EIGRP

Roots of EIGRP: IGRP-Developed in 1985 to overcome RIPv1’s limited hop count-Distance vector routing protocol-Metrics used by IGRP bandwidth (used by default) Delay (used by default) Reliability (not used by default) Load (not used by default)-Discontinued support starting with IOS 12.2(13)T & 12.2(R1s4)S

OSPF2

OSPF requires more resources from routerFast convergenceLess overhead – good for large networksLSAs sent in LSUs – only the changes are

flooded to the networkSupports VLSMComplex to configure!

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EIGRP – Cisco proprietary routing protocolKey featuresNeighbor and topology tablesFeasible Distance & Successor RoutesEIGRP packet typesSetting up EIGRP

SummaryBackground & History

–EIGRP is a derivative of IGRP EIGRP is a Cisco proprietary distance vector routing

protocol released in 1994

EIGRP terms and characteristics–EIGPR uses RTP to transmit & receive EIGRP packets–EIGRP has 5 packet type: Hello packets Update packets Acknowledgement packets Query packets Reply packets–Supports VLSM & CIDR

EIGRP

The Algorithm–EIGRP uses the Diffusing Update Algorithm (DUAL). –EIGRP does not send periodic updates and route entries do not age out. –Only changes in the routing information, such as a new link or a link becoming unavailable cause a routing update to occur. –EIGRP routing updates are still vectors of distances transmitted to directly connected neighbors.

EIGRP

Path Determination–EIGRP's DUAL maintains a topology table separate from the routing table, which includes both the best path to a destination network and any backup paths that DUAL has determined to be loop-free.

EIGRP7

–If a route becomes unavailable, DUAL will search its topology table for a valid backup path. • If one exists, that route is immediately

entered into the routing table. • If one does not exist, DUAL performs a

network discovery process to see if there happens to be a backup path that did not meet the requirement of the feasibility condition.

EIGRPConvergence

–EIGRP does not use holddown timers. –Instead, loop-free paths are achieved through a system of route calculations (diffusing computations) that are performed in a coordinated fashion among the routers. –The detail of how this is done is beyond the scope of this course, but the result is faster convergence than traditional distance vector routing protocols.

EIGRPEIGRP Message Format EIGRP Header

Data link frame header - contains source and destination MAC addressIP packet header - contains source & destination IP addressEIGRP packet header - contains AS numberType/Length/Field - data portion of EIGRP messageIn the IP packet header, the protocol field is set to 88

to indicate EIGRP the destination address is

set to the multicast 224.0.0.10.

If the EIGRP packet is encapsulated in an Ethernet frame, the destination MAC address

is also a multicast address: 01-00-5E-00-00-0A.

EIGRP

EIGRP packet header contains–Opcode field• Update• Query• Reply• Hello

–Autonomous System number• The AS number is used to track

multiple instances of EIGRP. EIGRP Parameters contains

–Weights• EIGRP uses for its composite

metric. • By default, only bandwidth and

delay are weighted. Both are set to 1.

• The other K values are set to zero.

–Hold time• The amount of time the EIGRP

neighbor receiving this message should wait before considering the advertising router to be down.

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EIGRP

TLV: IP internal contains (EIGRP routes within an autonomous system)

–Metric field (Delay and Bandwidth)• Delay is calculated as the

sum of delays from source to destination in units of 10 microseconds.

• Bandwidth is the lowest configured bandwidth of any interface along the route.

–Subnet mask field• The subnet mask is specified

as the prefix length or the number of network bits in the subnet mask.

• 255.255.255.0 is 24

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–Destination field• the address of the destination network.• Although only 24 bits are shown in this figure. • If a network address is longer than 24 bits, then the

Destination field is extended for another 32 bits TLV: IP external contains

–Fields used when external routes are imported into EIGRP routing process– import or redistribute a route into EIGRP.

EIGRPProtocol Dependent Modules

(PDM) EIGRP uses PDM to route several

different protocols i.e. IP, IPX & AppleTalk

PDMs are responsible for the specific routing task for each network layer protocol

–As you can see in the figure, EIGRP uses different EIGRP packets and maintains separate neighbor, topology, and routing tables for each Network layer protocol. • The IP-EIGRP module is

responsible for sending and receiving EIGRP packets that are encapsulated in IP and for using DUAL to build and maintain the IP routing table.

• The IPX EIGRP module is responsible for exchanging routing information about IPX networks with other IPX EIGRP routers.

• Apple-Talk EIGRP is for Apple-talk

EIGRP

Reliable Transport Protocol (RTP) Purpose of RTP

–Used by EIGRP to transmit and receive EIGRP packets– EIGRP was designed as a Network layer independent routing protocol; therefore, it cannot use the services of UDP or TCP because IPX and Appletalk do not use protocols from the TCP/IP protocol suite.

