Switching Basics and Intermediate Routing CCNA 3 Chapter 2

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Oct 29, 2013 (3 years and 9 months ago)

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Switching Basics and Intermediate
Routing CCNA 3

Chapter 2

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States


Link
-
state routing algorithms, also known
as
shortest path first

(
SPF
) algorithms,
build a complex database of topology
information


The algorithms compute the shortest path
between nodes


Maintains full knowledge of distant routers
and how they interconnect

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States



Link
-
state routing uses
link
-
state advertisements

(
LSAs
)


A basic building block that describes a router’s local
topology and is distributed to all other routers in the
area


Link
-
state routing uses a
topological database

(or
link
-
state database
)


The set of all links learned from the flooding of LSAs


Synchronized with all other routers in the area

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States


OSPF and
Intermediate System
-
to
-
Intermediate
System

(
IS
-
IS
) are link
-
state routing protocols


Collect routing information from all other routers in the
area


Each router calculates all the best paths to all
destinations in the network


Because each router calculates best paths, they are
less likely to propagate incorrect information learned
from a neighboring router

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States


Link
-
state routing protocols were designed
to overcome the limitations of distance
vector routing protocols


Respond quickly to network changes


Send only triggered updates


Send periodic updates at long intervals, such
as every 30 minutes


A hello mechanism determines reachability of
neighbors

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States

Link
-
State Routing Relies on Complex Mechanisms to
Permit Stable, Synchronous and High
-
Speed Routing

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States


When a failure occurs in a network:


Link
-
state protocols flood LSAs; use a special
multicast address


Each link
-
state router takes a copy of the LSA,
updates its topological database, and forwards the
LSA to neighboring routers


All link
-
state routers in the area recalculate their
routing tables using the Dijkstra SPF algorithm


A link is similar to an interface on a router


The state of the link is a description of the interface
and its relation to its neighboring routers

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States

OSPF Uses a Two
-
Layer Hierarchy

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States

Two primary elements exist in the two
-
layer hierarchy

1.
Area: A grouping of contiguous networks


Areas are logical subdivisions of the autonomous system


Each area must be connected directly to the backbone area
(known as area 0)

2.
Autonomous System (AS): A collection of networks
under a common administration


Share a common routing strategy


Can be logically subdivided into multiple areas

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Link
-
State Routing Overview

Maintaining Routing Information Via Link States


The
backbone area

is the transition area


All other areas communicate through it


All
non
-
backbone areas

are connected to it


These can be configured as a stub area, a
totally stubby area, or a not
-
so
-
stubby area
(NSSA) (not covered in this curriculum) to
reduce the sizes of the link
-
state database and
the routing table

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Link
-
State Routing Overview

Link
-
State Routing Protocol Algorithms


Link
-
State Routing Protocol Algorithms:


Rely on SPF protocols to maintain a complex
database of the network topology


Develop and maintain a full knowledge of the network
routers and how they interconnect


Use LSAs to exchange information with other routers


Each router that has exchanged LSAs constructs a
topological database


The SPF algorithm is used to compute reachability to
destination networks


A routing table is built from this information, containing only
lowest
-
cost routes

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Link
-
State Routing Overview

Link
-
State Routing Protocol Algorithms


(continued):


LSA exchanges are triggered events


Greatly speed up convergence process


No need to wait for a series of timers to expire before the
networked routers can begin to converge

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Link
-
State Routing Overview

Link
-
State Routing Protocol Algorithms


Cost Metric
Determines
Shortest Path
for Link
-
State
Routing
Protocols

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Link
-
State Routing Overview

Link
-
State Routing Protocol Algorithms


Next Hops and
Costs for
Destination
Routes
(Previous Slide)

