The Four Elements of EIGRP

7/17/2019 The Four Elements of EIGRP

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Expert Reference Series of White Papers

The Four Elements

of EIGRP

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The Four Elements of EIGRPRaymond B. Dooley, CCSI, Global Knowledge Course Director

IntroductionAll advanced IP routing protocols (OSPF, EIGRP, IS-IS) have several elements that are always present. This isbecause the function of all routing protocols is the same, which is to find the best path to an IP destinationaddress.

The four common elements of Enhanced Interior Gateway Routing Protocol (EIGRP) are:

1. The operations, processes, and rules for the exchange of packets to accomplish the routing protocolobjective.

2. The construction and maintenance of the routing protocol database.

3. The construction and use of the data structures created by elements one and two.

4. The configuration and verification of the entire process.

The purpose of this paper is to provide an overview of the elements for the implementation of EIGRP.

EIGRP with Diffusing Update Algorithm (DUAL)

EIGRP Key Technologies

– EIGRP

• Runs directly above the IP layer

– Neighbor discovery and recovery

• Uses Hello packets between neighbors

– Reliable Transport Protocol

• Guaranteed, ordered EIGRP packet delivery to all

neighbors

• Used for flooding

88—EIGRP

6—TCP

17—UDP

FrameHeader

Frame Payload C

R

CIPHeader

ProtocolNumber

Packet Payload

EIGRP was developed by Cisco as a proprietary routing protocol for Cisco devices. Cisco has recently made EIGRPopen. Everything for a routing protocol starts with the initial exchange of packets. The packets are transported in

an IP datagram using protocol number 88.

Copyright ©2014 Global Knowledge Training LLC. All rights reserved. 2



EIGRP Packets

– Hello: Establish neighbor relationships

– Update: Send routing updates

– Query: Ask neighbors about routing information

– Query Reply: Respond to query about routing

information

– ACK: Acknowledge a reliable packet

The graphic shows the EIGRP packet types which include hello, update, query, query reply, andacknowledgement. The use of the information carried in these packets will be described later. The exchange ofpackets facilitates the following:

• A neighbor adjacency is established

• A database of reachable networks is constructed on each router

From the database, additional data structures are created—Neighbor Table, Interface Table, Routing InformationBase (RIB), Forwarding Information Base (FIB), and Adjacency database.

EIGRP is a reliable protocol because the query, query reply, and update packets are acknowledged and

retransmitted if necessary when errors are detected.




The graphic shows an initial exchange of hello packets to form a neighbor relationship between two routerssometimes called an adjacency. For the neighbor relationship to be formed, several parameters in the hellopacket must match at both ends. The default interval for the EIGRP hello is five seconds with a dead interval of

15 seconds. If 15 seconds go by without a hello, the neighbor relationship is torn down.

From this beginning, the remaining operations will take place one by one.

The Cisco DUAL algorithm defines all the rules and processes that make this work, which include:

•

Timers for retransmission of lost or corrupted packets

• Pacing parameters for how many updates can be sent one after the other

• Sequencing numbers for updates and acknowledgements to keep everything in rhythm




Verifying EIGRP Neighbors (Cont.)

1. Neighbor index

2. Neighbor IP address

3. Interface on which the neighbor is reachable

4. Remaining hold time

5. Neighbor uptime

6. Smooth round-trip time

7. Retransmission timeout

8. Number of packets to send to neighbor

9. Last sequence received

R1#show ip eigrp neighbors

IP-EIGRP neighbors for process 110H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

0 192.168.1.102 Se0/0/1 10 00:07:22 10 2280 0 5

1 2 3 4 5 6 7 8 9

All of these parameters can be verified with the show ip eigrp interface and show ip eigrp neighbor commands.

EIGRP Metric

– The use of metric components is represented by K

values

– Metric components are:

• Bandwidth (K1)

• Delay (K3)

• Reliability (K4 and K5)

• Loading (K2)

– MTU is included in the update but not used for

metric calculation

The primary purpose of any IP routing protocol is to provide the “best” path to an IP destination address (“best”is expressed as a number called a metric).




The metric is calculated in a different way by each routing protocol. With EIGRP, the DUAL algorithm defines theEIGRP composite metric as shown in the graphic. The variables in the EIGRP metric calculation are shown in thefollowing list:

• Lowest bandwidth of a routing hop in the end-to-end path is expressed in Kbps.

