Chapter 7 Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF)

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Chapter 7 Enhanced IGRP (EIGRP) and Open Shortest Path First (OSPF)

EIGRP Features and Operation

Classless Protocol

vector protocol

Utilizes autonomous system


Set of contiguous routers that run the same routing protocol and share the same
ng information

Supports VLSM and summarization

Considered a hybrid protocol


It sends traditional distance
vector updates containing information about
networks plus the cost of reaching them from the perspective of the advertising


It synchronize
s routing tables between neighbor at start
up and then sends
specific updates only when topology changes occur.

Designed for large networks

Maximum hop count is 255 (default is 100)

Efficient neighbor discovery

Communication via Reliable Transport Protoc
ol (RTP)

Best path selection via Diffusing Update Algorithm (DUAL)

Dependent Module

Supports multiple network layer protocol

dependent models (PDMs)

Each EIGRP PDM will maintain a separate series of tables containing the routing
ation in relation to a specific protocol

Neighbor Discovery

Before EIGRP routers become neighbors they must meet three conditions:

Hello or ACK received

AS numbers match

Identical metrics (K values)

To maintain neighborship relationship EIGRP routers mus
t continue to receive Hellos
from their neighbors

Link state protocols use hello messages to establish neighborship, also called

Link state do not normally send out periodic route updates

EIGRP routers that don’t belong to the same AS do not a
utomatically share routing
information and they don’t become neighbors

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

Only changes to routin
g table propagated from then on

Routing Table Terminology


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. Considered the best path in
the routin
g table. It is reported by the neighbor.


Reported/advertised distance
: This is the metric of a remote network, as reported by a


Neighbor table
: Each router keeps state information about adjacent neighbors. When a
newly discovered neighbor is le
arned, the address and interface of that neighbor is
recorded in the neighbor table/RAM. Remember there is one neighbor table for each
protocol dependent module. Sequence numbers are used to match acknowledgments with
update packets.


Topology table
: The t
opology table is populated by the protocol
dependent modules and
acted upon by the Diffusing Update Algorithm (DUAL). Contains all destinations advertised
by neighboring routers: each destination address and a list of neighbors that have
advertised the des


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 maintain up to six feasible
successors in the topology table. Only the best metric is
copied and placed in the routing
table. *Is a backup route


: A successor router (think successful!) is the best route to a remote network. A
successful router is used by EIGRP to forward traffic to a destination and is stored in the
routing table.

Reliable Transport Protocol (RTP)

Proprietary protocol utilized by EIGRP

Manages the communication of messages between EIGRP routers

Reliability is a key factor

Each EIGRP router is aware of who its neighbors are

It first sends a multicast message to
its neighbors to get a list

If that fails it sends a unicast with the same data

If no response after sixteen attempts the neighbor is declared dead

The way EIGRP routers keep track of information they send is by using sequence
numbers per packet

g Update Algorithm (DUAL)

EIGRP algorithm

Used for selecting and maintaining the best path to each remote network



Backup route determination if one is available


Support of VLSMs


Dynamic route recoveries


Queries for an alternate route if no route
can be found

Provides the fastest route conversion time among all protocols

There are two reasons for this:


EIGRP routers maintain a copy of all of their neighbors routes


If there is not a viable alternative in the topology table, EIGRP router very quic
asked their neighbors for help

Using EIGRP to Support Large Networks

Support for multiple ASes on a single router

Support for VLSM and summarization

Route discovery and maintenance

Multiple ASes

Routers that have the same AS share the same routes

a large network you should divide the network into multiple distinct EIGRP ASes

Internal EIGRP route: has an administrative distance of 90.


These are routes originating within a specific AS by EIGRP routers that are
members of the same AS.

External EIGRP route: has an administrative distance of 170.


These routes appear within EIGRP route tables courtesy of either manual or
automatic redistribution, and they represent networks that originated outside of
the EIGRP autonomous system.

ting from IGRP to EIGRP


Use redistribution


Use the same autonomous system number for both protocols

VLSM Support and Summarization

Conserves address space

Supports the use of discontiguous subnets

Discontiguous network: one that has two or more subn
etworks of a classical network
connection together by different classful networks.


