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OSPF
BSCI Module 3
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Purpose of this Lesson
Coverage of topics new to the OSPF module of BSCI.
Whats new in this module?
Some new terminology and acronyms
More detailed explanation of LSAs and the Link-state Database
Several new OSPF configuration and verification commands
OSPF authentication configuration and verification commands
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New Terminology
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Minimizes routingtable entries
Localizes impact of
a topology changewithin an area
Detailed LSAflooding stops atthe area boundary
Requires ahierarchicalnetwork design
OSPF Areas
Review of OSPF area characteristics:
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Transit Area
Also known asBackbone Area 0
Regular Area
Also known asNonbackbone areas
OSPF Areas
New terminology for areas:
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OSPF Database
OSPF maintains three databases
Adjacency Database (show ip ospf neighbor)
Link-state Database (show ip ospf database)
Forwarding Database (show ip route)
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What is LSDB?
LSDB is an acronym for Link-state Database.
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LSAs and the Link-state Database
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LSA Sequence Numbering
Each LSA in the LSDB maintains a sequence number.
The sequence numbering scheme is a 4-byte numberthat begins with 0x80000001 and ends with0x7FFFFFFF.
OSPF floods each LSA every 30 minutes to maintainproper database synchronization. Each time the LSA isflooded, the sequence number is incremented by one.
Ultimately, an LSA sequence number will wrap aroundto 0x80000001. When this occurs, the existing LSA isprematurely aged to maxage (one hour) and flushed.
When a router encounters two instances of an LSA, itmust determine which is more recent. The LSA havingthe newer (higher) LS sequence number is more
recent.
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The OSPF Link-State Database
The show ip ospf database command displaysthe current LSDB for the local router.
The next slide discusses the purpose of the Age andSeq# fields highlighted below.
RTC#show ip ospf database
OSPF Router with ID (192.168.1.253) (Process ID 3)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count192.168.1.249 192.168.1.249 1705 0x80000005 0x00D5B0 5
192.168.1.253 192.168.1.253 1578 0x80000006 0x009F91 5
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Link-State Data Structures: LSAOperation
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Basic OSPFConfiguration
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Configuring OSPF
An excellent resource for information on the manydifferent OSPF configurations is the Cisco white paper,Configuring OSPF, which can be downloaded fromthe Cisco website:
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.html
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.html7/31/2019 BSCI Module 3 OSPF Edited
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router ospfprocess-id
Router(config)#
Enable one or more OSPF routing processes.
Configuring Basic OSPF
network ip-address wildcard-maskarea area-id
Router(config-router)#
Define the interfaces that OSPF will run on.
Router(config-if)#
ip ospfprocess-idarea area-id[secondaries none]
Optional method to enable OSPF explicitly on aninterface.
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Configuring OSPF for Multiple Areas
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OSPF Router ID
The router is known to OSPF by the OSPF router ID number. LSDBs use the OSPF router ID to differentiate one router from
the next.
By default, the router ID is the highest IP address on an active
interface at the moment of OSPF process startup.If no interface is up when the OSPF process starts, you will get the following error message:
p5r2(config)#router ospf 1
2w1d: %OSPF-4-NORTRID: OSPF process 1 cannot start.
A loopback interface can override the OSPF router ID. If a
loopback interface exists, the router ID is the highest IP addresson any active loopback interface.
The OSPF router-idcommand can be used to override theOSPF router ID.
Using a loopback interface or a router-idcommand isrecommended for stability.
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router-idip-address
This command is configured under the router ospf [process-id] command.
Any unique arbitrary 32-bit value in an IP address format (dotted
decimal) can be used.
If this command is used on an OSPF process that is already active,then the new router ID takes effect after the next reload or after amanual restarting of the OSPF process using:
OSPF router-idCommandRouter(config-router)#
Router(config)#router ospf 1
Router(config-router)#router-id 172.16.1.1
Router#clear ip ospf process
Router#clear ip ospf process
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OSPF over NBMA Topology Modes ofOperation
RFC 2328-compliant modes are as follows:Nonbroadcast (NBMA)
Point-to-multipoint
Additional modes from Cisco are as follows:
Point-to-multipoint nonbroadcast
Broadcast
Point-to-point
ip ospf network [{broadcast | non-broadcast | point-to-
multipoint [non-broadcast] | point-to-point}]
This interface command defines OSPF network type.
Router(config-if)#
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RFC-compliant Non-broadcast Mode
One IP subnet. Neighbors must be manually
configured.
DR and BDR elected.
DR and BDR need to have fullconnectivity with all otherrouters.
Typically used in a full meshtopology.
RTB(config-if)#ip ospf network non-broadcast--------RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0RTB(config-router)#neighbor 3.1.1.1RTB(config-router)#neighbor 3.1.1.3
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RFC-compliant Point-to-Multipoint Mode
One IP subnet. Uses multicast OSPF hello
packet to automaticallydiscover neighbors.
DR and BDR not required
Typically used in a partial-mesh or hub-and-spoketopology.
