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1EIGRP

EIGRP

MODULE ID: ICCNEIG001

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2EIGRP

Topics Covered

1. Features & Operations2. Various tables maintained by EIGRP3. EIGRP Terminologies4. Algorithm used5. EIGRP Metric6. EIGRP Packet types7. Configuring & Verifying8. Authentication

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EIGRP Features and Operation

Referred to as a hybrid routing protocol as it has characteristics of both distance-vector and link-state protocols.

Sends traditional distance-vector updates containing information about networks plus the cost of reaching them

Link-state characteristics it synchronizes routing tables between neighbors at startup, and then sends specific updates only when topology changes occur.

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EIGRP Features and Operation Suitable for very large networks. Maximum hop count of 255. powerful features of EIGRP

Support for IP, IPX, and AppleTalk via protocol-dependent modules

Classless Support for VLSM/CIDR Support for summaries and discontiguous

networks Efficient neighbor discovery Communication via Reliable Transport Protocol

(RTP) Best path selection via Diffusing Update Algorithm

(DUAL)

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Protocol-Dependent Modules

EIGRP supports different Network layer

protocols through the use of protocol-

dependent modules (PDMs)

Each EIGRP PDM maintains a separate

series of tables containing the routing

information that applies to a specific protocol.

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Neighbor Discovery

EIGRP routers must become neighbors before exchanging routes with each other. Three conditions for neighborship establishment:

Hello or ACK received AS numbers match Identical metrics (K values)

To maintain the neighborship relationship, EIGRP routers must continue receiving Hellos from their neighbors.

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Neighbor Discovery

EIGRP routers belonging to different autonomous systems (AS) do not automatically share routing information and they dont become neighbors.

EIGRP advertises its entire routing table only when it discovers a new neighbor and forms an adjacency with it through the exchange of Hello packets.

When this happens, both neighbors advertise their entire routing tables to one another.

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Neighbor Table

The neighbor table is maintained by means of Hello packets.

Hello packets are multicast announcements that the router is alive.

Hello packets place the router into adjacent routers neighbor tables.

Reciprocal hellos build the local neighbor table. Once neighbor tables are built, hellos continue periodically to maintain neighborship

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Neighbor Table

The Layer 3 address of the neighbor The interface through which the neighbors Hello was

heard. The holdtime, which is three times the value of the Hello timer by default. The uptime, or period since the router first heard from the neighbor. The sequence number. Retransmission timeout (RTO) Smooth Round Trip Time (SRTT) which calculates the

RTO. SRTT is the time (in milliseconds) that it takes a packet to be sent to a neighbor and a reply to be received.

The number of packets in a queue

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Topology TableThe topology table has a record not only of feasiblesuccessors and successors but also of all received routes with the following information Whether the route is passive or active. Whether an update has been sent to the neighbor. Whether a query packet has been sent to the neighbor. If

this field is positive, at least one route will be marked as active.

Whether a query packet has been sent; if so, another field will track whether any replies have been received from the neighbor.

That a reply packet has been sent in response to a query packet received from a neighbor.

Prefixes, masks, interface, next-hop, and feasible and advertised distances for remote networks.

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Routing Table

The routing table is built from the topology table using

DUAL

Successor paths are transferred to the routing table

and used as the basis for routing decisions

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Definitions of related terms

Feasible distance This is the best metric along all paths to a

remote network, including the metric to the neighbor that is advertising that remote network.

This is the route in the routing table, because it is considered the best path.

Reported distance This is the metric of a remote network, as

reported by a neighbor.

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Definitions of related terms

Feasible successor A feasible successor is a path whose reported

distance is less than the feasible distance, and it is considered a backup route.

EIGRP will keep up to six feasible successors in the topology table.

Only the successor with the best metric is placed in the routing table.

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Definitions of related terms

Successor A successor route is the best route to a remote

network. A successor route is used by EIGRP to

forward traffic to a destination and is stored in the routing table.

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Feasible Distance/Advertised Distance

PUNE MUMBAI DELHI SRINAGAR

BHOPAL

200 1100 500

1500 500

Eg:- Mumbai to Srinagar Path Selection

AD

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Unequal Cost Load Balancing

All IP routing protocols on Cisco routers support equal-cost load sharing. EIGRP is unique in its support for unequal-cost load sharing.

EIGRP does proportional unequal-cost load sharing EIGRP will pass a relative portion of the traffic to each interface.

Eg:-A 384 kbps link would get 60 percent and the 256 kbps link would handle 40 percent of the traffic.

This allows all links to a destination to be used to carry data without saturating the slower links or limiting the faster links

Unequal-cost load balancing takes the best FD and multiplies it by variance. Any other path with an FD less than this product is used for load sharing

Maximum 6 feasible successors can do load balancing Default is 4

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Reliable Transport Protocol (RTP)

EIGRP uses a proprietary protocol, called Reliable Transport Protocol (RTP) , to manage the communication of messages between EIGRP-speaking routers.

Cisco has designed a mechanism that uses multicasts and unicasts to deliver updates quickly, and to track the receipt of the data.

When EIGRP sends multicast traffic, it uses the Class D address 224.0.0.10.

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Diffusing Update Algorithm (DUAL)

EIGRP uses Diffusing Update Algorithm (DUAL) for selecting and maintaining the best path to each remote network.

