About EIGRP

EIGRP is a hybrid protocol.
Hybrid Routing is a combination of the both distance-vector and link-state routing.
Hybrid routing protocols use distance-vectors for more accurate metrics to determine the best paths to destination networks, and report routing information only when there is a change in the topology of the network.
They have the nature of hybrid routing allows for rapid convergence (as with the link state protocols), but compare with link-state protocol it requires much less processing power and memory.

EIGRP stands for Enhanced Interior Gateway Routing Protocol, EIGRP is a hybrid routing protocol developed by Cisco. EIGRP uses "Distributed update algorithm" or "DUAL", it combines the Link-state and Distance-Vector algorithms. EIGRP is considered balanced hybrid protocol. Unlike IGRP that only understands major network numbers, EIGRP can be configured to use subnet masks and supernet masks. The format of the EIGRP routing table is different from IGRP routing table.
Advantages of EIGRP are:

Fast convergence, Support for multiple protocols, requires less bandwidth than Distance-vector protocols because only partial updates are broadcasted and only when changes in topology are detected. EIGRP does not use periodic broadcasts. Improved loop detection and prevention.

Here are some of EIGRP's improvements over IGRP:

• DUAL
• Incremental updates
• Reduced bandwidth usage
• Support for multiple network layer protocols (IP, IPX, AppleTalk)
• Support for variable-length subnet masks (VLSMs), discontiguous networks, and classless routing
• Advanced distance vector capabilities
• Automatic route summarization on major network boundaries

EIGRP uses bandwidth and delay by default to calculate  metric. Metric can also be configured to use reliability, load, and MTU.

EIGRP Summary Route & NULL 0

EIGRP Summary Route & Null 0

 As many of us learned and done many labs and seen same thing in real time also. Whenever we configure summary route in EIGRP ,It creates a summary route in own router pointing towards Null 0. Many of you will be having question why?

Everybody knows answer also…But then also some confusion. Here I am trying to explain in simple way.

As EIGRP is having flexibility to configure summary route anywhere in network. No restrictions like OSPF (only ABR and ASBR). That is add on advantage for a network administrator to manage his network.

As we configure summary route in particular interface. That means we want to advertise a summary address to that neighbor instead of full routing table. As we configure Summary route on our router(originating router) the neighboring router will receive a summary address route pointing towards our router. But same time in our router one summary route will be created pointing towards Null 0.

Imagine the Packet flow, As the request will generate for a specific network which comes under summary address it will come to our router(originator) and if that time that concern network is unreachable then what will happen, Packet will process like -First it will look the longest prefix (exact route) and longest prefix is not available. Then packet will go to summary route and as summary route is pointing towards Null 0 interface so the packet will be dropped.

Suppose if the Summary route is not configured, then what will be situation. The packet will go to somewhere else where it should not go (May be from default route).

So in EIGRP, Null 0 works as Loop prevention Mechanism.

I think now it’s very clear that, In EIGRP the summary route pointing towards Null0 is a mechanism of Loop prevention in network.