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Link
-
State Routing


Benefits of

Link
-
State Routing


Link
-
state protocols use cost metrics to choose
paths


Cost metric reflects the capacity of the links


Routing updates are less frequent


Network can be segmented into area hierarchies


Limits the scope of route changes


Link
-
state protocols send only updates of a
topology change


Use triggered, flooded updates which lead to faster
convergence times

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Link
-
State Routing


Benefits of

Link
-
State Routing


Each router has a complete and synchronized
picture of the network


Difficult for routing loops to occur


LSAs are sequenced and aged


Routers always base their routing information on the
most recent set of information


With careful design work, size of link
-
state
databases can be minimized


Smaller Dijkstra calculations and faster convergence

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Link
-
State Routing


Limitations

of

Link
-
State Routing


In addition to a routing table, link
-
state
protocols require:


A topological database


An
adjacency database


Lists all the relationships formed between
neighboring routers for the purpose of exchanging
routing information


A
forwarding table


A data structure of a stripped down association
between network prefixes and next hops

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Link
-
State Routing


Limitations

of

Link
-
State Routing


Dijkstra’s algorithm requires CPU cycles to
calculate best paths through the network


If the network is large or unstable, this can require a
significant amount of CPU time


Not a problem for most modern routers


A strict hierarchical network design is required to
divide the network into smaller areas


Reduces the excessive use of memory and CPU
cycles


Reduces size of topology tables and Dijkstra
calculations


Areas must be contiguous at all times

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Link
-
State Routing


Limitations

of

Link
-
State Routing


Although configuration of link
-
state networks is
usually simple, configuring a large network can
be challenging


Trouble
-
shooting is usually easier, as every
router has a copy of the topology


However, interpreting the information requires a good
understanding of link
-
state routing concepts


Link
-
state protocols usually scale to bigger
networks than distance vector protocols

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Link
-
State Routing


Limitations

of

Link
-
State Routing


Link
-
state routing raises two concerns:


During the initial discovery process, link
-
state
routing protocols flood the network with LSAs


Significantly decreases the network’s capability to
transport data


This is temporary, but noticeable


Link
-
state routing is both memory
-

and
processor
-
intensive


Greater demand requires higher
-
end routers that
cost more

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Single
-
Area OSPF Concepts


OSPF was developed by the Interior
Gateway Protocol (IGP) group of the
Internet Engineering Task Force

(
IETF
)


Created in mid 1990s because RIP was
unable to serve large, heterogeneous
networks


OSPF has two primary characteristics:


Protocol is an open standard, not proprietary


Based on the SPF algorithm

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Single
-
Area OSPF Concepts

Comparing OSPF with Distance Vector Routing
Protocols


OSPF is a link
-
state protocol, RIP and IGRP are
distance vector protocols


Distance vector protocols send all, or a portion of,
their routing table in updates to their neighbors


A link is an interface on a router


The state of the link describes the interface and its
relationship to neighboring routers


Can include IP address, subnet mask, type of network


The collection of link states forms a link
-
state
database

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Single
-
Area OSPF Concepts

Comparing OSPF with Distance Vector Routing
Protocols


An OSPF router sends LSA packets to
periodically advertise its link states instead
of sending routing table updates


Information about attached interfaces and
metrics are included


LSAs are flooded to all routers in the area


As OSPF routers accumulate link
-
state
information, they use the SPF algorithm to
calculate the shortest path to each destination

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Single
-
Area OSPF Concepts

Comparing OSPF with Distance Vector Routing
Protocols


A topological (link
-
state) database is an
overall picture of networks in relationship
to routers


Contains the collection of LSAs received from
all routers in the same area


Database is pieced together from the LSAs


Routers in the same area have identical
topological databases

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Single
-
Area OSPF Concepts

Comparing OSPF with Distance Vector Routing
Protocols


OSPF can operate within a hierarchy


The largest entity is the Autonomous System
(AS):


A collection of networks under a common
administration that share a common routing
strategy


An AS can be divided into several areas, which are
groups of contiguous networks and attached hosts

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Single
-
Area OSPF Concepts

OSPF Hierarchical Routing


OSPF’s capability to separate a large
network into multiple areas is known as
hierarchical routing


