Cisco CCNA – EIGRP Concepts

Every EIGRP packet includes the EIGRP Packet Header. The important fields in this header are the Opcode field and the Autonomous System Number field. The Opcode specifies the packet type, which can be Update, Query, Reply and Hello. The Autonomous System (AS) Number specifies the EIGRP routing process. Unlike RIP, a Cisco router can run multiple instances of EIGRP. The AS number is used to track multiple instances of EIGRP.

Reliable Transport Protocol (RTP) is used by EIGRP for the delivery and reception of EIGRP packets. RTP sends packets to either unicast address or the reserved multicast address

As we already told you, EIGRP uses five different packet types:

  • Hello packets – are used to discover neighbors and form adjacencies with them. These packets are multicast and use unreliable delivery and are sent typically every 5 seconds.
  • Update packets – are used to propagate the routing information. Update packets are sent only when a change in the topology occurs and only to those routers who require them. EIGRP update packets use reliable delivery and are sent to the multicast address when multiple routers require them, and to the unicast address when they are required by a single router.
  • Acknowledgement (ACK) packets – are sent when reliable delivery is used. They contain a nonzero acknowledgement number and are always sent to the unicast address.
  • Query and Replay packets – are used by DUAL when searching for networks and other tasks. Queries are sent to the multicast address and use reliable delivery, while replies are sent as unicast and they are also using reliable delivery.

In EIGRP, the default administrative distance (AD) is 170 when used as an Exterior Gateway Protocol (EGP), 90 when used as an IGP and 5 for EIGRP summary routes.

EIGRP also supports authentication to ensure the information transmitted over the network is authentic and from a trusted source.