Open Shortest Path First (Ospf)

Front 1

LSA Type 1

Back 1

Router LSA: Lists all of a router's links and their state. These LSAs are flooded within the area they originated.

Front 2

LSA Type 2

Back 2

Network LSA: Produced by the DR on every multiaccess network. These LSAs list all attached routers, including the DR itself; they are flooded within the originating area.

Front 3

LSA Type 3

Back 3

Network Summary: Originated ABRs; sent into an area to advertise destinations outside the area; flooded throughout the autonomous system.

Front 4

LSA Type 4

Back 4

ASBR Summary: Also originated by ABRs; the destination advertised is an ASBR; flooded throughout the autonomous system.

Front 5

LSA Type 5

Back 5

AS External: Originated by ASBRs and advertises an external destination or a default route to an external destination; flooded throughout the autonomous system.

Front 6

LSA Type 7

Back 6

NSSA External: Originated by ASBRs in not-so-stubby areas.

Front 7

RFC-Compliant mode: Nonbroadcast multiaccess (NBMA)

Back 7

■ One IP subnet required.
■ Must manually configure neighbors—neighbor <address> [priority <number>] [poll-interval <number>]
■ DR/BDR election.
■ DR/BDR need full connectivity with all routers.
■ Sometimes used in partial mesh.
■ Frame Relay and ATM networks default to this type.

Front 8

RFC-Compliant mode: Point-to-multipoint

Back 8

■ One IP subnet required.
■ Hello packets used to discover neighbors.
■ DR/BDR not required.
■ Sometimes used in partial mesh.

Front 9

Cisco mode: Point-to-multipoint nonbroadcast

Back 9

■ Used if interface does not support multicast capabilities.
■ Neighbors must be manually configured.
■ DR/BDR election is not required.

Front 10

Cisco mode: Broadcast

Back 10

■ Makes WAN appear as LAN.
■ One IP subnet required.
■ Hellos discover neighbors.
■ DR/BDR elected.
■ Requires full mesh.

Front 11

Cisco mode: Point-to-point

Back 11

■ One IP subnet required.
■ No DR/BDR election.
■ Interfaces can be LAN or WAN.

Front 12

OSPF Packet Type 1

Back 12

Hello—These packets are used to build adjacencies

Front 13

OSPF Packet Type 2

Back 13

Database Description (DBD)—Checks for database synchronization between routers

Front 14

OSPF Packet Type 3

Back 14

Link-State Request (LSR)—Requests link state specifics from the router

Front 15

OSPF Packet Type 4

Back 15

Link-State Update (LSU)—Sends requested link-state records

Front 16

OSPF Packet Type 5

Back 16

Link-State Acknowledgment (LSA)—Acknowledges the
other packet types

Front 17

All Shortest Path First Routers Multicast Address

Back 17

224.0.0.5

Front 18

Designated Router [DR]/Backup Designated Router [BDR] Multicast Address

Back 18

224.0.0.6

Front 19

Link State Summaries

Back 19

Sent every 30 minutes to ensure synchronization

Front 20

Link State Entries Max Age

Back 20

60 minutes

Front 21

Router Type: O

Back 21

OSPF intra-area (router LSA)—Networks from within the same area as the router; Type 1 LSAs are used to advertise.

Front 22

Router Type: O IA

Back 22

OSPF interarea (summary LSA)—These are networks outside of the area of the router, but within the autonomous system; Type 3 LSAs are used to advertise.

Front 23

Router Type: O E1

Back 23

Type 1 external routes—Networks outside of the autonomous system; advertised by Type 5 LSAs; calculate cost by adding the external cost to the internal cost of each link that the packet crosses; used when multiple ASBRs are advertising the external route.

Front 24

Router Type: O E2

Back 24

Type 2 external routes—Networks outside of the autonomous system; advertised by Type 5 LSAs; cost is always the external cost only. This is the default type on Cisco routers.

Front 25

Stub Area

Back 25

An area into which external LSAs are not flooded (Type 4 and 5 LSAs are blocked). Adjacencies do not form with any other router not marked as stub. Virtual links cannot be configured within a stub. To configure an area as stub, use the following command:

Router(config-router)# area <area-id> stub

Front 26

Totally Stubby Area

Back 26

Also block summary (Type 3 LSAs); the only exception is a single Type 3 default LSA. To configure an area as totally stubby, use the following command:

Router(config-router)# area <area-id> stub [no-summary]

The no-summary keyword needs to be used on the ABR only. You can change the cost of the default route sent into the totally stubby area using the following router configuration command (default cost is 1):

area <area-id> default-cost <cost>

Front 27

Not-So-Stubby Area

Back 27

Allows external routes to be advertised into the stub area Type 7 LSAs. A not-so-stubby area (NSSA) ASBR generates the Type 7 LSA, and an NSSA ABR translates it into a Type 5 LSA, which gets sent into the OSPF domain. To configure an NSSA, use the following router configuration command:

area <area-id> nssa [no-redistribution] [defaultinformation-originate]

The area area-id nssa command is used on all routers in the area; it is used in place of the area stub command.

Front 28

LSA Types Summary

Back 28

Front 29

Configure interarea route summarization on an ABR

Back 29

area <area-id> range <address> <mask>

Front 30

Configure route summarization on an ASBR

Back 30

summary-address <address> <mask> [not-advertise] [tag <tag>]

The not-advertise optional keyword suppresses routes that match the specified prefix. The tag value can be used as a “match” value for controlling redistribution with route maps on the ABR.

Front 31

Clear Text Authentication

Back 31

Step 1. Enable area authentication on all routers in the area; use the following router configuration command:

area <area_id> authentication

Step 2. Enter the clear-text password on the interface in interface configuration mode:

ip ospf authentication-key <password>

Front 32

MD5 Authentication

Back 32

Step 1. Enable MD5 area authentication on all routers in the area using router configuration mode:

area <area_id> authentication
<message-digest>

Step 2. Set the key and password on the interfaces using interface configuration mode:

ip ospf message-digest-key <key_value> md5 <password>

Front 33

Calculate Cost Formula

Back 33

cost = reference bandwidth/bandwidth

The default reference bandwidth is 100 Mbps. The bandwidth value is
that which is configured on the interface using the bandwidth command.

Front 34

Administrative Distance

Back 34

Three different administrative distance values are possible for OSPF—intra-area routes, interarea routes, and external routes. By default, all are set to 110; these can be changed with the following router configuration command:

distance ospf {[intra-area <dist1>] [inter-area <dist2>] [external <dist3>]}

Front 35

Command to Reduce LSA Flooding

Back 35

Reduces the flooding of LSAs in stable topologies by setting LSAs to “do not age”; this is accomplished with the following interface configuration command on a per-interface basis:

ip ospf flood-reduction