Hierarchical routing enables you to separate a
large internetwork (AS) into smaller
internetworks called areas


Routing still occurs between areas


Many of the minute internal routing operations,
such as recalculating the database, are kept within
an area

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Single
-
Area OSPF Concepts

OSPF Hierarchical Routing


OSPF Uses
Areas to
Provide
Hierarchy

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Single
-
Area OSPF Concepts

OSPF Hierarchical Routing


OSPF’s hierarchical topology possibilities
have the following advantages:


Reduced frequency of SPF calculations


Smaller routing tables


Reduced link
-
state update overhead

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Single
-
Area OSPF Concepts

Dijkstra’s Algorithm


In Dijkstra’s algorithm, the best path is the
lowest cost path


Named for
Edsger Wybe Dijkstra
, a Dutch
computer scientist


Each link has a cost


Each node has a name


Each node has a complete topological
database

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Single
-
Area OSPF Concepts

Dijkstra’s Algorithm

Dijkstra’s Algorithm Uses Cost Metric

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Single
-
Area OSPF Concepts

Dijkstra’s Algorithm


Dijkstra’s algorithm places each router at the
root of a tree


Calculates the shortest path to each node based on
the cumulative cost to reach the destination


Each router has its own view of the topology


Each router uses the information in its topological
database to calculate a shortest
-
path tree, with itself
as the root


The router uses this tree to route network traffic

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Single
-
Area OSPF Concepts

Dijkstra’s Algorithm


The cost, or metric, of an interface
indicates the overhead that is required to
send packets across that interface


The
OSPF cost

of an interface is inversely
proportional to that interface’s bandwidth


Higher bandwidth equals lower cost


Cost = 100,000,000 / bandwidth in bps

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Single
-
Area OSPF Concepts

Dijkstra’s Algorithm

Shortest Path is Measured from Each Root Node
to Build a Shortest Path Tree

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Single
-
Area OSPF Configuration

Basic OSPF Configuration


The
router ospf

command takes a
process identifier as an argument:


Router (config)#
router ospf

process
-
id


The process ID is a locally significant number
between 1 and 65,535 that you select to
identify the routing process


It does not need to match the OSPF process ID on
other OSPF routers

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Single
-
Area OSPF Configuration

Basic OSPF Configuration


The
network

command identifies which IP
networks on the router are part of the OSPF
network:


Router(config
-
router)#
network

address

wildcard
-
mask

area

area
-
id
(all on one command line)

Parameters of a
network

Command

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Single
-
Area OSPF Configuration

Basic OSPF Configuration


The
wildcard mask

is sometimes called an
inverse mask

because it is the inverse of the
subnet mask for the network


This is not required; many network administrators use the
0.0.0.0 option to match the interface


Basis OSPF Network with Each Router in Area 0

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Single
-
Area OSPF Configuration

Basic OSPF Configuration


Using the
network

statement in
OSPF

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Single
-
Area OSPF Configuration

Basic OSPF Configuration


A router uses the
OSPF hello protocol

to
establish neighbor relationships


Hello packets let other routers know they are still functional


On networks supporting more than two routers
(
multiaccess networks
), such as Ethernet
networks, the hello protocol elects:


A
designated router
(
DR
)


Generates LSAs


Manages
link
-
state synchronization


A
backup designated router

(
BDR
)


Becomes the DR if the existing DR fails

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Single
-
Area OSPF Configuration

Loopback Interfaces


The OSPF router ID is the number by which the
router is known to OSPF


To modify the OSPF router ID to a loopback
address use this command:


Router(config)#
interface loopback

number


The highest IP address on an active interface of
a router at startup can be overridden by using a
loopback address


OSPF is more reliable if a loopback interface is configured
because a loopback interface is always active

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Single
-
Area OSPF Configuration

Modifying the OSPF Cost Metric


OSPF uses cost as the metric to
determine the best route


Cost is associated with the output side of an
interface


It is calculated with the formula


cost = 100,000,000/bandwidth in bps


The lower the cost, the more likely the route is to be
used

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Single
-
Area OSPF Configuration

