Cisco 3- EIGRP Perrine and Brierley Page 110/4/2015 Module 3 EIGRP

Cisco 3- EIGRP

Perrine and Brierley Page 104/19/23

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EIGRP

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EIGRP

Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco proprietary routing protocol based on IGRP.

EIGRP supports CIDR, and hence VLSM.

Compared to IGRP, EIGRP boasts faster convergence times, improved scalability and superior handling of routing loops.

Technically, EIGRP is an advanced distance-vector routing protocol that relies on features commonly associated with link-state protocols. Some of OSPF’s best traits, such as partial updates & neighbor discovery, are similarly put to use by EIGRP.

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EIGRP

Recall that OSPF’s strengths include scalability and multi-vendor support. So if your core routers are a mixed bag of products from several different vendors, OSPF and RIP may be your only options.

But OSPF’s benefits, especially it’s hierarchical design, come at a price: administrative complexity.

EIGRP is an ideal choice for large, multiprotocol networks built primarily on Cisco routers (says Cisco).

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Comparison of OSPF and EIGRP

OSPF EIGRP

Supports CIDR & VLSM, rapid convergence, partial updates, neighbor discovery.

Supports CIDR & VLSM, rapid convergence, partial updates, neighbor discovery

Enables the admin to define route summarization.

Uses automatic route summarization & user-defined route summaries.

Is an open standard; has multi-vendor support

Is proprietary; can only be used with Cisco routers.

Is scalable; admin defined ‘areas’ provide manageable hierarchy.

Is scalable; no hierarchical domains exist

Is difficult to implement Is easy to implement.

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EIGRP and IGRP Compatibility

EIGRP offers multiprotocol support and IGRP does not.

EIGRP scales IGRP’s metric by a factor of 256. That’s because EIGRP uses a metric that is 32 bits long, & IGRP uses a 24-bit metric. By dividing or multiplying by 256, EIGRP can easily exchange information with IGRP.

EIGRP imposes a max hop limit of 224, where IGRP has 255.

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Sharing or redistribution, is automatic between IGRP & EIGRP as long as both processes use the same AS number.

metric = [K1 x bandwidth + K2 x bandwidth) / (256ms – load) + (K3 x delay)] x

[K5 / (reliability+K4)]

Where by default: K1 = 1 K2 = 0 K3 = 1 K4 = 0 K5 = 0

EIGRP and IGRP Compatibility

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EIGRP tags routes learned from IGRP as external because they did not originate from EIGRP routers. External EIGRP routes are denoted by EX in the routing table.

But IGRP cannot differentiate between internal & external routes

EIGRP vs. IGRP

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EIGRP

EIGRP is an advanced distance vector routing protocol, but has advantages over simple distance vector protocols:

• rapid convergence

By using an routing algorithm, Diffusing Update Algorithm (DUAL) which guarantees loop-free operation & allows all routers involved in a topology change to synchronize at the same time.

• partial bounded updates

EIGRP routers make partial, incremental updates, & unlike OSPF, the routers send these partial updates only to the routers that need the information. This is called bounded updates.

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EIGRP cont.

• minimal consumption of bandwidth when the network is stable

•No timed routing updates - instead small hello packets. •Exchanged a regular intervals •don’t use a significant amount of bandwidth.

• support for VLSM and CIDR

• multiple network-layer support

EIGRP supports IP, IPX and AppleTalk via protocol-dependent modules (PDM).

• complete independence from routed protocols

PDM protect EIGRP from painstaking revision.

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EIGRP Terminology & Tables

EIGRP routers keep route and topology information in RAM for quick access.

EIGRP terms and tables:

neighbor table

Maintains a neighbor table that list adjacent routers. There is a neighbor table for each protocol that EIGRP supports.

topology table

Topology table for each configured network protocol. All learned routes to a destination are maintained in the topology table.

routing table

EIGRP chooses the best (successor) routes to a destination from the topology table & places these routes in the routing table.

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successora route selected as the primary route used to reach a destination. Successors are kept in the routing table.

feasible successorIs a backup route. They are kept in the topology table.

EIGRP routers establish adjacencies with neighbor routers by sending hello packets, sent every 5 seconds (default).

By forming adjacencies, EIGRP routers do the following:

• dynamically learn of new routes that join their network

• identify routers that become either unreachable or inoperable

• rediscover routers that had previously been unreachable.