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Characteristics of RTP–Involves both reliable & unreliable delivery of EIGRP packet Reliable delivery requires acknowledgment from destination Unreliable delivery does not require an acknowledgement from

destination–Packets can be sent Unicast Multicast

–Using address 224.0.0.10

EIGRP

EIGRP’s 5 Packet TypesHello packets

–Used to discover & form adjacencies with neighbors–EIGRP hello packets are multicasts and use unreliable delivery.

•Hello•Update•ACK•Query•Reply

EIGRP Update packets

–Update packets are used to propagate routing information–Update packets are sent only when necessary.–EIGRP updates are sent only to those routers that require it. –When a new neighbor is discovered, unicast update packets are sent so that the neighbor can build up its topology table. –In other cases, such as a link-cost change, updates are multicast. –Updates always are transmitted reliably

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Acknowledgement packets–Used to acknowledge receipt of update, query & reply packets–An acknowledgment packet is a hello packet that has no data. –EIGRP acknowledgement packets are always sent as an unreliable unicast

EIGRP

Query & Reply packetsUsed by DUAL for searching for networksQueries and replies use reliable delivery.Query packets can use

MulticastReply packet use only

unicast

EIGRP

Query Update Reply Hello Acknowledge

Reliable Reliable Reliable Unreliable(not require acknowledgment )

Unreliable(a hello packet that has no data )

multicast Multicast & unicast

unicast multicast unicast

EIGRPPurpose of Hello Protocol

–To discover neighbors & establish adjacencies with neighbor routers

Characteristics of hello protocol–Time interval for sending hello packet 5 seconds - high bandwidth (greater than T1) 60 seconds - multipoint circuits T1 bandwidth or slower -Holdtime

This is the maximum time router should wait before declaring a neighbor down

Default holdtime

–3 times hello interval

»15 seconds

»180 seconds

EIGRP Updates

EIGRP only sends update when there is a change in route status

Partial update–A partial update includes only the route information that has changed – the whole routing table is NOT sent

Bounded update–When a route changes, only those devices that are impacted will be notified of the change

EIGRP’s use of partial bounded updates minimizes use of bandwidth

EIGRP

Diffusing Update Algorithm (DUAL)–Purpose• EIGRP’s primary method for preventing routing loops• And also hold-down timers and split horizon, too. –Advantage of using DUAL• Provides for fast convergence time by keeping a list of

loop-free backup routes–DUAL maintains a list of backup routes it has already determined to be loop-free. If the primary route in the routing table fails, the best backup route is immediately added to the routing table.

EIGRP

Administrative Distance (AD)–Defined as the trustworthiness of the source route

EIGRP default administrative distances–Summary routes = 5–Internal routes = 90–Imported routes = 170

EIGRP

Authentication EIGRP can

– Encrypt routing information– Authenticate routing information

It is good practice to authenticate transmitted routing information.

– This practice ensures that routers will only accept routing information from other routers that have been configured with the same password or authentication information.

Note: Authentication does not encrypt the router's routing table.

Basic EIGRP Configuration EIGRP autonomous system

number actually functions as a process ID

–The vast majority of companies and institutions with IP networks do not need an AS number–The ISP is responsible for the routing of packets within its autonomous system and between other autonomous systems.

Process ID represents an instance of the routing protocol running on a router

ExampleRouter(config)#router eigrp autonomous-

system

EIGRP Metric CalculationEIGRP Metrics Use the show

interfaces command to view metrics

EIGRP Metrics–Bandwidth – EIGRP uses a static bandwidth to calculate metric–Most serial interfaces use a default bandwidth value of 1.544Mbos (T1)–The value of the bandwidth may or may not reflect the actual SPEED of the interface. –If actual SPEED of the link differs from the default bandwidth value, then you should modify the bandwidth value,

The default bandwidth for ethernet is 10,000 Kbits. The default bandwidth for fastethernet is 100,000 Kbits.

EIGRP Metric Calculation

EIGRP MetricsDelay is the defined as

the measure of time it takes for a packet to traverse a route

–it is a static value based on link type to which interface is connected–The delay value, much like the bandwidth value, is a default value that can be changed by the network administrator manually.

EIGRP Metric CalculationReliability (not a default EIGRP metric)

–A measure of the likelihood that a link will fail or how often the link has experienced errors. –Measure dynamically & expressed as a fraction of 255• the higher the fraction the better the reliability–Reliability is calculated on a 5-minute weighted average to avoid the sudden impact of high (or low) error rates.

Condition/Network Status (not a default EIGRP metric)– A number that reflects how much traffic is using a link– Number is determined dynamically and is expressed as a fraction of 255 The lower the fraction the less the load on the linkThis value is calculated on a 5-minute weighted average to avoid the sudden impact of high (or low) channel usage.

