1. Myths and Truths about EIGRP
Peter Palúch, CCIE #23527, CCIP, CCAI Cisco CSC Designated VIP
Networking Academy Webinar February 29th, 2012

2. 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

3. 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 adjacencies
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

4. About EIGRP’s Nature (2)
What defines a DV protocol?
DV works by routers exchanging vectors, i.e. arrays, of distances to individual known networks
The CCNA-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
What 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

5. A Network Topology ExampleD B E R1 R3 R5 G C F R4

6. What would Router A see in LS and DV?B C B E R1 R3 R5
D, E, F, G
D, E, F, G C
D, E, F, G F G R4 R2 R3 R4
In Link-State Protocol
In Distance-Vector Protocol

7. 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!

8. 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 etc. – 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-Vector – absolutely!
Hybrid? No way 

9. 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 always advertised in their individual form
Delay and Hop Count are added to
Bandwidth, Reliability, MTU are minimized, Load is maximized

10. About EIGRP’s Metrics (2)
A couple of misconceptions exists about the K-values
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
K-values are carried in Hello packets and must match neighbor’s

11. About EIGRP’s Metrics (3)
The Hop Count is not used in best path selection
It is used as a safety net against count-to-infinity situations
Hop Count Limit can be configured in EIGRP configuration using the metric maximum-hops command
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 equal-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

12. 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

13. 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-linearly
Various IOS mechanisms depend on correct bandwidth setting
If EIGRP metric is to be influenced, the Delay should be modified

14. 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

15. 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? R2
/24
/24
/24
/24 R1 R3 R5
/24
/24
/24 R4

16. 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

17. 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.”

18. 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

19. 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 R2 5 22 25 10 R1 R3 R5 1 10 10 R4

20. 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 destination
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

21. 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-in-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

22. About EIGRP’s SIA state (2)
Will this circular topology lead to SIAs?

23. 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 textbooks
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

24. Small things to be aware of (1)
EIGRP has a concept of Router ID
Chosen in a similar way to OSPF
By eigrp router-id command
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 output
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 output

25. 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 network command just like any other directly connected network
This often leads to confusion that static routes do not need to be redistributed using the redistribute command
Especially dangerous when trying to inject a default route using network command
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 defined – it would have to be a static route via egress interface

26. 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 – 16,777,215
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 and instead, it overflows, possibly producing a smaller value

27. Conclusions 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

28. Thank you!
Peter Palúch