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43-481: EIGRP uses a feasibility condition to ensure that only loop-free routes are ever selected. The feasibility condition is conservative: when the condition is true, no loops can occur, but the condition might under some circumstances reject all routes to a destination although some are loop-free. When no feasible route to a destination is available, the DUAL algorithm invokes a diffusing computation to ensure that all traces of

86-573: 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 . EIGRP uses 224.0.0.10 as its multicast address and protocol number 88. Cisco Systems now classifies EIGRP as

129-574: A distance vector routing protocol, but it is normally said to be a hybrid routing protocol. 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: EIGRP associates six different vector metrics with each route and considers only four of

172-462: 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. The feasibility condition is a sufficient condition for loop freedom in EIGRP-routed network. It is used to select the successors and feasible successors that are guaranteed to be on

215-455: A feasible successor, its RD must be smaller than the FD of the successor. If this feasibility condition is met, there is no way that adding this route to the routing table could cause a loop. If all the successor routes to a destination fail, the feasible successor becomes the successor and is immediately added to the routing table. If there is no feasible successor in the topology table, a query process

258-455: A loop-free route to a destination. Its simplified formulation is strikingly simple: or in other words, It is important to realize that this condition is a sufficient, not a necessary, condition. That means that neighbors which satisfy this condition are guaranteed to be on a loop-free path to some destination, however, there may be also other neighbors on a loop-free path which do not satisfy this condition. However, such neighbors do not provide

301-529: 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 RFC   7868 in 2016, which did not advance to Internet Standards Track level, and allowed Cisco to retain control of

344-466: A relationship, known as an adjacency . The entire routing table is exchanged between both routers at this time. After the exchange has completed, only differential changes are sent. EIGRP is often considered a hybrid protocol because it also sends link state updates when link states change. EIGRP supports the following features: Example of setting up EIGRP on a Cisco IOS router for a private network . The 0.0.15.255 wildcard in this example indicates

387-542: 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

430-447: A single metric, using a formula which can be adjusted through the use of pre-set constants. By default, the IGRP composite metric is a sum of the segment delays and the lowest segment bandwidth. The maximum configurable hop count of IGRP-routed packets is 255 (default 100), and routing updates are broadcast every 90 seconds (by default). IGRP uses protocol number 9 for communication. IGRP

473-443: A subnetwork with a maximum of 4094 hosts—it is the bitwise complement of the subnet mask 255.255.240.0. The no auto-summary command prevents automatic route summarization on classful boundaries, which would otherwise result in routing loops in discontiguous networks. EIGRP is a distance vector & Link State routing protocol that uses the diffusing update algorithm (DUAL) (based on work from SRI International ) to improve

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516-411: 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

559-512: Is a distance vector interior gateway protocol (IGP) developed by Cisco . It is used by routers to exchange routing data within an autonomous system . IGRP is a proprietary protocol . IGRP was created in part to overcome the limitations of RIP (maximum hop count of only 15, and a single routing metric) when used within large networks. IGRP supports multiple metrics for each route, including bandwidth , delay , load , and reliability ; to compare two routes these metrics are combined into

602-492: Is considered a classful routing protocol. Because the protocol has no field for a subnet mask , the router assumes that all subnetwork addresses within the same Class A, Class B, or Class C network have the same subnet mask as the subnet mask configured for the interfaces in question. This contrasts with classless routing protocols that can use variable length subnet masks . Classful protocols have become less popular as they are wasteful of IP address space . In order to address

645-406: Is initiated to look for a new route. Legend: Now a client on router E wants to talk to a client on router A. That means a route between router A and router E must be available. This route is calculated as follows: The immediate neighbours of router E are router C and router D. DUAL in router E asks for the reported distance (RD) from routers C and D respectively to router A. The following are

688-408: Is kept as a "feasible successor", because its RD is less than the FD of the successor: Enhanced Interior Gateway Routing Protocol#Feasibility condition 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

731-423: 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

774-498: 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

817-462: Is that if K 5 {\displaystyle K_{5}} is set to zero, the term K 5 K 4 + Reliability {\displaystyle {\tfrac {K_{5}}{K_{4}+{\text{Reliability}}}}} is not used (i.e. taken as 1) . The default is for K 1 {\displaystyle K_{1}} and K 3 {\displaystyle K_{3}} to be set to 1, and

860-497: 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

903-419: 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

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946-540: 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

989-439: The K values as needed to consider the other Vector metrics. For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric: where the various constants ( K 1 {\displaystyle K_{1}} through K 5 {\displaystyle K_{5}} ) can be set by the user to produce varying behaviors. An important and unintuitive fact

1032-444: 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 RFC   7868 . EIGRP was developed 20 years ago, yet it is still one of

1075-400: 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

1118-411: 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

1161-430: The factor of 256 (effectively bit-shifting it 8 bits to the left), the value is extended into 32 bits, and vice versa. This way, redistributing information between EIGRP and IGRP involves simply dividing or multiplying the metric value by a factor of 256, which is done automatically. 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