Discontigious Networks

Does not work with RIPv1, RIPv2, IGRP, EIGRP

Does work with OSPF


Because by default OSPF does not auto

Can be made to work with EIGRP


Supports the manual creation of summaries at any and all EIGRP routers

Route Discovery and Maintenance

Routing by rumor

Neighbors are discovered via hello process and the link states on monitor

EIGRP routers maintai
ns three tables:


Neighborship table: Records information about routers with whom neighborship have
been formed,.


Topology table: stores the route advertisements about every route in the internetwork
received from each neighbor.


Route table: stores the rout
es that are currently used to make routing decisions. There
would be separate copies of each of these tables for each protocol that is actively being
supported by EIGRP

EIGRP Metrics

EIGRP uses four variables to compare routes and select the best possible











MTU size (redistribution)

By bandwidth and delay of the line are used by default**

Maximum Path and Hop Count

EIGRP can load
balance across or up to six links (equal or unequal)

Router eigrp 10


paths x

IGRP has the maximum hop count of 100

To change the hop count type
metric maximum
hops x

Hop count is not used in path metric calculation

However maximum hop count is used to limit the scope of the AS


command allows EIGRP to load
balance up t
o six on the equal cross links


The variance metric is set to one by default (only equal cost links will load

Configuring EIGRP

EIGRP commands are configured in two modes


Router configuration mode


Interface configuration mode

Customization of sum
maries, metrics, timers and bandwidth

To prevent an interface from receiving or sending Hello packets (forming adjacencies)


Command is
interface interface

using the

passive interface
command with RIP means it will prohibit the sending of route
dates but allow their receipt.

Redistribute command

RIP hop count must match EIGRP metric values (redistribution)

bandwidth metric, delay, reliability, load, and MTU

redistribute rip metric 10000000 20000 255 1 1500


redistribute eigrp 10 metr
ic 1


changing the metric to hop count

*The no


command (EIGRP) will advertise all subnets between routers

Show IP EIGRP neighbors


The H field indicates the order in which the neighbor was discovered.

The hold time is ho
w long this router will wait for a Hello packet to arrive from a specific

The uptime indicates how long the neighborship has been established.

The SRTT field is the smooth round
trip timer

an indication of the time it takes for a round

this router to its neighbor and back. This value is used to determine how long to wait after
a multicast for a reply from this neighbor. First a multitask is requesting from the neighbor, if
none received in time a unicast is initiated to complete the com

The Retransmission Time Out (RTO) field, which is the amount of time EIGRP waits before
retransmitting a packet from the retransmission queue to a neighbor.

The Q value indicates whether there are any outstanding messages in the queue
large values would indicate a problem.

The Seq field indicates the sequence number of the last update from that neighbor
that’s used to maintain synchronization and avoid duplicate or out
sequence processing of

Show IP EIGRP Topol
ogy Command

Routes denoted by a P, means that it is in a passive state


Passive state means that the route has maintained its path to the network


Active State or denoted by A, indicates that the router has lost its path to the network
and is looking for a r

In order for the route to be a feasible successor, its advertised distance must be less than the feasible
distance of the successor route.

Debug IP EIGRP Notification


This command output should show nothing

The only time you will s
ee output if there is a problem on the network or you added or deleted
a network from a router

Open Shortest Path First (OSPF) Basics

Open standard routing protocol

How it works: the shortest path tree is contrasted and then the routing table is calculat
ed with
the resulting best paths

Additional features


Consists of areas and autonomous systems


Minimizes routing update traffic


Allows scalability


Supports VLSM/CIDR


Has unlimited hop count


Allows multi
vendor deployment (open standard)

Does not auto

Uses bandwidth as best path metric

Fast conversions

Benefits of hierarchical design


To decrease routing overhead


To speed up convergence


To confine network instability to single areas of the network

ABR=Area Border Routers

All routers should connect t
o the backbone generally labeled area 0

The router that connects these ASes is called an Autonomous System Boundary Router (ASBR).