• Accumulated delay end-to-end expressed in tenths of microseconds

•

Load expressed as a fraction of 255 (1/255, etc...)

• Reliability expressed as a fraction of 255 (255/255 is start)

• MTU – Maximum Transmission Unit (default is 1500)

DUAL calculations use the bandwidth and delay variables in an equation to determine a “best” metric for eachdestination. This information is sent to each neighbor as part of the routing update.

Construction of the EIGRP Topology DatabaseAn EIGRP router is directly connected to one or more EIGRP routers in the same administrative domain, called anAutonomous System (AS). The Layer 2 connection between the routers is either an Ethernet or Wide Area

Network (WAN) connection.

Once the physical connection is in place EIGRP hellos will go back and forth every five seconds by default. With the proper matching of parameters (AS number, Security, and metric calculation consistency) in the helloprotocol, a neighbor relationship is established and a neighbor table and interface table are constructed.

At this point, each router sends all of its best routes to all IP destinations and all its neighbors (it’s like, “Here’s allI have.”). Sounds chaotic and it could be without a well-defined process, which starts with some definitions andrules as shown in the graphic.

Router 5 is making the routing domain aware of network 10.1.1.0/24. “Distance” is expressed as the compositemetric described earlier. Network 10.1.1.0 is advertised by R5 to Routers 1 and 4 with an advertised distance

(metric) of 500. The link from R5 to both R1 and R4 has a metric of 500. Therefore, the feasible distance to




10.1.1.0 from R1 and R4 is AD + Link Distance, or 500 + 500 = 1000. This FD of R1 and R4 now becomes an AD(advertised distance) as they send it to other neighbors.

In the drawing, AD1 is the advertisement from R1 to R4. R4 will then advertise the route to R3 with an AD(advertised distance) of 1000. Since the link from R1 to R3 is Ethernet, the link cost is 1000, so the feasible

distance (FD1) for R3 via R1 is 2000 (AD + LD, 1000 + 1000).

The route as advertised by R5 to R4 is AD2, and when the same process is used for the calculations at R3, FD2 will

be 500 + 500 + 500 + 1000 = 2500. In routing, the lowest metric is always preferred, and FD1 has a lower metric(2000 vs. 2500).

These results are shown in the graphic and are used to construct the EIGRP topology table, which is the

compilation of all the routes received from ALL of the EIGRP neighbors and their associated advertised andfeasible distances. The routing table will be described later.




EIGRP Topology Table

– The list of all routes learned from each EIGRPneighbor

– The source for the topology table: IP EIGRP

Neighbor TableR1#show ip eigrp topology

IP-EIGRP Topology Table for AS(1)/ID(172.30.13.1)

Codes: P - Passive, A - Active, U - Update, Q - Query, R - Reply,

r - reply Status, s - sia Status

P 192.168.1.0/24, 1 successors, FD is 2297856

via 10.1.115.5 (2297856/128256), Serial0/0/0.4


via 10.1.115.5 (2297856/128256), Serial0/0/0.4


via 10.1.115.5 (2297856/128256), Serial0/0/0.4

P 10.1.115.0/24, 1 successors, FD is 2169856via Connected, Serial0/0/0.4

<output omitted>

IP EIGRP Topology Table

Destination 1 FD and AD via Each Neighbor

An example of an EIGRP topology table is shown in the graphic. Note that the P on the left means passive orstable. Anything other than a P indicates that some sort of convergence event is in progress. Also note that thebest route is called a successor.

If multiple successors exist for the same network and all have an identical FD metric, load balancing (or EqualCost Multi-Pathing [ECMP]) will occur. The calculated successor will be copied to the Routing Table (or RoutingInformation Base [RIB]).

The outbound interface is shown, and in this case it is serial 0/0/0.4.

The fraction 2297856/128256 is the FD/AD for the destination.

Since EIGRP is an advance distance vector routing protocol rather than a link-state routing protocol, there is thepossibility of a routing malfunction called a routing loop. Routing loops are usually caused by the attempt toselect an alternate route that includes a failed component being advertised by a neighbor in error. Over theyears, numerous rules have been devised to solve this problem.

EIGRP implements the split horizon rule that prevents networks from being advertised out of the same interfacewhere they were learned in the first place.

The DUAL algorithm in EIGRP implements another rule called the feasible condition.

