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| {{refimprove|date=June 2013}}
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| {{technical|date=November 2013}}
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| {{IPstack}}
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| The '''Enhanced Interior Gateway Routing Protocol (EIGRP)''' is an advanced [[communications protocol]] that helps automate routing decisions on a [[computer network]]. The protocol was designed by [[Cisco Systems]] to accommodate [[classless inter-domain routing|key design changes]] to the underlying communications protocol used on the Internet known as [[IPv4]].<ref>http://www.differencebetween.net/technology/internet/difference-between-igrp-and-eigrp/</ref> EIGRP was originally a proprietary protocol, which meant that it was only available on Cisco routers, but became an open protocol in 2013, allowing any [[router (computing)|router]] manufacturer to use it.<ref>Cisco Systems (2013), [http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6554/ps6599/ps6630/qa_C67-726299.html Enhanced Interior Gateway Routing Protocol (EIGRP) Informational RFC Frequently Asked Questions], retrieved 14 September 2013</ref>
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| EIGRP allows a [[router (computing)|router]] to share information it knows about the network with neighbouring [[router (computing)|routers]] within the same logical area known as an [[Autonomous System (Internet)|autonomous system]]. Contrary to other well known routing protocols, such as [[routing information protocol]], EIGRP only shares information that a neighbouring router would not have, rather than sending all of its information. EIGRP is optimised to help reduce the workload of the router and the amount of data that needs to be transmitted between routers.
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| == Overview ==
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| The Enhanced Interior Gateway Routing Protocol (EIGRP) replaced the now-obsolete [[Interior Gateway Routing Protocol]] (IGRP) in 1993.<ref name=difference>http://www.differencebetween.net/technology/internet/difference-between-igrp-and-eigrp</ref> One of the major reasons for this was because the design of the [[Internet Protocol]] had been changed to support [[IPv4 address|classless IPv4 addresses]], which IGRP could not support.<ref name=difference />
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| Almost all [[router (computing)|routers]] contain a [[routing table]], which contains information needed by the router to forward traffic to a network. If the router does not contain a valid path to the network, the traffic is discarded by the router and will never reach its intended destination. EIGRP is a [[dynamic routing|dynamic routing protocol]] that allows routers to share the information with other routers that have EIGRP correctly configured. This eases the workload on a [[network administrator]] who does not have configure changes to the [[routing table]] manually.
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| In addition to the [[routing table]], EIGRP also uses the following tables to store information:-
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| * '''Neighbor Table''': The neighbor table keeps a record of the [[IP addresses]] of [[router (computing)|routers]] that have a direct physical connection with this router. Routers that are connected to this router indirectly, through another router are not recorded in this table as they are not considered neighbours.
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| * '''Topology Table: ''' The topology table stores routes that it has learned from neighbour routing tables. Unlike a routing table, the topology table does not store all routes, but only routes that have been determined by EIGRP. The topology table also records the metrics for each of the listed EIGRP routes, the feasible successor and the successors. Routes in the topology table are marked as "passive" or "active". Passive indicates that EIGRP has determined the path for the specific route and has finished processing. Active indicates that EIGRP is still trying to calculate the best path for the specific route. Routes in the topology table are not usable by the router until they are inserted into the routing table. The topology table is never used by the [[router (computing)|router]] to forward traffic. Routes in the topology table will not be inserted into the routing table if they are passive, are a feasible successor, or have a higher [[administrative distance]] than an equivalent path.<ref name=admin />
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| 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, 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 as it is no longer available. 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 the routing table was last updated. EIGRP does not send its routing table periodically, but will only sent routing table data when an actual change has occurred.
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| When a [[router (computing)|router]] running EIGRP is connected to another router also running EIGRP, information is exchanged between the two routers and a relationship is formed known as an adjacency. The entire routing table is exchanged between both routers at this time. After this has occurred, only differential changes are sent.
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| == Features ==
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| The Enhanced Interior Gateway Routing Protocol (EIGRP) supports the following features:<ref name="globalknowledge">{{citation |publisher=Global Knowledge Training LLC |year=2013 |title=Cisco Training White Paper |accessdate=17 September 2013 |url=http://www.globalknowledge.com/training/whitepaperdetail.asp?pageid=502&wpid=663}}</ref>
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| * Support for [[Classless Inter-Domain Routing]] (CIDR) and variable length subnet masking. Routes are not summarised at the classful network boundary unless auto summary is enabled.
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| * Support for load balancing on parallel links between sites.
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| * The ability to use different authentication passwords at different times.
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| * [[MD5]] authentication between two routers.
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| * Sends topology changes, rather than sending the entire routing table when a route is changed.
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| * Periodically checks if a route is available and propagates routing changes to neighbouring routers if any changes have occurred.
