Enhanced Interior Gateway Routing Protocol
The Enhanced Interior Gateway Routing Protocol ( EIGRP ) is an originally proprietary routing protocol published by Cisco in 1992 . Cisco announced in March 2013 that it would make this an open standard. The protocol was published under RFC 7868 . EIGRP is an improved version of the earlier IGRP , with which there is still compatibility .
EIGRP is an extended distance vector routing protocol (Advanced Distance Vector) which behaves like a link-state routing protocol when exchanging with neighboring routers and when storing routing information . Due to the extensive features that are more likely to be found in link-state protocols, EIGRP is therefore often classified as a balanced hybrid routing protocol. With the help of these link-state properties, EIGRP achieves a very fast convergence in relation to conventional distance-vector routing protocols and is immune to routing loops. In particular, the reliability in environments with GRE tunnels that are dynamically cross-linked by NHRP make EIGRP an interesting alternative to OSPF . By using BFD , convergence times in the range of a few milliseconds can now be achieved for many IGPs .
With EIGRP, neighboring routers are stored in a neighbor table . All routes that are announced via these neighbors are in turn collected in a topology table . With EIGRP, the best route to a target network is determined using the Diffusing Update Algorithm (DUAL) .
The calculation of the metric for different routes is based on the same procedure for EIGRP as for IGRP , but due to the longer field for storing the metric, EIGRP scales the value by a factor of 256 compared to IGRP. When manually converting values between EIGRP- and IGRP routers, this must be taken into account; in practice, the conversion is automatic.
As an innovation compared to IGRP , the Classless Inter-Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) methods are also supported. Since these features are now standard in modern networks, IGRP no longer plays a significant role in practice.
EIGRP primarily supports the Internet Protocol (IP) , IPX and Appletalk as routed protocols of the 3rd layer of the OSI reference model . Due to its modular structure, however, with the help of so-called protocol-dependent modules (PDM) it is possible to use newer protocols such as IPv6 without having to update EIGRP yourself.
A router that works with EIGRP manages a separate routing table, a neighborhood table and a topology table for each protocol of OSI layer 3.
However, the multi-protocol properties of EIGRP are no longer fully supported in the current versions of IOS . Thus only IP and IPX remain as protocols to be routed.
- Wolfgang Schulte: Handbook of the routing protocols of the networks , SVH Verlag, 2009, ISBN 978-3-8381-1066-0