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



Distant
-
vector protocol



Utilizes autonomous system

o

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



Supports VLSM and summarization



Considered a hybrid protocol

o

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

o

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)


Protocol
-
Dependent Module



Supports multiple network layer protocol



Protocol
-
dependent models (PDMs)



Each EIGRP PDM will maintain a separate series of tables containing the routing
inform
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
adjacencies



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

1.

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.

2.

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

3.

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.

4.

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
tination.

5.

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

6.

Successor
: 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


Diffusin
g Update Algorithm (DUAL)



EIGRP algorithm



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



Benefits

o

Backup route determination if one is available

o

Support of VLSMs

o

Dynamic route recoveries

o

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:

1.

EIGRP routers maintain a copy of all of their neighbors routes

2.

If there is not a viable alternative in the topology table, EIGRP router very quic
kly
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



In
a large network you should divide the network into multiple distinct EIGRP ASes
(Redistribution)



Internal EIGRP route: has an administrative distance of 90.

o

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.

o

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.



*Migra
ting from IGRP to EIGRP

o

Use redistribution

o

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.

o

172.16.10.0
----
10.3.1.0
----
172.16.20.0


Discontigious Networks



Does not work with RIPv1, RIPv2, IGRP, EIGRP



Does work with OSPF

o

Because by default OSPF does not auto
-
summ
arize



Can be made to work with EIGRP

o

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:

1.

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

2.

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

3.

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

path

1.

Bandwidth**

2.

Delay**

3.

Load

4.

Reliability

5.

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



maximum
-

paths x



E
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



*The
variance

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

o

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


Configuring EIGRP



EIGRP commands are configured in two modes

o

Router configuration mode

o

Interface configuration mode



Customization of sum
maries, metrics, timers and bandwidth



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

o

Command is
passive
-
interface interface



**
using the

passive interface
command with RIP means it will prohibit the sending of route
up
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



RIP
--
>EIGRP



redistribute eigrp 10 metr
ic 1



EIGRP
--
>RIP



changing the metric to hop count

*The no
auto
-

summary

command (EIGRP) will advertise all subnets between routers





Show IP EIGRP neighbors

command



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
neighbor.



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
-
trip
from

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
munication



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
-
consistent
ly
large values would indicate a problem.



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


Show IP EIGRP Topol
ogy Command



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

o

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

o

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


*
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

Command



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

o

Consists of areas and autonomous systems

o

Minimizes routing update traffic

o

Allows scalability

o

Supports VLSM/CIDR

o

Has unlimited hop count

o

Allows multi
-
vendor deployment (open standard)



Does not auto
-
summar
ize



Uses bandwidth as best path metric



Fast conversions



Benefits of hierarchical design

o

To decrease routing overhead

o

To speed up convergence

o

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



Link:

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
n.



Neighbor:

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



Adjacency:
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.
224.0.0.5. 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
-
s
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
-
access
net
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
-
access):

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.
Req
uirements: DR and BDR must be elected on such networks.



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



Point
-
to
-
point:
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
automatically.



Point
-
to
-
multipoint:
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

o

Network
10.0.0.0 0.255.255.255



*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 = 255.255.255.240



256
-
240
= 16 block size



Wildcard: 0.0.0.15



network 192.168.10.64 0.0.0.15 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


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


The
show ip ospf
Command



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.


The
show ip ospf database

Command



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
ike EIGRP




The
show ip ospf interface

Command



Displays all interface
-
related OSPF information



Key information displayed:



Interface IP address



Area assignment



Process ID



Router ID



Network type



Cost



Priority



DR/BDR election information (if applicable)



Hell
o and Dead timer intervals



Adjacent neighbor information


The
show ip ospf neighbor

Command



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



Elect
ions do not occur on point
-
to
-
point links


The
show ip protocols

Command



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
-
vector)


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

router.



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

1.

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.

2.

Authentication:

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.

3.

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
packets.



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.


Adjacencies



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
SPF



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
ration



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
interface


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
-
id
command



adds a new Router ID for a router



router ospf 1



router ospf 172.16.10.5



changes RID without rebooting


Loopback interface and
ro
uter
-
id



loopback/logical interface does not override
router
-
id
command


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
adjacency***



AD number takes priority when using two prot
ocols


Configuring EIGRP and OSPF Summary Routes



EIGRP auto
-
summarizes at classful boundaries



must use the
no auto
-
summary

command



OSPF and contiguous networks



use multiple areas


Summary



EIGRP



Uses the capabilities of the Reliable Transport Protocol (RTP)

to communicate between
neighbors



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