EIGRP principle (UP)

The metric calculation formula for EIGRP is basically the same as IGRP, but the metric calculation of EIGRP is igrp*256, which enables finer-grained metric identification.
Four components of EIGRP:

• Protocol-related modules
• Rtp
• Neighbor Discovery Mechanism
• Dual algorithm

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Protocol-related modules

EIGRP implements the protocol-related modules for ip,ipx and AppleTalk. EIGRP can, in many cases, automate route redistribution with other routing protocols:

• IPX EIGRP will automatically and IPX RIP protocol, NLSP protocol for route redistribution
• AppleTalk EIGRP will automatically route redistribution to the AppleTalk rtmp protocol
• If the IGRP process and the EIGRP process are within the same as, then IP eigrp automatically and IGRP will route the redistribution

Rtp

The essence of RTP lies in reliable and orderly transceiver.
reliable refers to the Cisco private "reliable multicast" algorithm. Use the 224.0.0.10 multicast address. And each reliable multicast packet neighbor will unicast a confirmation message to confirm the reliable multicast packet.
The ordered packets will have two serial numbers, one is the serial number assigned by the router that sent the packet, and the serial number is added 1 each time the new packet is sent. The other is the serial number that was used to make the acknowledgment, which is the serial number of the packet that was recently received for the destination router.
EIGRP is hosted with the IP Protocol, protocol number 88. The EIGRP groupings mainly have the following:

• Hello: used for neighbor discovery and recovery processes, periodic send, unreliable send mode
• ACK: Hello packets that do not contain data, single-advertisement send, unreliable send method
• Update: used to pass routing update information. The routing update information for EIGRP is only sent to the necessary routers when necessary. Reliable updates are used.
• Query and reply: Dual the packet used to manage the diffusion calculation. Query uses multicast or unicast, and reply is sent using unicast mode. Query and reply are reliable ways to send
• Request: originally intended for use by a routing server, but not implemented.

A reliable way to send out packets, 16 times retransmission without confirmation will break the neighbor relationship.

Neighbor Discovery and recovery

Because of the triggering update of EIGRP routing information, the neighbor Discovery and maintenance mechanism of EIGRP is very important. On a network with a bandwidth greater than 1544Kbps, the Hello packet is sent every 5s.

You can use interface commands ip hello-interval eigrp to make changes

The Hello packet has hold time, which is typically 3 times times that of the Hello interval. The hold timer expires when the neighbor is declared unreachable and notifies the dual that the neighbor is missing.

You can use interface commands ip hold-time eigrp to change
The fast convergence of EIGRP is also due to the fast neighbor detection and maintenance mechanism

Neighbor table

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The neighbor table mainly records the address of the neighbor and the interface that receives the neighbor Hello packet.
The specific meaning has been studied in NP notes, and is not written. Focus on the RTO (the RTO is the time, in milliseconds, and that's the router will wait for an acknowledgment of a U Nicast Packet sent after a multicast have failed) not very good translation, the spirit of understanding it

EIGRP principle (UP)