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| * Runs separate [[routing]] processes for [[Internet Protocol]] (IP), [[IPv6]], [[IPX]] and [[AppleTalk]] through the use of [[protocol-dependent module]]s (PDMs).
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| * Backwards compatibility with the IGRP routing protocols.<ref name=intro>http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml</ref>
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| ==Configuration==
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| ===Cisco IOS example===
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| Example of setting up EIGRP on a Cisco IOS router for a [[private network]]. The 0.0.15.255 [[wildcard character|wildcard]] in this example indicates a subnetwork with a maximum of 4094 hosts—it is the [[bitwise operation#NOT|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.
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| <code>
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| Router> '''enable'''
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| Router# '''config terminal'''
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| Router(config)# '''router eigrp 1'''
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| Router(config-router)# '''network 10.201.96.0 ?'''
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| A.B.C.D EIGRP wild card bits
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| <cr>
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| Router(config-router)# '''network 10.201.96.0 0.0.15.255'''
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| Router(config-router)# '''no auto-summary'''
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| Router(config-router)# '''end'''</code>
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| == Technical Details ==
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| EIGRP is a [[distance vector]] [[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 four metrics; load, delay, reliability and MTU.{{citation needed|date=November 2013}}
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| 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.<ref name=admin>Cisco Systems (2013), [http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080094195.shtml What is Administrative Distance?], retrieved 14 September 2013</ref>
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| 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 neighbours completely.<ref>{{cite web |url=http://packetlife.net/blog/2009/jan/17/rtp-eigrp/ |publisher=Packet Life |title=RTP in EIGRP |date=2009-01-17}}</ref><ref>Shamim,Aziz,Liu, Martey 2002, Troubleshooting IP Protocols, Cisco Press USA, accessed 23 November 2013, <http://books.google.com.au/books?id=fzBOZDGBDDgC&lpg=PA214&ots=eyWtKIr1dc&dq=reliable%20transport%20protocol%20eigrp&pg=PA214#v=onepage&q=reliable%20transport%20protocol%20eigrp&f=true></ref> The reliable transport protocol also contains other mechanisms to maximise efficiency and support [[multicast]]ing.<ref name="globalknowledge"/> EIGRP uses [[List of IP protocol numbers|protocol number]] 88.<ref name="globalknowledge"/>
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| === Distance vector routing protocol ===
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| Cisco Systems now classifies EIGRP as a distance vector routing protocol.<ref name=intro /> While EIGRP is an advanced routing protocol that combines many of the features from both link-state and distance-vector routing protocol, EIGRP's DUAL algorithm contains many features which make it more of a distance vector routing protocol rather than a link-state routing protocol.<ref>Ashraf, Muhammad Irfan, et al. "[http://www.ijcsmr.org/vol2issue4/paper346.pdf Comparative Analysis of Link State and Hybrid Routing Protocols]."</ref> Despite this, EIGRP does contains many differences to most other distance-vector routing protocols, including:-<ref name=Albrightson>Albrightson, R., Garcia-Luna-Aceves, J. J., & Boyle, J. (1994, May). EIGRP a fast routing protocol based on distance vectors. In Proc. Networld/Interop (Vol. 94, pp. 136-147).</ref>
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| * The use of explicit hello packets to discover and maintain adjacencies between routers.
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| * The use of a reliable protocol to transport routing updates.
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| * The use of a feasibility condition to select a loop-free path.
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| * The use of diffusing computations to involve the affected part of network into computing a new shortest path.
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| ===EIGRP composite and vector metrics===
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| EIGRP associates six different vector metrics with each route and considers only four of the vector metrics in computing the Composite metric:
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| <code>
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| Router1> '''enable'''
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| Router1# '''show ip eigrp topology 10.0.0.1 255.255.255.255'''
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| IP-EIGRP topology entry for 10.0.0.1/32
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| State is Passive, Query origin flag is 1, 1 Successor(s), FD is 40640000
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| Routing Descriptor Blocks:
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| 10.0.0.1 (Serial0/0/0), from 10.0.0.1, Send flag is 0x0
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| Composite metric is (40640000/128256), Route is Internal
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| Vector metric:
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| Minimum bandwidth is 64 Kbit
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| Total delay is 25000 microseconds
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| Reliability is 255/255
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| Load is 197/255
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| Minimum MTU is 576
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| Hop count is 2</code>
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| ;Bandwidth: Minimum Bandwidth (in kilobits per second) along the path from router to destination network.
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| ;Load: Number in range 1 to 255; 255 being saturated
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| ;Total Delay: Delay, in 10s of microseconds, along the path from router to destination network
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| ;Reliability: Number in range 1 to 255; 255 being the most reliable
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| ;MTU:Minimum path [[Maximum Transmission Unit]] (MTU) (never used in the metric calculation)
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| ;Hop Count: Number of routers a packet passes through when routing to a remote network, used to limit the EIGRP AS. EIGRP maintains a hop count for every route, however, the hop count is not used in metric calculation. It is only verified against a predefined maximum on an EIGRP router (by default it is set to 100 and can be changed to any value between 1 and 255). Routes having a hop count higher than the maximum will be advertised as unreachable by an EIGRP router.
