Myths and Truths about EIGRP

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Myths and Truths

about EIGRP

Peter Pal
úch, CCIE
#23527, CCIP, CCAI

Cisco CSC Designated VIP 2011

Networking Academy Webinar

February 29
, 2012


Rationale behind this session

EIGRP has been supported since 1992 in IOS 9.21

Despite its long existence, EIGRP still proves to be little
known and poorly understood when it comes to details

There are many materials about EIGRP published

Sadly, many of them are superficial

Even more sadly, quite a few of them contain incomplete,
inaccurate or outright misleading and incorrect information

Certain topics are notorious for this

Not even official materials have been spared

This session would like to dispel some of the most
common myths and superstitions about EIGRP


About EIGRP’s Nature (1)

EIGRP was described in the past as “balanced hybrid”

Combining Distance
Vector (DV) and Link
State (LS) properties

To decide what EIGRP really is, we need to look at what
classifies the protocol as either DV or LS

What defines a LS protocol?

LS works by routers exchanging
topological elements
, i.e. exact
information about
routers themselves

and their

Addresses, networks and metrics are merely attributes of routers
and their interconnections

Each router knows all other routers and their connections
precisely; all routers have identical link
state database

Using the link
state database of any single router, administrator
can draw the diagram of the entire network


About EIGRP’s Nature (2)

What defines a DV protocol?

DV works by routers exchanging
, i.e. arrays,
of distances
to individual known

level explanation of the DV as exchanging “metrics and
directions” is perhaps illustrative but imprecise

No topological information (i.e. what are the individual routers,
how are they interconnected) is ever exchanged or known

does not

influence the routing protocol’s type?

Type of metrics used

Full vs. incremental updates

Periodic vs. event
based updates

Usage of a Hello mechanism and establishing neighborships

Usage of a reliable transport mechanism


A Network Topology Example














What would Router A see in LS and DV?

















D, E,

F, G

In Link
State Protocol

In Distance
Vector Protocol





What information does EIGRP carry?

In EIGRP, routing information is
carried inside Update packets

In an Update, after header, an array
of records follows:

Type and size of the record in the
array (IP internal, IP external, …)

Next Hop

Metric Values (Delay, Bandwidth,
MTU, Hop Count, Reliability, Load)

Address information (prefix, netmask)

This is a
vector of distances


About EIGRP’s Nature (3)

EIGRP carries detailed information about a particular path’s
properties but there is no topological information

No matter how diverse the component metrics are, they are only
metrics and do not reveal the complete picture of the network

Addition of mechanisms formerly typical for LS protocols does
not make EIGRP a hybrid protocol

A hybrid protocol would need to maintain a LS database for a limited
part of network, describing the rest using a DV approach

This is the principle of areas in LS protocols

Hello mechanism, neighborships, event
driven incremental updates

all these only increase the effectivity of EIGRP communication

They do not change the nature of routing information itself

The conclusion about EIGRP’s nature:

Advanced Distance


Hybrid? No way


About EIGRP’s Metrics (1)

EIGRP’s metric system includes six components

Bandwidth (static, min along path)

Delay (static, cumulative)

Reliability (dynamic, min along path)

Load (dynamic, max along path)

MTU (dynamic, min along path)

Hop Count (dynamic, cumulative)

EIGRP uses a weighted formula to compute a single
composite metric using the first four components

MTU and Hop Count are not used in this formula

All metrics are

advertised in their individual form

Delay and Hop Count are added to

Bandwidth, Reliability, MTU are minimized, Load is maximized


About EIGRP’s Metrics (2)

A couple of misconceptions exists about the K

The K
values are sometimes confused with metrics themselves

The K
values are thought to control the individual metrics, in
particular, K5 is thought to control the MTU usage

The K
values are simply weights applying to diverse
metric components

MTU is not included in the formula

Regardless of K
values settings, Update packets always contain
all metric components

values are carried in Hello packets and must match neighbor’s


About EIGRP’s Metrics (3)

