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The OSPF has a hierarchy system that separates a single Autonomous System into individual areas, while EIGRP has not a hierarchy system, it prefer to handle with an entire Autonomous System, right?

What are advantages of using a Hierarchy System? What if there, Why EIGRP has not a Hierarchy System?

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This has to do with the algorithms used. OSPF and IS-IS use Dijkstra's algorithm which requires a hierarchy. EIGRP uses a proprietary algorithm (DUAL) which does not. The DUAL algorithm is diffusing (the "D" in DUAL) so that it doesn't require a hierarchy.

EIGRP can be set up in a sort of hierarchy with stub areas, but it doesn't fundamentally need a hierarchy the way OSPF or IS-IS do.

With Dijkstra's algorithm, each router has a complete understanding of an area, but DUAL only knows about the next hop for a route.

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  • That understanding of an area has some relation with CPU overload? I mean, OSPF has handle with parts of their Autonomous System, while EIGRP handle with an entire. Reworking the questions, Is there any "visible difference" on CPU usage?
    – TMoraes
    Nov 20 '15 at 0:01
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    Well, an EIGRP router has a handle on its direct neighbors, not the entire AS. I really don't think there is a noticeable difference on CPU load between the routing protocols. You really have to worry more about things like traffic load, PBR, encryption, QoS, etc. The actual routing protocols don't particularly put a load on the CPU, with the exception of BGP loading and processing an initial large table (like the whole Internet table).
    – Ron Maupin
    Nov 20 '15 at 0:09
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    One other point is that by breaking up an AS into areas, flapping links or bouncing routers have less of an affect on the entire AS. Propagating every little change over the AS when there are problems can cause temporary black holes for traffic while the changes travel around the AS. OSPF areas tend to contain this disruption to a single area.
    – Ron Maupin
    Nov 20 '15 at 0:19
  • The last questions for avoiding "Open Chat" Well, an EIGRP router has a handle on its direct neighbors, not the entire AS Would OSPF do the same? and What is "flapping links"?
    – TMoraes
    Nov 20 '15 at 0:21
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    An OSPF router has a handle on the entire area, but an EIGRP router really only knows about its direct neighbors. A flapping link is one that goes up and down multiple times in a relatively short time period. The can happen with a bad interface or cable, and can be a real problem with WAN circuits. Every time the link changes state, the new state must be propagated over the entire area (OSPF) or the entire AS (EIGRP). BGP has had a special way of handling this, and other protocols or services have been adding things to mitigate the problem.
    – Ron Maupin
    Nov 20 '15 at 0:27

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