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| ====The K values====
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| There are five K values used in the Composite metric calculation – K1 through K5. The K values only act as multipliers or modifiers in the composite metric calculation. K1 is not equal to Bandwidth, etc.
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| 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 the K values as needed to consider the other Vector metrics.
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| For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric:
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| : <math>\bigg [ \bigg ( K_1 \cdot {\text{Bandwidth}}_{E} + \frac{K_2 \cdot {\text{Bandwidth}}_{E}}{256-\text{Load}} + K_3 \cdot {\text{Delay}}_{E}
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| \bigg )
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| \cdot \frac {K_5}{K_4 + \text{Reliability}} \bigg ] \cdot 256</math>
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| where the various constants (<math>K_1</math> through <math>K_5</math>) can be set by the user to produce varying behaviors. An important and unintuitive fact is that if <math>K_5</math> is set to zero, the term <math>\tfrac {K_5}{K_4 + \text{Reliability}}</math> '''is not used (i.e. taken as 1)'''.
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| The default is for <math>K_1</math> and <math>K_3</math> to be set to 1, and the rest to zero, effectively reducing the above formula to <math>({\text{Bandwidth}}_{E} + \text{Delay}_{E}) \cdot 256</math>.
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| Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent [[routing loop]]s 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.
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| EIGRP scales the interface ''Bandwidth'' and ''Delay'' configuration values with following calculations:
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| : <math>{\text{Bandwidth}}_{E}</math> = 10<sup>7</sup> / Value of the ''bandwidth'' interface command
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| : <math>{\text{Delay}}_{E}</math> = Value of the ''delay'' interface command
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| 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<sup>7</sup> kbit/s (i.e. 10 Gbit/s) by the interface bandwidth statement value yields a result that is used in 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.
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| [[IGRP]] uses 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 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.
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| ===Feasible successor===
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| 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|feasibility condition]].
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| 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.
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| 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.
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| 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.
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| 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.
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| ===Active and passive state===
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| 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|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 remains in the ''passive'' state.
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| ===Feasibility condition===
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| 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 a loop-free route to a destination. Its simplified formulation is strikingly simple:
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| :''If, for a destination, a neighbor router advertises a distance that is strictly lower than our feasible distance, then this neighbor lies on a loop-free route to this destination.''
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| or in other words,
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| :''If, for a destination, a neighbor router tells us that it is closer to the destination than we have ever been, then this neighbor lies on a loop-free route to this destination.''
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| 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 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.
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| ==References==
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| {{reflist|colwidth=30em}}
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| * {{Citation
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| | last = Cisco Systems
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| | author-link = Cisco Systems
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| | title = Enhanced Interior Gateway Routing Protocol
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| | date = 2005-09-09
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| | url = http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml
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| | id = Document ID 16406
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| | accessdate = 2008-04-27 }}.
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| * {{Citation
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| | last = Cisco Systems
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| | author-link = Cisco Systems
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| | title = Internetworking Technology Handbook: Enhanced Interior Gateway Routing Protocol (EIGRP)
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| | date = n.d.
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| | url = http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Enhanced_IGRP.html
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| | accessdate = 2008-04-27 }}.
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| * {{Citation
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| | last = Cisco Systems
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| | author-link = Cisco Systems
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| | title = Introduction to EIGRP
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| | date = 2005-08-10
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| | url = http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml
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| | id = Document ID 13669
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| | accessdate = 2008-04-27 }}.
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| * {{Citation
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| | last = Lammle
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| | first = Todd
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| | title = CCNA Cisco Certified Network Associate Study Guide
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| | place = [[Indianapolis, Indiana]]
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| | publisher = [[Wiley Publishing]]
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| | year = 2007
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| | edition = Sixth
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| | isbn = 978-0-470-11008-9 }}.
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| * {{Citation
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| | last = Cisco Systems
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| | author-link = Cisco Systems
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| | title = EIGRP Information Draft
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| | date = 2013-02-18
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| | url = http://www.ietf.org/staging/draft-savage-eigrp-00.txt
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| | id = rfc number not yet assigned
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| | accessdate = 2013-02-18 }}.
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| ==External links==
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| * {{cite web |url=http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml |title=Introduction to EIGRP |publisher=Cisco Systems |date=2005-08-10}}
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| [[Category:Cisco protocols]]
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| [[Category:Routing protocols]]
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| [[he:Interior Gateway Routing Protocol#EIGRP]]
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