Modifying the OSPF Cost Metric

OSPF Cost Values

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Single
-
Area OSPF Configuration

Modifying the OSPF Cost Metric


It is essential for proper OSPF operation that
the correct interface bandwidth is set:


Router(config)#
interface serial 0


Router(config
-
if)#
bandwidth 56


Cost can be changed to influence the outcome of OSPF
cost calculation


When costs are from different vendors are unequal, might want
to make change to match costs


Might need to change cost to account for Gigabit Ethernet


Use this command to change cost:


Router(config
-
if)#
ip ospf cost

number


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Single
-
Area OSPF Configuration

OSPF Authentication


A router trusts the information that is coming from a
router that should be sending it the information


To guarantee this trust, routers in a specific area can be
configured to authenticate each other with
OSPF
authentication


Each interface can present an authentication key that the router
uses to send OSPF information to other routers on the segment


The key, known as a password, is a shared secret between the
routers


The key can be up to eight characters long


The key generates the authentication data in the OSPF header

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Single
-
Area OSPF Configuration

OSPF Authentication


Use the following syntax to configure OSPF
authentication:


Router(config
-
if)#
ip ospf authentication
-
key

password


After the password is configured, authentication
must be enabled:


Router(config
-
router)#
area

area
-
number

authentication


With simple authentication, the password is sent as
plain text (security risk)


Configure encryption of the password

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Single
-
Area OSPF Configuration

OSPF Authentication


Authentication password encryption syntax:


Router(config
-
if)#
ip ospf message
-
digest
-
key

key
-
id encryption
-
type md5

key (all on one line!)


The
key
-
id

is an identifier with a value of between 1 and 255


The
encryption
-
type

refers to the type of encryption, where 0 means
none and 7 means proprietary


The following is configured in router configuration mode on
a router with an interface in the area
area
-
id


Router(config
-
router)#
area

area
-
id

authentication message
-
digest


MD5 creates a message digest, which is scrambled data
based on the password and the message contents


If the digests match, the receiving router trusts the data

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Single
-
Area OSPF Configuration

OSPF Network Types and OSPF Timers


OSPF interfaces automatically recognize three
OSPF

network types
:


Broadcast multiaccess, such as Ethernet


Point
-
to
-
point networks


Nonbroadcast multiaccess networks (NBMA), such as Frame Relay


An administrator can manually configure a fourth OSPF network
type: point
-
to
-
multipoint


In a multiaccess network, it is not known in advance how
many routers will be connected


In point
-
to
-
point networks, only two routers will be
connected

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Single
-
Area OSPF Configuration

OSPF Network Types and OSPF Timers


In a broadcast multiaccess network segment, many
routers can be connected


If every router has to establish adjacency with every
other router, [
n

* (
n
-
1) / 2] adjacencies need to be formed


For 5 routers the formula would be 5*(5
-
1) / 2 = 5*4 / 2 = 20 / 2 =
10 adjacencies


Routers hold an election for a DR router


This router becomes adjacent to all other routers in the
broadcast segment


All other routers send their link
-
state information to the DR


The DR sends link
-
state information to all other routers on the
segment by using the 224.0.0.5 multicast address

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Single
-
Area OSPF Configuration

OSPF Network Types and OSPF Timers


Despite the gain in efficiency that electing a DR
provides, a disadvantage exists:


The DR is a single point of failure


A second router is elected the BDR to take over in
case the DR fails


To make sure that both the DR and BDR see the
link states that all routers send on the segment, the
224.0.0.6 multicast address is used


On point
-
to
-
point networks, no DR or BDR is
elected; both routers become fully adjacent

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Single
-
Area OSPF Configuration

OSPF Network Types and OSPF Timers

OSPF Network Type, Characteristics, and DR Election

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Single
-
Area OSPF Configuration

