Enhanced Interior Gateway Routing Protocol

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Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol loosely based on their original IGRP. EIGRP is an advanced distance-vector routing protocol, with optimizations to minimize both the routing instability incurred after topology changes, as well as the use of bandwidth and processing power in the router. Routers that support EIGRP will automatically redistribute route information to IGRP neighbors by converting the 32 bit EIGRP metric to the 24 bit IGRP metric. Most of the routing optimizations are based on the Diffusing Update Algorithm (DUAL) work from SRI, which guarantees loop-free operation and provides a mechanism for fast convergence.

Contents

[edit] Basic operation

The data EIGRP collects is stored in three tables:

  • Neighbor Table: Stores data about the neighboring routers, i.e. those directly accessible through directly connected interfaces.
  • Topology Table: Confusingly named, this table does not store an overview of the complete network topology; rather, it effectively contains only the aggregation of the routing tables gathered from all directly connected neighbors. This table contains a list of destination networks in the EIGRP-routed network together with their respective metrics. Also for every destination, a successor and a feasible successor are identified and stored in the table if they exist. Every destination in the topology table can be marked either as "Passive", which is the state when the routing has stabilized and the router knows the route to the destination, or "Active" when the topology has changed and the router is in the process of (actively) updating its route to that destination.

Unlike most other distance vector protocols, EIGRP does not rely on periodic route dumps in order to maintain its topology table. Routing information is exchanged only upon the establishment of new neighbor adjacencies, after which only changes are sent.

[edit] Multiple metrics

EIGRP associates five different metrics with each route:

  • Total Delay (in 10s of microseconds)
  • Minimum Bandwidth (in kilobits per second)
  • Reliability (number in range 1 to 255; 255 being most reliable)
  • Load (number in range 1 to 255; 255 being saturated)
  • Minimum path Maximum Transmission Unit (MTU) (though not actually used in the calculation)

For the purposes of comparing routes, these are combined together in a weighted formula to produce a single overall metric:

\bigg [ \bigg ( K_1 \cdot \text{Bandwidth} + \frac{K_2 \cdot \text{Bandwidth}}{256-\text{Load}} + K_3 \cdot \text{Delay} \bigg ) \cdot \frac {K_5}{K_4 + \text{Reliability}} \bigg ] \cdot 256

where the various constants (K1 through K5) can be set by the user to produce varying behaviors. An important and totally non-obvious fact is that if K5 is set to zero, the term \tfrac {K_5}{K_4 + \text{Reliability}} is not used (i.e. taken as 1).

The default is for K1 and K3 to be set to 1, and the rest to zero, effectively reducing the above formula to (Bandwidth + Delay) * 256.

Obviously, these constants must be set to the same value on all routers in an EIGRP system, or permanent routing loops will probably 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 Bandwidth and Delay metrics with following calculations:

Bandwidth for EIGRP = 107 / Interface Bandwidth
Delay for EIGRP = Interface Delay / 10

On Cisco routers, the interface bandwidth is a configurable static parameter expressed in kilobits per second. Dividing a value of 107 kbit/s (i.e. 10 Gbit/s) by the interface bandwidth yields a value that is used in the weighted formula. Analogously, the interface delay is a configurable static parameter expressed in microseconds. Dividing this interface delay value by 10 yields a delay in units of tens of microseconds that is used in the weighted formula.

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.

EIGRP also 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 224). Routes having a hop count higher than the maximum will be advertised as unreachable by an EIGRP router.

[edit] Successor

A successor for a particular destination is a next hop router that satisfies these two conditions:

  • it provides the least distance to that destination
  • it is guaranteed not to be a part of some routing loop

The first condition can be satisfied by comparing metrics from all neighboring routers that advertise that particular destination, increasing the metrics by the cost of the link to that respective neighbor, and selecting the neighbor that yields the least total distance. The second condition can be satisfied by testing a so-called Feasibility Condition for every neighbor advertising that destination. There can be multiple successors for a destination, depending on the actual topology.


The successors for a destination are recorded in the topology table and afterwards they are used to populate the routing table as next-hops for that destination.

[edit] Feasible Successor

A feasible successor for a particular destination is a next hop router that satisfies this condition:

This condition is also 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 routers 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 die. 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 (eg. 12.4), this range is between 1 and 16.

[edit] Active and Passive State

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 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 remains in the Passive state.

[edit] Advertised Distance and Feasible Distance

Advertised Distance (AD) is the distance to a particular destination as reported by a router to its neighbors. This distance is sometimes also called a Reported Distance and is equal to the current lowest total distance through a successor.

A Feasible Distance (FD) is the lowest known distance from a router to a particular destination since the last time the route went from Active to Passive state. It can be expressed in other words as a historically lowest known distance to a particular destination. While a route remains in Passive state, the FD is updated only if the actual distance to the destination decreases, otherwise it stays at its present value. On the other hand, if a router needs to enter Active state for that destination, the FD will be updated with a new value after the router transitions back from Active to Passive state. This is the only case when the FD can be increased. The transition from Active to Passive state in effect marks the start of a new history for that route.