EIGRP Metric Calculation

EIGRP uses the lowest bandwidth (BW)in its metric calculation

Calculated BW = reference BW / lowest BW(kbps)

Delay – EIGRP uses the cumulative sum of all outgoing interfaces

Calculated Delay = the sum of outgoing interface delays

EIGRP Metric = calculated BW + calculated delay

DUAL Concepts

The Diffusing Update Algorithm (DUAL) is used to prevent looping

–Successor–Feasible Distance (FD)–Feasible Successor (FS)–Reported Distance (RD) or Advertised Distance (AD)–Feasible Condition or Feasibility Condition (FC)

DUAL Concepts

SuccessorThe best least cost route to a destination found in the routing table

Feasible distanceThe lowest calculated metric along a path to a destination network

2 commands can be used to find the “successor” and “feasible distance”:

–show ip route–show ip eigrp topology

1) If feasible successor exists: If current successor route fails, feasible successor becomes the

current successor, i.e. the current route. Routing of packets continue with little delay.

2) If no feasible successor exists: This may be because the Reported Distance is greater than the

Feasible Distance. Before this route can be installed, it must be placed in the active

state and recomputed. Routing of packets continue but with more of a delay.

What if the successor fails?

DUAL Concepts

Feasibility Condition (FC)–Met when a neighbor’s reported distance (RD) is less than the local router’s FD to the same destination network–The reported distance is simply an EIGRP neighbor's feasible distance to the same destination network. –The reported distance is the metric that a router reports to a neighbor about its own cost to that network.

DUAL Concepts Reported distance (RD)

–The metric that a router reports to a neighbor about its own cost to that network

DUAL Concepts

The centerpiece of EIGRP is DUAL and its EIGRP route-calculation engine. The actual name of this technology is DUAL Finite State Machine (FSM).

Finite Sate Machine (FSM)–An abstract machine that defines a set of possible states something can go through, what event causes those states and what events result form those states–FSMs are used to describe how a device, computer program, or routing algorithm will react to a set of input events–Selects a best loop-free path to a destination–Selects alternate routes by using information in EIGRP tables

Fine-Tuning EIGRPEIGRP bandwidth utilization

–By default, EIGRP uses only up to 50% of interface bandwidth for EIGRP information• This prevents the EIGRP process from over-utilizing a link

and not allowing enough bandwidth for the routing of normal traffic.

SummaryDUAL

–Purpose of DUAL To prevent routing loops–Successor Primary route to a destination–Feasible successor Backup route to a destination–Feasible distance Lowest calculated metric to a destination–Reported distance The distance towards a destination as

advertised by an upstream neighbor

SummaryChoosing the best route

–After router has received all updates from directly connected neighbors, it can calculate its DUAL 1st metric is calculated for each route 2nd route with lowest metric is designated

successor & is placed in routing table 3rd feasible successor is found

–Criteria for feasible successor: it must have lower reported distance to the destination than the installed route’s feasible distance–Feasible routes are maintained in topology table

EIGRP42

Offers multiprotocol operation (IP, IPX etc)EIGRP multiplies IGRP metric by 256Uses 32-bit metric c.f. IGRP 24-bit metricMetric = bandwidth / delayEIGRP BW = (107/ BW) x 256EIGRP delay = (delay/10) x 256Max hop = 224 c.f. IGRP 256

EIGRP key features43

EIGRP automatically shares routing information with IGRP and vice versa

EIGRP tags routes from other protocols as externalRapid convergence from Diffused Updating Algorithm

(DUAL)Guaranteed no loops – all routers running EIGRP update at

the same time if change occursEfficient use of BW – partial incremental updates onlySent only to routers that need information – not all routers

Good use of Network Bandwidth44

Partial bounded updates, unlike OSPFNo timed routing updatesRIP = 30s; IGRP = 90s – full routing tablesEIGRP uses small hello packetsUnreliable – no acknowledgementsSupports VLSM & CIDR unlike IGRPUses PDMs to support IP, IPX, Appletalk

EIGRP Router Tables45

Neighbour table lists adjacent routersOne neighbour table per supported protocolSimilar to OSPF adjacency databaseHolds neighbour address and interface dataHold time = time to wait before considering

neighbour to be ‘down’ = 3 x hello interval Multicast to 224.0.0.10

Smooth Round Trip Timer SRTT

Neighbour Discovery and Rediscovery46

Setting up EIGRP47

Needs Autonomous System Number

Router(config)#router eigrp 123Router(config-router)#net 192.168.1.1 0.0.0.3Router(config-router)#net 22.22.22.0 0.0.0.255Router(config-router)#exit

Uses Wildcard Mask

Summary48

EIGRP similar to OSPF - Has neighbour table Sends hello packets

Has topology table (1 for each protocol)Uses DUAL on Neighbour & Topology tablesElects successors and feasible successors (FS)FS used in times of trouble – fast selectionVerification and debug commands