OSPF Network Types and OSPF Timers


OSPF uses:


Hello intervals


Default of 10 seconds on broadcast networks


Default of 30 seconds on nonbroadcast networks


Dead intervals (4 times the hellow interval by default)


Default of 40 seconds on broadcast networks


Default of 120 seconds on nonbroadcast networks


To change the default times:


Router(config
-
if)#
ip ospf hello
-
interval

seconds


Router(config
-
if)#
ip ospf dead
-
interval

seconds

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Single
-
Area OSPF Configuration

Propagating a Default Route


OSPF routing ensures loop
-
free paths to every
network in the routing domain


To reach networks outside the domain, either OSPF must
know about the network or OSPF must have a default
route


To have an entry for every network in the world would require
enormous resources for each router


A practical alternative is to add a default route to the OSPF router
connected to the outside network


This default route can be redistributed to each router in the AS
through normal OSPF updates

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Single
-
Area OSPF Configuration

Propagating a Default Route


To configure a static default route:


Router(config)#
ip route 0.0.0.0 0.0.0.0

[
interface

|
next
hop address
]


This is referred to as the quad
-
zero route


Any destination network address is matched


To propagate this route to all the routers in a normal
OSPF area:


Router(config
-
router)#
default
-
information originate


All routers in the OSPF area learn a default route provided that
the interface of the border router to the gateway router is active

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Single
-
Area OSPF Configuration

Verifying OSPF Configuration


Several
show

commands display information about
OSPF configuration:


Display parameters about timers, filters, metrics and
networks:
show ip protocols


Display the routes that are known to the router:
show ip
route


Verify that interfaces have been configured in the
intended areas:
show ip ospf interface


Display OSPF neighbor information on a per
-
interface
basis:
show ip ospf neighbor

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Single
-
Area OSPF Configuration

Troubleshooting OSPF

Output from the
debug ip ospf events

Command

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Single
-
Area OSPF Configuration

Troubleshooting OSPF


The
debug ip ospf events

output might appear if:


The IP subnet masks for routers on the same network do not match


The OSPF hello interval does not match that configured for a
neighbor


The OSPF dead interval does not match that configured for a
neighbor


If a router configured for OSPF does not see a router on an
attached network


Make sure both routers are configured with the same subnet mask,
OSPF hello and dead intervals


Make sure both neighbors are part of the same area type

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Single
-
Area OSPF Configuration

Troubleshooting OSPF

Sample Output from the
debug ip ospf packet

Command

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Single
-
Area OSPF Configuration

Troubleshooting OSPF

Fields in
debug ip ospf packet

Output

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Single
-
Area OSPF Configuration

Troubleshooting OSPF

Fields in
debug ip ospf packet

Output (continued)

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Summary


Link
-
state routing protocols such as OSPF and IS
-
IS quickly
and reliably propagate routing information within an AS


Link
-
state routing protocols build link
-
state databases, which
are synchronized with link
-
state advertisements (LSAs)


The link
-
state protocol then applies Dijkstra’s algorithm (SPF) to
determine the best path(s) to each destination, which are then
installed in the routing table


OSPF is the most commonly deployed link
-
state protocol


Employs DRs and BDRs on broadcast segments to optimize
propagation of link
-
state information


Each link uses hello and dead interval timers depending on OSPF
network type: broadcast multiaccess, NBMA, point
-
to
-
point, point
-
to
-
multipoint

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Summary


OSPF is configured by:


Defining which interfaces will participate in a given OSPF process for
a specific area


Use the
network

statements coupled with inverse masks


Inverse masks are often created to exactly match the subnet mask of the
network associated with the given link, or they can be defined simply
with a 0.0.0.0 mask to exactly match their interface ID


Verifying OSPF configurations is done with these
commands:
show ip protocol
,
show ip route
,
show ip
ospf interface
,
show ip ospf neighbor


Troubleshooting OSPF is done with these commands:
debug ip ospf events
,
debug ip ospf packets