Since there is a topology database recording the best routes, the next best routes, the next after that, (and soforth), the feasible condition is applied to make sure that none of the alternative routes could cause a loop. Theability to change to a “second best” route immediately upon the failure of a route is a very good feature for fast

convergence, but it must always be loop-free.

The feasible condition is that the advertised distance (AD) of an alternative route must be less than the current

feasible distance (FD) being used to reach the route.




When this condition is met, the alternate route is called a feasible successor (FS).

If an alternate route exists and does not meet the feasible condition, then it may still be selected and installed asa route after an EIGRP query process in order to insure that it is loop-free.

DUAL Operation

– The topology table is changed when:

• The cost or state of a directly connected link changes

• An EIGRP packet (update, query, reply) is received

• A neighbor is lost

– DUAL computes an alternate path if the primary

(successor) is lost

• Local computation: a feasible successor is present inthe topology—the route is passive

• DUAL recomputation: no feasible successor is present

in the topology—the route is active

The last bullet in the graphic indicates an ACTIVE condition. The EIGRP query process is part of the ACTIVEcondition.

A query packet is sent to each neighbor asking about the lost route. Each query is acknowledged and a replymust be received for all queries, otherwise, convergence is not complete.




A runaway query process is one of the things that can slow down EIGRP. Many rules and features scope (limit) this process, such as proper route summarization and the EIGRP stub router feature (neither will be described indetail).

Completing the Remaining EIGRP Data Structures

EIGRP Routing Update Process

– EIGRP neighbors discover and maintainadjacencies through use of hello packets.

– On the initial discovery of a neighbor, full routingupdates are exchanged.

– After the initial exchange, only triggeredincremental updates are exchanged as long as theadjacency is not lost or reset.

– All received routes are stored in the topologytable.

– Only successors are advertised to neighbors.

The graphic provides a review of all the EIGRP processes described so far.




From the topology, all the successors (best routes) are copied to the Routing Information Base (RIB), which is alsocalled the routing table. This dynamic table is maintained as part of the routing process memory by incrementalrouting updates after the initial “loading” process. The exchange of information necessary to maintain this tableis called the control plane.

Once the tables are converged and stable, it is necessary to quickly deal with changes in network topology (suchas link failures and device failures). With all the real-time protocols in modern networks (voice, video, multicast),repair times are measured in milliseconds. Over the years, Cisco has evolved multiple features in the Cisco IOS toaddress this issue.




EIGRP: DATA STRUCTURES

Control Plane

Routing Protocol

IP Routing Table (RIB)

IP Forwarding Table (FIB)

Exchange of

Routing Information

CEFData Plane

Stored in fast

memory cache or

hardware

The latest feature in this evolution is Cisco Express Forwarding (CEF), which copies all the contents of the RIB(routing table) to a fast “cache” in the fastest memory location possible, such as a line module in a 6500 or Nexusmulti-layer switch. This table is called the Forwarding Information Base (FIB) and it is located in the data plane.

This table is consulted by the router when packets arrive on an interface and a routing decision is required. TheFIB is refreshed by the RIB regularly. The only time the RIB is consulted is when a FIB fails to provide a routingentry.




The Configuration and Verification of Implementing

EIGRPThe configuration of EIGRP on Cisco devices does not require a thorough understanding of all the underlyingmathematical process that make EIGRP work, so the commands are simple. Verification and troubleshooting can

be a bit more challenging, however. The skill is in understanding the output of verification commands.

The graphic shows an EIGRP configuration for a simple network.

Global command for EIGRP is router eigrp [AS number]. Remember, the AS number is a parameter that mustmatch in the hello packets in order for neighbors to be recognized.

The network statement specifies which interfaces will participate in EIGRP. The wildcard (inverse) mask can beused to identify interfaces more specifically. It is used in the same way here that it is in an access control list (ACL)with a wildcard mask providing match/don’t care logic to permit or deny specific networks.




Verifying EIGRP: show ip eigrp

neighbors

R1#show ip eigrp neighborsIP-EIGRP neighbors for process 100

H Address Interface Hold Uptime SRTT RTO Q Seq

(sec) (ms) Cnt Num

0 192.168.1.102 Se0/0/1 10 00:07:22 10 2280 0 5

R1#

As described earlier, the show ip eigrp neighbor command is a logical way to verify that the EIGRP router is“seeing” its neighbors.