EIGRP Terminology

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EIGRP Technologies cont.

Reliable Transport Protocol (RTP) is a transport layer (layer 4) protocol that can guarantee ordered delivery of EIGRP packets to all neighbors. To stay independent of IP, EIGRP uses its own proprietary transport-layer protocol to guarantee delivery of routing information.

EIGRP uses RTP to provide reliable or unreliable service as the situation warrants. Hello packets are not required to be reliable delivery. RTP supports both unicasting & multicasting , and it can multicast and unicast to different peers simultaneously.

The centerpiece of EIGRP is DUAL, the EIGRP route calculation engine. It uses a finite state machine.

DUAL tracks all the routes advertised by neighbors and uses the composite metric of each route to compare them. DUAL also guarantees that each path is loop-free.

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EIGRP Technologies cont.

A successor is a neighboring router that is currently used for packet forwarding; it provides the least cost route to the destination and is not part of the routing loop

A feasible successor provides the next lowest cost path without introducing routing loops.

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EIGRP Data Structure

Neighbor Table

Routing Table

Topology Table

Reported distance (RD)

Feasible distance (FD)

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Neighbor Table

the most important table in EIGRP. The neighbor relationships in the neighbor table are basis for all EIGRP routing updates and convergence activity. supports reliable, sequenced delivery of packets.

Routing Table

Contain the routes installed by DUAL as the best loop free paths to a given destination. It canmaintain up to 4 routes per destination.

Topology Table

stores all the information it needs to calculate a set of distances and vectors to all reachable destinations.

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Reported distance (RD)

The distance reported by an adjacent neighbor to a specific destination.

Feasible distance (FD)

The lowest calculated metric to each destination.

Table is sorted with the successor routes at the top, followed by feasible successors.

At the bottom are what DUAL believes are routing loops.

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To see the topology table, use the command:

Router# show ip eigrp topology [all]

Example of output:

P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33

via 10.2.0.2 (2681856/2169856), Serial1

Viewing (2681856/2169856), 2681856 is the FD, and 2169856 is the RD.

If no feasible successors to the destination exist, DUAL places the route in the active state.

Entries in the topology table can be in one of 2 states

• passive route – route that is stable and available for use

• active route – route in the process of being recomputed by DUAL.

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Internal routes originate from within the EIGRP AS.

External routes originate from outside the system.

Routes learned (redistributed) from other routing protocols such as RIP, OSPF and IGRP are external.

Static routes originating from outside the EIGRP AS and redistributed inside are also external routes.

NOTE:

The internal administrative distance of EIGRP is 90.

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EIGRP Packet Types

EIGRP relies on 5 packet types to maintain its various tables and establish complex relationships with neighbor routers.

The 5 packet types:

1. Hello

2. Acknowledgment

3. Update

4. Query

5. Reply

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Hello Packets

EIGRP relies on hello packets to discover, verify & rediscover neighbor routers. The default hello interval depends on the bandwidth of the interface:

Bandwidth Example Link Default Hello Default Hold

Interval Time

Less than 1.544Mbps Multipoint Frame Relay 60 sec 180 sec

Greater than 1.544Mbps T1, Ethernet 5 sec 15 sec

Keep the hold timer 3 times the hello interval. EIGRP hello packets are multicast. On IP networks,

EIGRP routers send hellos to the multicasts IP address 224.0.0.10.

Recall that OSPF requires neighbor routers to have the same hello & dead intervals to communicate. EIGRP has no such restriction.

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Acknowledgement Packets

An EIGRP router uses acknowledgement packets to indicate receipt of any EIGRP packet during a reliable exchange.

To be reliable, a sender’s message must be acknowledged by the recipient.

Hello packets are always sent unreliable, & require no acknowledgement.

Update Packets

Update packets are used when a router discovers a new neighbor. They are also used when a router detects a topology change.

All update packets are sent reliably.

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Query and Reply Packets

EIGRP routers use query packets whenever they need specific information from one or all of its neighbors. A reply packet is used to respond to a query.

Query can be multicast or unicast

Replies are always unicast.

Both packet types are sent reliably.

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EIGRP Convergence (DUAL)

DUAL’s sophisticated algorithm results in EIGRP’s exceptional fast convergence, says Cisco

A router’s topology table includes a list of all routes advertised by the neighbors. For each network, the router keeps the real (computed) cost of getting to that network & also keeps the advertised cost (reported distance) from its neighbor.

