Enhanced Interior Gateway Routing Protocol - Wikipedia, the free encyclopedia

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Enhanced Interior Gateway Routing Protocol
From Wikipedia, the free encyclopedia

Enhanced Interior Gateway Routing Protocol - (EIGRP) Internet Protocol Suite
is a Cisco proprietary routing protocol loosely based on their
Application Layer
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 BGP · DHCP · DNS · FTP · HTTP · IMAP ·
IRC · LDAP · MGCP · NNTP · NTP · POP ·
of bandwidth and processing power in the router. Routers
RIP · RPC · RTP · SIP · SMTP · SNMP ·
that support EIGRP will automatically redistribute route SSH · Telnet · TLS/SSL · XMPP ·
information to IGRP neighbors by converting the 32 bit
EIGRP metric to the 24 bit IGRP metric. Most of the routing (more)
optimizations are based on the Diffusing Update Algorithm
(DUAL) work from SRI, which guarantees loop-free Transport Layer
operation and provides a mechanism for fast convergence.
TCP · UDP · DCCP · SCTP · RSVP · ECN ·

(more)
Contents Internet Layer

1 Basic operation IP (IPv4, IPv6) · ICMP · ICMPv6 · IGMP ·
2 EIGRP Composite and Vector metrics IPsec ·
3 Successor
4 Feasible Successor (more)
5 Active and Passive State
Link Layer
6 Reported Distance and Feasible Distance
7 Feasibility Condition ARP/InARP · NDP · OSPF ·
8 EIGRP classification as a distance-vector Tunnels (L2TP) · PPP · Media Access
9 Other details
Control (Ethernet, DSL, ISDN, FDDI) ·
10 References
(more)
11 External links

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.

Routing table: Stores the actual routes to all destinations; the routing table is populated from the
topology table with every destination network that has its successor and optionally feasible successor
identified (if unequal-cost load-balancing is enabled using the variance command). The successors and
feasible successors serve as the next hop routers for these destinations.

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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. Also, it uses route tagging.

EIGRP Composite and Vector metrics
EIGRP associates six (6) different vector metrics with each route and considers only four (4) of the vector
metrics in computing the Composite metric:

Router>show ip eigrp topology 10.0.0.1 255.255.255.255
IP-EIGRP topology entry for 10.0.0.1/32
State is Passive, Query origin flag is 1, 1 Successor(s) , FD is 40640000
Routing Descriptor Blocks:
10.0.0.1 (Serial0/0/0) , from 10.0.0.1, Send flag is 0x0
Composite metric is (40640000/128256) , Route is Internal
Vector metric:
Minimum bandwidth is 64 Kbit
Total delay is 25000 microseconds
Reliability is 255/255
Load is 197/255
Minimum MTU is 576
Hop count is 1

Bandwidth

Minimum Bandwidth (in kilobits per second) along the path from router to destination network

Load

Load (number in range 1 to 255; 255 being saturated)

Delay

Total Delay (in 10s of microseconds) along the path from router to destination network

Reliability

Reliability (number in range 1 to 255; 255 being the most reliable)

MTU

Minimum path Maximum Transmission Unit (MTU) (never used in the metric calculation)

Hop Count

Number of routers a packet passes through when routing to a remote network, used to limit the EIGRP
AS.

The K Values There are five (5) 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.

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.

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

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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 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
(setting this only affects metric calculation and not actual line bandwidth). Dividing a value of 107 kbit/s (i.e.
10 Gbit/s) by the interface bandwidth statement 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 255). Routes having a hop count higher than the maximum will be
advertised as unreachable by an EIGRP router.

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

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the routing table as next-hops for that destination.

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

it is guaranteed not to be a part of some routing loop

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
(e.g. 12.4), this range is between 1 and 16.

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.

Reported Distance and Feasible Distance
Reported Distance (RD) is the total metric along a path to a destination network as advertised by an
upstream neighbor.[1] This distance is sometimes also called a Advertised Distance (AD) and is equal to the
current lowest total distance through a successor for a neighboring router.

A Feasible Distance (FD) is the lowest known distance from a router to a particular destination. This is the
Reported Distance (RD) + the cost to reach the neighboring router from which the RD was sent.[1] It is
important to note that this metric represents 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,

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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 RD 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 RD 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 RD gets updated to 12, however, the
FD will remain at the value of 8. Therefore, the values of RD 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 RD and FD to the new shortest path length, in this case, 10.

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 RD < 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 (http://www.soe.ucsc.edu
/research/ccrg/publications/jj.dual.ton93.pdf) .

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.

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.

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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 native 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.

Other details
EIGRP supports Classless Inter-Domain Routing (CIDR), allowing the use of variable-length subnet
masks—one of the protocol's improvements over its predecessor.

EIGRP is not usable in applications where routers need to know the exact network topology (for example,
traffic engineering in MPLS).[citation needed]

EIGRP can run separate routing processes for Internet Protocol (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 for a private network. 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
<cr>
Router(config-router)# network 10.201.96.0 0.0.15.255
Router(config-router)# no auto-summary
Router(config-router)# end

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References
1. ^ a b http://www.cisco.com/en/US/tech/tk365
/technologies_white_paper09186a0080094cb7.shtml#feasibleandreported

Cisco Systems (2005-09-09), Enhanced Interior Gateway Routing Protocol (http://www.cisco.com
/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml) , Document ID 16406,
http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094cb7.shtml,
retrieved 2008-04-27.
Cisco Systems (n.d.), Internetworking Technology Handbook: Enhanced Interior Gateway Routing
Protocol (EIGRP) (http://www.cisco.com/en/US/docs/internetworking/technology/handbook
/Enhanced_IGRP.html) , http://www.cisco.com/en/US/docs/internetworking/technology/handbook
/Enhanced_IGRP.html, retrieved 2008-04-27.
Cisco Systems (2005-08-10), Introduction to EIGRP (http://www.cisco.com/en/US/tech/tk365
/technologies_tech_note09186a0080093f07.shtml) , Document ID 13669, http://www.cisco.com
/en/US/tech/tk365/technologies_tech_note09186a0080093f07.shtml, retrieved 2008-04-27.
Lammle, Todd (2007), CCNA Cisco Certified Network Associate Study Guide (Sixth ed.),
Indianapolis, Indiana: Wiley Publishing, ISBN 978-0-470-11008-9.

External links
Cisco IOS IPv6 Configuration Guide, Release 12.4: Implementing EIGRP for IPv6
(http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/ip6-eigrp.html)
EIGRP—A Fast Routing Protocol Based on Distance Vectors (http://www.cse.ucsc.edu/research
/ccrg/publications/interop94.pdf)
IGRP Metric (http://www.cisco.com/en/US/tech/tk365
/technologies_tech_note09186a008009405c.shtml)
Loop-free Routing Using Diffusing Computations (http://www.soe.ucsc.edu/research/ccrg/publications
/jj.dual.ton93.pdf)
Termination Detection for Diffusing Computations (http://www.cs.utexas.edu/users/EWD/ewd06xx
/EWD687a.PDF)
What you need to know about EIGRP (http://www.setup32.com/network-administration/networking
/know-eigrp.php)
EIGRP IP default network command (http://blog.ipexpert.com/2010/03/29/eigrp-ip-default-network-
command/)
EIGRP route selection animation (http://www.visualland.net/view.php?cid=1243&protocol=EIGRP&
title=5.%20Route%20Selection&ctype=2)
Retrieved from "http://en.wikipedia.org/wiki/Enhanced_Interior_Gateway_Routing_Protocol"
Categories: Cisco protocols | Routing protocols

This page was last modified on 2 April 2011 at 23:23.
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