The Hop Count is not used in best path selection

It is used as a safety net against count
infinity situations

Hop Count Limit can be configured in EIGRP configuration using
metric maximum


Prefixes with a higher Hop Count will not be accepted

Information about MTU usage is conflicting

Some materials maintain that MTU is unused

Some other suggest that the MTU is used in cases where multiple
cost paths are available

In discussions with four independent Cisco employees, it has
been confirmed that using the MTU was an idea considered but
ultimately never actually implemented


About EIGRP’s Metrics (4)

Reliability and Load metrics are unused by default

By tweaking the K
values, they can be utilized

However, Reliability and Load metrics in EIGRP do not
behave as intuitively expected

Reliability and Load counters on interfaces are updated steadily

Simply taking them into EIGRP would necessitate sending
Updates every moment these counters change

This could create extensive churn in routing tables, possibly leading
to ever increasing swings in these metrics values

In reality, advertised values of Reliability and Load always
correspond to their state in the moment of sending the update

They are set when advertising a route

Updates are not sent as a result of Reliability and Load changes


About EIGRP’s Metrics (5)

Thus, Reliability and Load are largely useless in EIGRP

Why are they even included, then?

The reason is backward compatibility and seamless
interoperability with IGRP that originally introduced them

If both IGRP and EIGRP were configured on a router in the same
AS, automatic redistribution between IGRP and EIGRP took place

Out of all EIGRP’s metrics, only Bandwidth and Delay are
usable in a reasonable way

Bandwidth should never be used to influence routing decisions

It behaves non

Various IOS mechanisms depend on correct bandwidth setting

If EIGRP metric is to be influenced, the Delay should be modified


About EIGRP’s Terminology (1)

EIGRP uses an arcane terminology that is sometimes
troublesome to be used or understood in a proper way

Autonomous System: really a process number

Topology Table: does not contain any topological information

Successor and Feasible Successor: these are routers, not routes

Advertised Distance: its acronym of AD often gets confused with
Administrative Distance, a totally unrelated concept

Feasible Distance: it is not the current best distance, neither a
total distance through a particular neighbor

The Feasible Distance (FD) is one of the most
misunderstood concepts in EIGRP

Before diving into details about FD, let’s make a small
experiment in the network topology


Network Topology with Addressing

Assume that, initially, all links are configured with the same Delay=10
and K
values are 0, 0, 1, 0, 0 (on R5, the stub network’s Delay=1)

Now, assume that the Delay on links from R1 to R2
R4 gets increased to
20, resulting in the metric growup

What happens to FD?







About EIGRP’s Terminology (2)

FD is not the current best path metric

FD is not the total distance via a particular neighbor

Rather, FD is a record of the smallest distance to the
destination since the last time the route went Passive

In other words, the FD is the historical minimum of the distance to
the particular destination

The history here ends and starts anew with the route transitioning
from Active to Passive state

During Passive state, the FD may only decrease

The only way to increase FD is to engage in a diffusing
computation and reset the FD after finishing it

FD is a local variable associated with each known prefix and is
never sent to any other EIGRP router


About EIGRP’s Terminology (3)

Using this notion of the FD, the Feasibility Condition can
be rewritten as:

“If a neighbor is closer to the destination than I have ever been, it
is guaranteedly not on a routing loop that would eventually lead
back to me.”


About EIGRP’s Feasible Successors (1)

To recall:

A successor must meet two constraints: pass the FC check and
provide the least total distance to the destination

A feasible successor must only pass the FC check

A FS provides a convenient back
up next hop in case the
current successor fails

It is widely believed that if the successor fails and at least one
feasible successor is known, it will always be used

However, there are situations in which a feasible
successor is known

and yet, when successor fails,
router will enter Active mode instead of using the FS


Network Topology with Modified Metrics

Let’s focus on the route from R1 to the network behind R5

FD = 21

R4: successor, Total Distance = 21*256 = 5376, RD = 11*256 = 2816

R3: feasible successor, Total Distance = 36*256 = 9216, RD = 11*256 = 2816

R2: fails FC check, Total Distance = 28*256 = 7168, RD = 23*256 = 5888














About EIGRP’s Feasible Successors (2)