Verifying EIGRP: show ip route eigrp

R1#show ip route eigrp

D 172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:07:01, Serial0/0/1172.16.0.0/16 is variably subnetted, 2 subnets, 2 masks

D 172.16.0.0/16 is a summary, 00:05:13, Null0

192.168.1.0/24 is variably subnetted, 2 subnets, 2 masks


R1#show ip route

<output omitted>

Gateway of last resort is not set

D 172.17.0.0/16 [90/40514560] via 192.168.1.102, 00:06:55, Serial0/0/1



C 172.16.1.0/24 is directly connected, FastEthernet0/0


C 192.168.1.96/27 is directly connected, Serial0/0/1


The show ip route output will verify that EIGRP control plane information is being received from neighbors.Debug commands may provide more detail on this process, but they must be used with caution.




Verifying EIGRP: show ip protocols

R1#show ip protocols

Routing Protocol is "eigrp 100"

Outgoing update filter list for all interfaces is not setIncoming update filter list for all interfaces is not set

Default networks flagged in outgoing updates

Default networks accepted from incoming updates

EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0

EIGRP maximum hopcount 100

EIGRP maximum metric variance 1

Redistributing: eigrp 100

EIGRP NSF-aware route hold timer is 240s

<output omitted>

Maximum path: 4

Routing for Networks:

172.16.1.0/24

192.168.1.0

Routing Information Sources:

Gateway Distance Last Update

(this router) 90 00:09:38

Gateway Distance Last Update192.168.1.102 90 00:09:40

Distance: internal 90 external 170

Show ip protocols is perhaps the single most productive show command for verifying the entire EIGRP processbecause it can show:

• Possible update filters in place

• Interfaces that are participating based on network statements

• Default administrative distance (a method to distinguish between information received from twodifferent routing protocols)

• Neighbors discovered

•

Passive interfaces (interfaces where EIGRP has been disabled by command)

• Route redistribution (exchange of routes with other routing domains





interfaces

R1#show ip eigrp interfacesIP-EIGRP interfaces for process 100

Xmit Queue Mean Pacing Time Multicast Pending

Interface Peers Un/Reliable SRTT Un/Reliable Flow Timer Routes

Fa0/0 0 0/0 0 0/10 0 0

Se0/0/1 1 0/0 10 10/380 424 0


topology

R1#show ip eigrp topology

IP-EIGRP Topology Table for AS(100)/ID(192.168.1.101)

Codes: P - Passive, A - Active, U - Update, Q - Query, R -

Reply,

r - reply Status, s - sia Status

P 192.168.1.96/27, 1 successors, FD is 40512000via Connected, Serial0/0/1


via Summary (40512000/0), Null0


via Summary (28160/0), Null0


via Connected, FastEthernet0/0


via 192.168.1.102 (40514560/28160), Serial0/0/1

The show ip eigrp interface and show ip eigrp topology were described earlier and are important verification

tools as well.




Additional EIGRP features not described in detail in this paper include:

• EIGRP authentication using a key chain and MD5 hash

• Passive interfaces

• Unequal cost load balancing

•

Issues with NBMA and multipoint WANs

• EIGRP over MPLS

• Route redistribution

• Route filtering

• Troubleshooting

• EIGRP stub

• Route summarization

• EIGRP design

ConclusionEIGRP has been used successfully for many years by small, medium, and large enterprises that use mostly Ciscodevices. It converges as fast as any other internal gateway protocol and provides features that facilitate design,implementation, and troubleshooting. Since it is proprietary, excellent support is provided by Cisco. So, this is aviable choice for an enterprise routing protocol along with OSPF and IS-IS.

Learn MoreLearn more about how you can improve productivity, enhance efficiency, and sharpen your competitive edge through training.

ROUTE - Implementing Cisco IP Routing v1.0

ARCH - Designing Cisco Network Service Architectures v2.1

SPNGN1 - Building Cisco Service Provider Next-Generation Networks, Part 1

SPNGN2 - Building Cisco Service Provider Next-Generation Networks, Part 2

Visit www.globalknowledge.com or call 1-800-COURSES (1-800-268-7737) to speak with a Global Knowledge

training advisor.

About the AuthorRay Dooley, BS, MBA, CCSI, CCNA, CCNP, CCDA, CCDP, SE, FE, has been a network professional in severalcapacities for over 30 years. He is a Global Knowledge Course Director for CCDA, ARCH, SWITCH, ROUTE,TSHOOT, and ICMI. He has done course development for Global Knowledge, Cisco Systems, and GE.


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The Four Elements of EIGRP