Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced version of IGRP. IGRP is Cisco"s Interior Gateway Routing Protocol used in TCP/IP and OSI internets. It is regarded as an interior gateway protocol (IGP) but has also been used extensively as an exterior gateway protocol for inter-domain routing.
Key capabilities that distinguish Enhanced IGRP (EIGRP) from other routing protocols include fast convergence, support for variable-length subnet mask, support for partial updates, and support for multiple network layer protocols.
A router running EIGRP stores all its neighbors" routing tables so that it can quickly adapt to alternate routes. If no appropriate route exists, EIGRP queries its neighbors to discover an alternate route. These queries propagate until an alternate route is found.
The support of EIGRP for variable-length subnet masks permits routes to be automatically summarized on a network number boundary. In addition, EIGRP can be configured to summarize on any bit boundary at any interface.
EIGRP does not make periodic updates. Instead, EIGRP sends partial updates only when the metric for a route changes. Propagation of partial updates is automatically bounded so that only those routers that need the information are updated. As a result of these two capabilities, EIGRP consumes significantly less bandwidth than IGRP.
Protocol Structure
| 8 bits | 16 bits | 32 bits |
| Version | Opcode | Checksum |
| Flags | ||
| Sequence number | ||
| Acknowledge number | ||
| Asystem: Autonomous system number | ||
| Type | Length | |
- Version -- The version of EIGRP.
- Opcode COperation code indicating the message type: 1 Update. 2 Reserved. 3 Query. 4 Hello. 5 IPX-SAP.
- checksum C IP checksum which is computed using the same checksum algorithm as a UDP checksum
- Flag -- Initialization bit and is used in establishing a new neighbor relationship
- Sequence number -- used to send messages reliably
- Acknowledge number -- used to send messages reliably
- Asystem -- Autonomous system number. A gateway can participate in more than one autonomous system where each system runs its own IGRP. For each autonomous system, there are completely separate routing tables. This field allows the gateway to select which set of routing tables to use.
- Type C Value in the type field: 1 EIGRP Parameters. 2 Reserved. 3 Sequence. 4 Software version. 5 Next Multicast sequence.
- Length -- Length of the frame.
Related Terms: IP, TCP, IGRP, RIP, EGP, BGP
Sponsor Source:EIGRP is a Cisco protocol.
Reference Links: http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/en_igrp.htm : Enhanced IGRP