For example, if the route to a newly discovered destination X went from Active to Passive state with a total distance of 10, the router sets the AD and FD to 10. Later this distance decreases from 10 to 8. The distance remains in the Passive state (because distance decrease never violates the Feasibility Condition) and the router updates the AD and FD to 8. Even later, the distance increases to 12 but in such a way that there is still a valid successor or feasible successor available. In this case, the AD gets updated to 12, however, the FD will remain at the value of 8. Therefore, the values of AD and FD can be different. Finally, the actual successor fails and no other feasible successor is currently identified. Therefore, the router has to transition to Active state and ask its neighbors for a new route to the destination X. Assuming that the newly found path to that destination has a total distance of 100, the router will transition back to Passive state and update both its AD and FD to the new shortest path length, in this case, 100.

[edit] Feasibility Condition

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:

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.

or in other words,

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.

In exact terms, every neighbor that satisfies the relation AD < FD for a particular destination is on a loop-free route to that destination.

This condition is also called the Source Node Condition and is one of more equivalent conditions that were proposed and proven by Dr. J. J. Garcia-Luna-Aceves at SRI. The paper proposing the Source Node Condition and the Diffusing Update Algorithm algorithm itself can be found here.

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-evalued for possible usage if the router transitions to Active state for that destination.

[edit] EIGRP classification as a distance-vector

In the past, EIGRP was described in various Cisco marketing materials as a balanced hybrid routing protocol, allegedly combining the best features from link-state and distance-vector protocols. This description is not correct from a principal point of view. By definition:

  • Distance-vector routing protocols are based on a distributed form of Bellman-Ford algorithm to find shortest paths. They work by exchanging a vector of distances to all destinations known to each node. No further topological information is ever exchanged. Thus, each node knows about all destinations present in the network and it knows the resulting distance to each destination via every of the node's neighbors. However, the node does not have any idea of the actual network topology, nor does the node need it.
  • Link-state routing protocols are based on algorithms to find shortest paths in a graph (the most often used algorithm is Dijkstra's algorithm). They work by exchanging a description of each node and its exact connections to its neighbors (in essence, each node describes its adjacencies to neighboring nodes and this information is flooded throughout the network). Therefore, each node knows the exact network topology, i.e. it has a graph representation of the network. Using this graph, each node computes the shortest paths from itself to each available destination.

The EIGRP routers exchange messages that contain information about bandwidth, delay, load, reliability and MTU of the path to each destination as known by the advertising router. Each router uses these parameters to compute the resulting distance to a destination. No further topological information is present in the messages. This principle fully corresponds to the operation of distance-vector protocols. Therefore, EIGRP is in essence a distance-vector protocol.

It is true that EIGRP uses a number of techniques not present in naïve distance-vector protocols, notably

  • 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 network into computing a new shortest path

None of these techniques, however, makes any difference to the basic principles of EIGRP, which exchanges a vector of distances to each known destination network without full knowledge of the network topology, and, as a matter of fact, similar techniques have been used in other distance-vector protocols (notably DSDV and AODV). While EIGRP is indeed an advanced distance-vector routing protocol, it is not a hybrid protocol.

An example of a true hybrid routing protocol would be the multi-area Open Shortest Path First (OSPF) protocol. The intra-area routing in OSPF is done using the link-state approach, as each area knows its precise internal topology. Inter-area routing in OSPF is done using the distance-vector approach—the networks outside an area are known only by their distance, not by their exact topology.

[edit] Other details

EIGRP is able to deal with Classless Inter-Domain Routing (CIDR), allowing the use of variable-length subnet masks—one of the protocol's main advantages over its predecessor. Its main disadvantage is that it runs only on Cisco equipment, which may lead to an organization being locked in to this vendor. Also, EIGRP is not usable in applications where routers need to know the exact network topology (for example, traffic engineering in MPLS).

EIGRP can run separate routing processes for IP, IPv6, IPX and AppleTalk through the use of protocol-dependent modules (PDMs). However, this does not facilitate translation between protocols.

Example of setting up EIGRP on a Cisco IOS router using classful IP addressing:

Router> enable
Router# config terminal
Router(config)# router eigrp ?
  <1-65535>  Autonomous system number
Router(config)# router eigrp 1
Router(config-router)# network 192.168.0.0
Router(config-router)# end

Example of setting up EIGRP on a Cisco IOS router using classless IP addressing. The 0.0.15.255 wildcard in this example indicates 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.

Router> enable
Router# config terminal
Router(config)# router eigrp 1
Router(config-router)# network 10.201.96.0 ?
  A.B.C.D  EIGRP wild card bits
Router(config-router)# network 10.201.96.0 0.0.15.255
Router(config-router)# no auto-summary
Router(config-router)# end

[edit] References

[edit] External Links