This algorithm allows for the following: Backup route determination if one is available Support of Variable-Length Subnet Masks

(VLSMs) Dynamic route recoveries Queries for an alternate route if no route can

be found DUAL provides EIGRP the fastest route

convergence time among all protocols.

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Multiple ASes

EIGRP uses autonomous system numbers to identify the collection of routers that share route information.

Only routers that have the same autonomous system numbers share routes.

In large networks, the complicated topology and route tables, slows convergence during diffusing computation operations.

Its possible to divide the network into multiple distinct EIGRP autonomous systems, or ASes.

Each AS is populated by a contiguous series of routers, and route information can be shared among the different ASes via redistribution.

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VLSM Support and Summarization

EIGRP supports the use of Variable-Length Subnet Masks.

EIGRP also supports the use of discontiguous subnets,

A discontiguous network is one that has two or more subnetworks of a classful network connected together by different classful networks.

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A discontiguous network

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EIGRP auto summarization

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EIGRP Metrics

EIGRP can use a combination of four metrics: Bandwidth Delay Load Reliability MTU

EIGRP uses only bandwidth and delay of the line to determine the best path to a remote network by default.

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Metric calculation

By default, K1=K3=1 and K2=K4=K5=0. Those who followed the math will note that when K5=0the metric is always zero. Because this is not useful, EIGRP simply ignores everything outsidethe parentheses. Therefore, given the default K-values the equation becomesMetric= 256 (bandwidth +delay )Actually:metric = slowest bandwidth + sum of all delays

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EIGRP Packet Types

Response to a queryReply

Needing specific info from a neighbor

Query

When a router discovers a new neighbor or a topology change

Update

Indicates receipt of an EIGRP during a reliable packet

Acknowledgement (unicast)

Discovers, verifies, rediscovers neighbor routers

Hello multicast every 5 seconds

DefinitionPacket

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Configuring EIGRP

Two modes from which EIGRP commands are entered: router configuration mode and interface configuration mode

Router configuration mode enables the protocol determines which networks will run EIGRP sets global characteristics

Interface configuration mode allows customization of Summaries Metrics Timers Bandwidth

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Configuring EIGRP

Configuring involves enabling the following 3 items The EIGRP processThe routing protocol needs to be

started on the router. The EIGRP autonomous system numberRouters in

the same administrative domain should be identified and grouped by a common AS number. This 16-bit number is arbitrary. Organizations that have a BGP AS will sometimes use that number; others just make up a number or use AS 1. The significance of the AS is that a router will not become a neighbor with a router in a foreign AS.

Participating router interfacesYou can activate EIGRP on some or all interfaces. For instance, a router might run EIGRP on internal interfaces and BGP on external interfaces

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EIGRP Configuration

Router#config tRouter(config)#interface Router(config-if)#ip address

Router(config)#router eigrp 100

Router(config-router)#network 172.17.0.0

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EIGRP Authentication

EIGRP supports two forms of authentication: simple passwords and MD5 hashes

Simple passwords are not secure, because any listener can see this traffic and read the key value.

Hashed keys, sent as MD5 values, are secure because a listener cannot use the value in one transmission to compute the key.

Using MD5 Authentication, the router generates a hash value for every EIGRP transmission and checks the hash of every received EIGRP packet.

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Other optional commands

no auto-summary Turns off automatic classful summarization. ip summary-address Manually configures summarization. variance Configures unequal proportional load balancing. ip hello-interval eigrp autonomous-systemnumber seconds

Changes the hello frequency. ip hold-time eigrp autonomous-system-number seconds

Changes the length of time before a neighbor is considered dead.

bandwidth Changes the bandwidth setting on an interface, which is used to calculate the EIGRP metric and the maximum percentage of bandwidth to which EIGRP traffic will limit itself.

ip bandwidth-percent eigrp as-number Changes the maximum amount of bandwidth EIGRP traffic will use. The default is 50%

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Verification

show ip route

show ip protocols

show ip eigrp neighbors

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Summary

1. EIGRP Features2. Neighbor discovery & Neighbor Tables3. Topology Table & Routing Table4. Feasible & Reporting Distance5. Successor & Feasible Successor6. RTP & DUAL7. EIGRP Metric & EIGRP Packet Types8. Summarisation9. Configuration & Verification

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References

Books Hand book on Internetworking by CISCO Building Scalable Cisco Internetworks by

CISCO CCNA Study guide by Todd Lamle

URLs http://www.cisco.com http://en.wikipedia.org

Slide 1Slide 2EIGRP Features and OperationSlide 4Protocol-Dependent ModulesNeighbor DiscoverySlide 7Neighbor TableSlide 9Topology TableRouting TableDefinitions of related termsSlide 13Slide 14Feasible Distance/Advertised DistanceUnequal Cost Load BalancingReliable Transport Protocol (RTP)Diffusing Update Algorithm (DUAL)Multiple ASesVLSM Support and SummarizationA discontiguous networkEIGRP auto summarizationEIGRP MetricsMetric calculationEIGRP Packet TypesConfiguring EIGRPSlide 27EIGRP ConfigurationEIGRP AuthenticationOther optional commandsVerificationSummaryReferences