OSPF Terminology


is a network or router interface assigned to any given network. Contains state information
(up or dow
n) and IP addresses

Router ID:

an IP address used to identify the router. The highest IP address of all configured
loopback interfaces is chosen by default. If no loopback is configured, the highest IP address of
all active and physical interface is chose


two or more routers that have an interface on a common network (Point to point
serial link)

a relationship between two OSPF routers that permits the direct exchange of route
updates requirements (very picky!!!) : type of network and

configuration of routers.

Hello Protocol:

provides dynamic neighbor discovery and maintains neighbor relationships. topology database

Neighborship Database
: is a list of all OSPF routers for which Hello packets have been seen.
Router ID and sta
te are listed.

Topological Database
: contains information from all of the LSA packets that have been received
for an area. The main purpose is to compute the shortest path to every network.

Link State Advertisement:
is an OSPF data packet containing link
tate and routing information
that’s shared among OSPF routers. An OSPF router will exchange LSA packets only with routers
to which it has established adjacencies.

Designated Router:

(DR) is elected whenever OSPF are connected to the same multi
work. The DR is decided by the router with the highest priority. In the event of a tie, router
ID is used to break the tie.

Backup Designated Router: (BDR)
is a hot standby for DR on multi
access links. The BDR
receives all routing update from OSPF adjacen
t routers but does not flood LSA updates.

OSPF areas:
a grouping of contiguous networks and routers. Routers in the same area share a
common Area ID. Area ID is associated with specific interfaces on the router, hence different
interface can belong to diff
erent areas. However, same areas have the same topology table.

Broadcast (multi

allow multiple devices to connect/access the same network as well as
provide a broadcast ability in which a single packet is delivered to all nodes on the network.
uirements: DR and BDR must be elected on such networks.

broadcast (multi
: (NBMA) These networks allow for multi
access but have no
broadcast ability. Frame relay, X.25, and ATM.

type of network topology consisting of a direct co
nnection between two routers
via a single communication path. DRs and BDRs are not needed. Neighbors are discovered

type of network topology consisting of a series of connection between a
single interface on a one router

and multiple destination routers. DRs and BDRs are not needed.

SPF Tree Calculation

Shortest path first algorithm

RFC 2338

Configuring OSPF

Router ospf x

X= process ID

Process ID is locally significant

OSPF using wild card for network mask



*OSPF router will only become neighbors if their interfaces share a network that’s configured to
that belong to the same area number.

Wildcards and OSPF

Wildcards: always one less than the block size.

/28 =

= 16 block size


network area 0

Verifying OSPF Configuration

sh ip route output

O denotes OSPF internal routes

C denotes directly connected networks

* uses bandwidth to determine the best path to a network

tant note: OSPF can load
balance only across links of equal costs. It can’t load
balance across
cost links as EIGRP can.

show ip ospf

Lists: Router ID, area information, SPF statistics, and LSA timer information

Displays OSPF informat
ion for one or all OSPF processes running on the router.

show ip ospf database


Gives you information about the number of routers in the internetwork (AS) plus the
neighboring router’s ID (topology database)

shows only routers, not every link l

show ip ospf interface


Displays all interface
related OSPF information

Key information displayed:

Interface IP address

Area assignment

Process ID

Router ID

Network type



DR/BDR election information (if applicable)

o and Dead timer intervals

Adjacent neighbor information

show ip ospf neighbor


Summarizes the pertinent OSPF information regarding neighbors and the adjacency state.

Also shows if a DR or BDR has been elected

Vital in production networks

ions do not occur on point
point links

show ip protocols


Overview of the actual operation of all currently running protocols.