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The best path is the path with the lowest metric route that is calculated by adding the metric between the next-hop router and the destination (the reported distance) to the metric between the local router and the next-hop router.

This computed cost, or distance is the FD.

The next-hop router(s) are selected as the best path is the successor. If several routes have the same FD, then there can be several successors to a destination.

EIGRP Convergence (DUAL)

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RTA

RTX

RTZ

FDDI

Network24

RTY

COST

Serial10

100 100

10

20

1

Neighbor Computed Cost to 24 Report Distance to 24

RTX 40 30 RTY 31 21 RTZ 230 220

10

For RTA, RTY is the successor to Network 24, because it has the lowest computed cost (31). Hence RTA’s FD=31.

If RTY goes down, then is there a feasible successor – or not? If so, what is it?

RTA reaches Network 24 via:

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RTA

RTX

RTZ

FDDI

Network24

RTY

COST

Serial10

100 100

10

20

1

Neighbor Computed Cost to 24 Report Distance to 24

RTX 40 30 RTY 31 21 RTZ 230 220

10

Feasible successor =

1. It’s RD < RTA’s FD AND

2. Has the lowest computed cost of all other routesDUAL calculates the feasible successor to be through RTX

Remember: RTA’s FD=31

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RTA

RTX

RTZ

FDDI

Network24

RTY

COST

Serial10

100 100

10

20

1

Neighbor Computed Cost to 24 Reported Distance to 24

RTX 40 30 RTY 31 21 RTZ 230 220

10

Feasible successor =

1. It’s RD < RTA’s FD AND

2. Has the lowest computed cost of all other routes

Suppose RTX also goes down. Is there a feasible successor for RTA to Network 24?

NO! DUAL will set the route to Network 24 from passive to active state, and RTA will query its neighbor’s about Network 24 again.

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Consider the output from the command ‘show ip eigrp topology all’:

P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33via 10.2.0.2 (2681856/2169856), Serial0via 10.2.1.2 (2681856/2169856), Serial1via 10.2.2.2 (2891856/2769856), Ethernet 0

Remember:Successor is the route that has the best lowest cost or FD.Feasible successor =

It’s RD < link’s FD AND has the lowest computed cost of all other routes

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Consider the output from ‘show ip eigrp topology all’:

P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33via 10.2.0.2 (2681856/2169856), Serial0via 10.2.1.2 (2681856/2169856), Serial1via 10.2.2.2 (2891856/2769856), Ethernet 0

What are the successor(s)?

10.2.0.2 AND 10.2.1.2

What are the feasible successor(s)?

NONE

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EIGRP Operation

The 4 key steps to EIGRP operation are:

1. building the neighbor table

2. discovering routes

3. choosing routes

4. maintaining routes

Choose the routes based on these 5 factors:

1. bandwidth

2. delay

3. reliability

4. load

5. MTU (maximum transmission unit)

So, unless otherwise configured by an administrator, bandwidth & delay are the only 2 factors that determine EIGRP’s metric value.

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Configuring EIGRP

Configuring EIGRP for IP Networks

Router(config)# router eigrp autonomous-system-number

Router(config-router)# network network-number

network-number is the NETWORK address of the interface of the router

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Configuring EIGRP

The ip bandwidth-percent command configures the percentage of bandwidth that EIGRP can use on an interface.

By default, EIGRP is set to use up to 50 percent of the bandwidth of an interface to exchange routing information.

The command relies on the bandwidth of an interface.

Some cases the engineer sets the bandwidth to a lower number than the actual bandwidth of the link (in order to manipulate the routing metric).

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Summarizing EIGRP routes for IP

EIGRP automatically summarizes routes at a classful boundary.

For discontiguous subnetworks, you do NOT want summarization.

The command that turns summarization off on EIGRP is:

Router(config-router)# no auto-summary

Hence, with summarization turned off, EIGRP routers will advertise subnets.

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Manual summary routes are configured on a per-interface basis by:

Router(config-if)#ip summary-address eigrp autonomous-system-number ip-address mask administrative-distance

By default, EIGRP summary routes have an administrative distance of 5. The value can range between 1 and 255.

Summarizing EIGRP routes for IP

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Cisco 3- EIGRP Perrine and Brierley Page 110/4/2015 Module 3 EIGRP