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Myths and Truths about EIGRP Peter Palúch, CCIE #23527, CCIP, CCAI Cisco CSC Designated VIP 2011-2012 Networking Academy Webinar February 29 th, 2012.

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Presentation on theme: "Myths and Truths about EIGRP Peter Palúch, CCIE #23527, CCIP, CCAI Cisco CSC Designated VIP 2011-2012 Networking Academy Webinar February 29 th, 2012."— Presentation transcript:

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

2 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 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 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 5 A Network Topology Example R1 R2 R3 R4 R5 A B C D E F G

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

7 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 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 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 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 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 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 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 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 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? R1 R2 R3 R4 R / / / / / / /24

16 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 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 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 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 R1 R2 R3 R4 R

20 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 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 22 About EIGRP’s SIA state (2) Will this circular topology lead to SIAs?

23 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 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 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 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 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 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 28 Thank you! Peter Palúch

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