Assume that R4 as a successor fails

R1 will determine that while R3 is a FS, R2 has even better total
metric towards the destination but it does not pass the FC check

R1 will move the route into Active state and send queries

After receiving all replies and going Passive, R1 will reset the FD
and set it to the metric of the newly found route towards the

The FS, although identified, will not be used in this case

Satisfying ourselves with the current FS would make us stay with
using a worse route than what is objectively available

The true logic regarding the use of FS

After successor fails, find the neighbor providing the next least
cost route to the destination

If it is a FS, start using it, otherwise, enter the Active state


About EIGRP’s SIA state (1)

The process of diffusing computation in EIGRP depends on
correct and timely arrival of Replies to all Queries

Once a router sends a Query, it must wait to receive Replies from all
queried neighbors before starting to send Replies itself

Delays in waiting for a single Reply will slow down the entire process
of convergence to a new path

One of the most sensitive weak spots in EIGRP

In extreme situation, if a Reply never comes, a route in Active state
can never reach the Passive state

EIGRP uses a so
called Active timer to bound the diffusing computation
to the duration of max. 3 minutes

If a diffusing computation does not finish in this time, the router declares
a so
called Stuck
Active state and resets the adjacency with the
unresponsive neighbor

Lossy and oversubscribed links, overloaded neighbors, misbehaving
EIGRP implementation, overly redundant or deep network topology:
all of this contributes to the risk of creating a SIA state


About EIGRP’s SIA state (2)

Will this circular topology lead to SIAs?


About EIGRP’s SIA state (3)

In circular topologies, Queries may be forwarded in a way
that they eventually reach back to the router that started
the diffusing computation

It is a popular belief that this will cause a deadlock and a SIA

This statement has even made it into Cisco Press CCNP

In reality, if a router in Active state receives a Query, it
just replies immediately with the same information it has
already advertised in its own Query

Hence, circular topologies are no more prone to SIA states than
any other

In fundamental EIGRP’s algorithms, there are no deadlocks or
race conditions


Small things to be aware of (1)

EIGRP has a concept of Router ID

Chosen in a similar way to OSPF

eigrp router

If not, then by the highest active loopback IP address

If not, then by the highest IP address on all active interfaces

Router ID is displayed in the
show ip eigrp topology

Router ID is used as a prevention against routing loops, mostly caused
by circular route redistribution

Router ID is attached to redistributed routes

If a router receives a route tagged with its own Router ID, it will ignore it

Formerly, the Router ID has been attached to external (redistributed)
routes only

In recent IOS versions, the Router ID is also added to internal routes and
evaluated accordingly

See the
show ip eigrp topology X.X.X.X


Small things to be aware of (2)

Static routes defined using egress interfaces are
considered by EIGRP as directly connected

They can be injected into EIGRP using the

just like any other directly connected network

This often leads to confusion that static routes do not need to be
redistributed using the


Especially dangerous when trying to inject a default route using


This will add all directly connected networks into EIGRP

(this is
probably not what you want)

Whether the default route will be injected as well depends on how it is

it would have to be a static route via egress interface


Small things to be aware of (3)

When configuring metrics, it is good to avoid values close
to the maximum value

Delay can be defined in the range of 1


The maximum delay value represents infinity and a network with
such delay value will be considered unreachable

Remember that delay is cumulative

The variance code in EIGRP has a small glitch

Variance of “V” allows to use all FS routes whose metric is in the
interval of <BM, V * BM> where BM is the current best metric

In certain networks, even if the variance seems to be sufficient,
FS routes are mysteriously not included into the routing table

It turns out that the EIGRP code suffers from a trivial unsigned
integer overflow: the product of V*BM is not capped at 2
1 and
instead, it overflows, possibly producing a smaller value



EIGRP is a unique protocol in many aspects

As a proprietary protocol, its details are understandably
less publicly known and understood

Great care must be taken not to propagate incorrect
information about its workings

Huge THANK YOU to Donald Slice, Edison Ortiz, Russ White and
Riccardo Simoni for clarifying some of the obscure facts


Thank you!

Peter Pal