It shows:

OSPF Process ID

OSPF Router ID

Type of OSPF area

networks and areas configured for OSPF

OSPF Router IDs

of neighbors

Does not show/use timers to keep the network stable (like distance

Debugging OSPF

debug ip ospf packet

Shows Hello packets being sent and received on your router

debug ip ospf hello

Shows Hello packets being sent and received on your


Includes area # and multicast address

debug ip ospf adj

Shows DR and DBR elections on a broadcast and non
broadcast multi
access network
(real time)

OSPF DR and BDR Elections

**The election process happens when a broadcast or non
broadcast multi
access network is
connected to a router and the link comes up.**

Neighborship criteria:

Routers that share a common segment become neighbors on that segment

Elected via Hello protocol (multicast)

Requirements for neighborship


Area ID:

The idea here is that

the two routers’ interfaces have to belong to the
same area on a particular segment. And of course, those interfaces have to
belong to the same subnet.



OSPF allows for the configuration of a password for a specific
area. It is optional ho
wever if authentication is used, the passwords have to be
the same on the segment.


Hello and Dead:

intervals OSPF exchanges Hello packets on each segment. This
is a keepalive system used by routers to acknowledge their existence on a
segment and for electi
ng a designated router (DR) on both broadcast and non
broadcast multi
access segments.

The Hello interval specifies the number of seconds between Hello

The Dead interval is the number of seconds that a router’s Hello packets
can go without being s
een before its neighbors declare the OSPF router
dead (down).

The intervals must be the same on the segment, if not routers will not
become neighbors.


Next step after neighborship process

Databases are exchanged (no Hello packets)

To minimize
overhead on a segment, OSPF elects one DR and BDR per multi
access segment

The DR and BDR act as a central point of contact for information exchange (reduce overhead)

DR and BDR Elections

Elections are accomplished via the Hello protocol

Hello packets a
re exchanged via IP multicast packets on each segment

Only broadcast and non
broadcast multi
access networks do elections**

Ethernet and Frame Relay

The router with the highest OSPF priority on a segment will become the DR for that segment**

Default value
is 1

value of 0 means it will not participate in the election process

If all routers have value of 1, the highest Router ID wins

Router ID is based on IP address of any interface at OSPF startup

OSPF and Loopback Interfaces

Use loopback interfaces with O

Loopback interfaces: are logical interfaces, which are virtual, software
only interfaces; they are
not real router interfaces

Having loopback interfaces ensures that that an interface is always active for OSPF processes***

Diagnostics and OSPF configu

By having a loopback interface, you ensure that it it has Router ID

Router ID is used to advertise the routes as well as elect the DR and BDR.

Prevent the highest IP address from becoming Router ID by using logical/loopback

Configuring Lo
opback Interfaces

sh ip ospf

int loopback 0

Each router has to be in a separate subnet

Can use any IP address we want as long as the addresses are never the same on any two routers

Use /32 to save subnets

Changing Router ID

either reboot router or delete
OSPF and re
create the database

reboot is better option

sets Router ID on logical addresses

router ospf process

adds a new Router ID for a router

router ospf 1

router ospf

changes RID without rebooting

Loopback interface and

loopback/logical interface does not override

OSPF and loopback interfaces

saves address space (not advertised)

does not appear in OSPF table (can't ping)

OSPF Interface Priorities

fixing elections for DR and BDR

Using priorities

instead of logical addresses

priority of 0 ensures it cannot participate

better to just change the priority on the desired interface***

ip ospf priority 2

must reload or shutdown DR & BDR to take effect***

debug ip ospf adj

to see the elections occur o
n a broadcast or non
broadcast multi
access network

Troubleshooting OSPF

look at address and wildcards

If two directly connected routers do not have the timers set the same, they will not form an

AD number takes priority when using two prot

Configuring EIGRP and OSPF Summary Routes

EIGRP auto
summarizes at classful boundaries

must use the
no auto


OSPF and contiguous networks

use multiple areas



Uses the capabilities of the Reliable Transport Protocol (RTP)

to communicate between

Utilizes the Diffusing Update Algorithm (DUAL) to compute the best path to each
remote network