have to be less?

thanks

As you may or may not know, EIGRP uses at least two tables: 1) Neighbor Table 2) Topology Table. The Neighbor table contains a list of directly connected neighbors. The Topology table is an extension of the Neighbor table to include routes that were learned from the EIGRP router's neighbor. After the topology table is stabilized, the DUAL process is run on the topology table to calculate feasible successors. After all the feasible successors are calculated, the best feasible successor is moved from the topology table to the routing table. The actual number of routes moved from the topology table to the routing table depends on factors such as the following: 1) equal cost load balancing 2) unequal cost load balancing 3) the number of allowed active EIGRP routes in the routing table.

What do you mean by "so what if a router has an AD...?" As you may or may not know, AD usually refers to "Administrative Distance" which is the number used by the router to determine which routing protocol's sub/network route to place into the routing table. Are you trying to understand the logic that's used by DUAL and the EIGRP router to determine which feasible successors gets moved from the topology table to the routing table?

Source:
1. Introduction to EIGRP [IP Routing] - Cisco Systems - http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml#feasible
• Mod Posts: 11,962 Mod
Not to be rude, but I think the question is a little beyond the scope of your knowledge tech-airman, but not sure. When talking about EIGRP the AD is Advertised Distance and the FD is Feasable Distance, not Administrative Distance. When calculating feasible succesors one of the terms it must meet is that in order to becomne a feasible succesor the AD must be less than the FD of the succesor.
An expert is a man who has made all the mistakes which can be made.
• Banned Posts: 6,308 ■■■■■■■■■□
In this case AD refers to the advertised distance as opposed to administrative distance. In some texts you will also find it mentioned as reported distance. It's all about the mechanism of DUAL as opposed to route selection with EIGRP as compared with say OSPF using administrative distance.
• Member Posts: 1,181 ■■■□□□□□□□
Yes, it is one of the primary tenets of EIGRP.
I usually hang out on 224.0.0.10 (FF02::A) and 224.0.0.5 (FF02::5) when I'm in a non-proprietary mood.

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