EIGRP интернет-хаттамасы: қағидалары мен ерекшеліктері

Internet protocol
Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector routing protocol that is used on a computer network for automating routing decisions and configuration. The protocol was designed by Cisco Systems as a proprietary protocol, available only on Cisco routers. In 2013, Cisco permitted other vendors to freely implement a limited version of EIGRP with some of its associated features such as High Availability (HA), while withholding other EIGRP features such as EIGRP stub, needed for DMVPN and large scale campus deployment. Information needed for implementation was published with informational status as in 2016, which did not advance to Internet Standards Track level, and allowed Cisco to retain control of the EIGRP protocol. EIGRP is used on a router to share routes with other routers within the same autonomous system. Unlike other well known routing protocols, such as RIP, EIGRP only sends incremental updates, reducing the workload on the router and the amount of data that needs to be transmitted. EIGRP replaced the Interior Gateway Routing Protocol (IGRP) in 1993. One of the major reasons for this was the change to classless IPv4 addresses in the Internet Protocol, which IGRP could not support.

EIGRP is a distance vector & Link State routing protocol that uses the diffusing update algorithm (DUAL) (based on work from SRI International) to improve the efficiency of the protocol and to help prevent calculation errors when attempting to determine the best path to a remote network. EIGRP determines the value of the path using five metrics: bandwidth, load, delay, reliability and MTU. EIGRP uses five different messages to communicate with its neighbor routers – Hello, Update, Query, Reply, and Acknowledgement. EIGRP routing information, exchanged to a router from another router within the same autonomous system, has a default administrative distance of 90. EIGRP routing information, that has come from an EIGRP enabled router outside the autonomous system, has a default administrative distance of 170. EIGRP does not operate using the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). This means that EIGRP does not use a port number to identify traffic. Rather, EIGRP is designed to work on top of Layer 3 (i. e. the IP protocol). Since EIGRP does not use TCP for communication, it implements Cisco's Reliable Transport Protocol (RTP) to ensure that EIGRP router updates are delivered to all neighbors completely. The Reliable Transport Protocol also contains other mechanisms to maximize efficiency and support multicasting. While EIGRP is an advanced routing protocol that combines many of the features of both link state and distance vector routing protocols, EIGRP's DUAL algorithm contains many features which make it more of a distance vector routing protocol than a link state routing protocol. Despite this, EIGRP contains many differences from most other distance vector routing protocols, including:

the use of explicit hello packets to discover and maintain adjacencies between routers. the use of a reliable protocol to transport routing updates. the use of a feasibility condition to select a loop free path. the use of diffusing computations to involve the affected part of the network into computing a new shortest path.

A feasible successor for a particular destination is a next hop router that is guaranteed not to be a part of a routing loop. This condition is verified by testing the feasibility condition. Thus, every successor is also a feasible successor. However, in most references about EIGRP the term feasible successor is used to denote only those routes which provide a loop free path but which are not successors (i. e. they do not provide the least distance). From this point of view, for a reachable destination, there is always at least one successor, however, there might not be any feasible successors. A feasible successor provides a working route to the same destination, although with a higher distance. At any time, a router can send a packet to a destination marked "Passive" through any of its successors or feasible successors without alerting them in the first place, and this packet will be delivered properly. Feasible successors are also recorded in the topology table. The feasible successor effectively provides a backup route in the case that existing successors become unavailable. Also, when performing unequal cost load balancing (balancing the network traffic in inverse proportion to the cost of the routes), the feasible successors are used as next hops in the routing table for the load balanced destination. By default, the total count of successors and feasible successors for a destination stored in the routing table is limited to four. This limit can be changed in the range from 1 to 6. In more recent versions of Cisco IOS (e. g. 12.4), this range is between 1 and 16.

A destination in the topology table can be marked either as passive or active. A passive state is a state when the router has identified the successor(s) for the destination. The destination changes to active state when the current successor no longer satisfies the feasibility condition and there are no feasible successors identified for that destination (i. e. no backup routes are available). The destination changes back from active to passive when the router received replies to all queries it has sent to its neighbors. Notice that if a successor stops satisfying the feasibility condition but there is at least one feasible successor available, the router will promote a feasible successor with the lowest total distance (the distance as reported by the feasible successor plus the cost of the link to this neighbor) to a new successor and the destination will remain in the passive state.

EIGRP features load balancing on paths with different costs. A multiplier, called variance, is used to determine which paths to include into load balancing. The variance is set to 1 by default, which means load balancing on equal cost paths. The maximum variance is 128. The minimum metric of a route is multiplied by the variance value. Each path with a metric that is smaller than the result is used in load balancing. With the functionality of the Unequal Path Cost Load Balancing on EIGRP, OSPF protocol is unable to design the network by Unequal Path Cost Load Balancing. Regarding the Unequal Path Cost Load Balancing function on industry usage, the network design can be flexible with the traffic management.

Cisco released details of the proprietary EIGRP routing protocol in an RFC in an effort to assist companies whose networks operate in a multi vendor environment. The protocol is described in EIGRP was developed 20 years ago, yet it is still one of the primary Cisco routing protocols due to its purported usability and scalability in comparison to other protocols. Cisco has stated that EIGRP is an open standard but they leave out several core details in the RFC definition which makes interoperability hard to set up between different vendors' routers when the protocol is used. Even Cisco NX OS for example does not support unequal cost load balancing. As of 2022 EIGRP has alpha support in FRRouting and it seems to be generally unsupported by other routing software.