1204-405: The interface Bandwidth and Delay configuration values with following calculations: On Cisco routers, the interface bandwidth is a configurable static parameter expressed in kilobits per second (setting this only affects metric calculation and not actual line bandwidth). Dividing a value of 10 kbit/s (i.e. 10 Gbit/s) by the interface bandwidth statement value yields a result that is used in

1247-481: The issues of address space and other factors, Cisco created EIGRP (Enhanced Interior Gateway Routing Protocol). EIGRP adds support for VLSM (variable length subnet mask) and adds the Diffusing Update Algorithm (DUAL) in order to improve routing and provide a loopless environment. EIGRP has completely replaced IGRP, making IGRP an obsolete routing protocol. In Cisco IOS versions 12.3 and greater, IGRP

1290-407: The path will become unavailable. EIGRP is designed to detect these changes and will attempt to find a new path to the destination. The old path that is no longer available is removed from the routing table. Unlike most distance vector routing protocols, EIGRP does not transmit all the data in the router's routing table when a change is made, but will only transmit the changes that have been made since

1333-412: The path with the lowest metric to reach the destination, and the redundant path is the path with the second lowest cost (if it meets the feasibility condition). There may be multiple successors and multiple feasible successors. Both successors and feasible successors are maintained in the topology table, but only the successors are added to the routing table and used to route packets. For a route to become

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1376-736: 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 . Interior Gateway Routing Protocol Interior Gateway Routing Protocol ( IGRP )

1419-408: The problematic route are eliminated from the network. At which point the normal Bellman–Ford algorithm is used to recover a new route. DUAL uses three separate tables for the route calculation. These tables are created using information exchanged between the EIGRP routers. The information is different than that exchanged by link-state routing protocols . In EIGRP, the information exchanged includes

1462-547: The rest to zero, effectively reducing the above formula to ( Bandwidth E + Delay E ) ⋅ 256 {\displaystyle ({\text{Bandwidth}}_{E}+{\text{Delay}}_{E})\cdot 256} . Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops may result. Cisco routers running EIGRP will not form an EIGRP adjacency and will complain about K-values mismatch until these values are identical on these routers. EIGRP scales

1505-400: The results: The route via C is therefore in the lowest cost. In the next step, the distance from router E to the neighbours are added to the reported distance to get the feasible distance (FD): DUAL therefore finds that the route via D has the least total cost. Then the route via D will be marked as "successor", equipped with passive status and registered in the routing table. The route via C

1548-402: The routes, the " metric " or cost of each route, and the information required to form a neighbor relationship (such as AS number, timers, and K values). The three tables and their functions in detail are as follows: DUAL evaluates the data received from other routers in the topology table and calculates the primary (successor) and secondary (feasible successor) routes. The primary path is usually

1591-421: The routing table was last updated. EIGRP does not send its routing table periodically, but will only send routing table data when an actual change has occurred. This behavior is more inline with link-state routing protocols , thus EIGRP is mostly considered a hybrid protocol. When a router running EIGRP is connected to another router also running EIGRP, information is exchanged between the two routers. They form

1634-407: The same basic formula for computing the overall metric, the only difference is that in IGRP, the formula does not contain the scaling factor of 256. In fact, this scaling factor was introduced as a simple means to facilitate backward compatility between EIGRP and IGRP: In IGRP, the overall metric is a 24-bit value while EIGRP uses a 32-bit value to express this metric. By multiplying a 24-bit value with

1677-427: The shortest path to a destination, therefore, not using them does not present any significant impairment of the network functionality. These neighbors will be re-evaluated for possible usage if the router transitions to Active state for that destination. 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

1720-465: The vector metrics in computing the Composite metric: The composite routing metric calculation uses five parameters, so-called K values, K1 through K5. These act as multipliers or modifiers in the composite metric calculation. K1 is not equal to Bandwidth, etc. By default, only total delay and minimum bandwidth are considered when EIGRP is started on a router, but an administrator can enable or disable all

1763-401: The weighted formula. The interface delay is a configurable static parameter expressed in tens of microseconds. EIGRP takes this value directly without scaling into the weighted formula. However, various show commands display the interface delay in microseconds. Therefore, if given a delay value in microseconds, it must first be divided by 10 before using it in the weighted formula. IGRP uses

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1806-412: The workload on a network administrator who does not have to configure changes to the routing table manually. In addition to the routing table , EIGRP uses the following tables to store information: Information in the topology table may be inserted into the router's routing table and can then be used to forward traffic. If the network changes (for example, a physical link fails or is disconnected),

1849-471: Was the change to classless IPv4 addresses in the Internet Protocol , which IGRP could not support. Almost all routers contain a routing table that contains rules by which traffic is forwarded in a network. If the router does not contain a valid path to the destination, the traffic is discarded. EIGRP is a dynamic routing protocol by which routers automatically share route information. This eases

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