RTB(config-if)#ip ospf network point-to-multipoint--------RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0
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Ciscos Point-to-Multipoint Non-broadcast mode
RTB(config-if)#ip ospf network point-to-multipoint non-broadcast--------RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0RTB(config-router)#neighbor 3.1.1.1 cost 10RTB(config-router)#neighbor 3.1.1.3 cost 20
Cisco extension to RFC-compliant
point-to-multipoint mode Must statically define neighbors,
like nonbroadcast mode
Like point-to-multipoint mode,DR/BDR not elected
Used in special cases whereneighborscannot be automaticallydiscovered
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Ciscos Broadcast Mode
Makes a WAN interfaceappear to be a LAN
One IP subnet
Uses multicast hellos todiscover neighbors
DR and BDR elected
Requires a full mesh.
RTB(config-if)#ip ospf network broadcast--------RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0
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One IP subnet per subinterface pair
No DR or BDR election
Used when only two routers need toform an adjacency on a pair ofinterfaces
Same properties as any physicalpoint-to-point physical interface
RTB(config)#interface serial 0/0.1RTB(config-subif)#ip address 3.1.1.2 255.255.255.0RTB(config-subif)#interface serial 0/0.2RTB(config-subif)#ip address 4.1.1.2 255.255.255.0--------RTB(config-router)#network 3.1.1.0 0.0.0.255 area 0RTB(config-router)#network 4.1.1.0 0.0.0.255 area 0
Ciscos Point-to-Point mode
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Using the neighbor Command
Used to statically define neighbor relationships in an
NBMA network
neighbor ip-address [priority number] [poll-interval
number] [cost number] [database-filter all]
Router(config-router)#
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OSPF over NBMA Topology Summary
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Special Treatment for LSAs on Virtual Links
LSAs usually age out after 30 minutes
LSAs learned across virtual links have the DoNotAge(DNA) option set
Required to prevent excessive flooding over virtual links
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Configuring and Verifying a Virtual Link
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LSA Types
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LSA Types
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The Link ID in the OSPF Database
RouterA#show ip ospf database
OSPF Router with ID (10.0.0.11) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.0.11 10.0.0.11 548 0x80000002 0x00401A 1
10.0.0.12 10.0.0.12 549 0x80000004 0x003A1B 1
100.100.100.100 100.100.100.100 548 0x800002D7 0x00EEA9 2
Net Link States (Area 0)Link ID ADV Router Age Seq# Checksum
172.31.1.3 100.100.100.100 549 0x80000001 0x004EC9
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.0.0 10.0.0.11 654 0x80000001 0x00FB11
10.1.0.0 10.0.0.12 601 0x80000001 0x00F516
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LSA Type 1: Router LSA
One router LSA (type 1) for every router in an area:
Includes list of directly attached links
Identified by the router ID of the originating router
Floods within its area only; does not cross ABR
Link-state ID depends on link type
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LSA Type 2: Network LSA
Advertised by the DR of the broadcast network
Floods within its area only; does not cross ABR
Link-state ID is the DR
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LSA Type 4: Summary LSA
Summary (type 4) LSAs are used to advertise an ASBR to
all other areas in the autonomous system. They are generated by the ABR of the originating area.
They are regenerated by all subsequent ABRs to floodthroughout the autonomous system.
Link-state ID is the router ID of the ASBR.
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LSA Type 5: External LSA
External (type 5) LSAs are used to advertise networks from
other autonomous systems.
Type 5 LSAs are advertised and owned by the originatingASBR.
The Link-state ID is the external network number.
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Interpreting the OSPF Database
RouterA#show ip ospf database
OSPF Router with ID (10.0.0.11) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
10.0.0.11 10.0.0.11 548 0x80000002 0x00401A 1
10.0.0.12 10.0.0.12 549 0x80000004 0x003A1B 1
100.100.100.100 100.100.100.100 548 0x800002D7 0x00EEA9 2
Net Link States (Area 0)Link ID ADV Router Age Seq# Checksum
172.31.1.3 100.100.100.100 549 0x80000001 0x004EC9
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.0.0 10.0.0.11 654 0x80000001 0x00FB11
10.1.0.0 10.0.0.12 601 0x80000001 0x00F516
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Th l P t
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Themax-lsa Parameters
Parameter Description
maximum-numberMaximum number of non-self-generated LSAs that the OSPF process can keepin the OSPF LSBD.
threshold-
percentage
(Optional) The percentage of the maximum LSA number, as specified by themaximum-numberargument, at which a warning message is logged. Thedefault is 75 percent.
warning-only(Optional) Specifies that only a warning message is sent when the maximumlimit for LSAs is exceeded; the OSPF process never enters ignore state.Disabled by default.
ignore-timeminutes
(Optional) Specifies the time to ignore all neighbors after the maximum limit ofLSAs has been exceeded. The default is 5 minutes.
ignore-countcount-number
(Optional) Specifies the number of times that the OSPF process canconsecutively be placed into the ignore state. The default is five times.
reset-time
minutes
(Optional) Specifies the time, in minutes, after which the ignore count is reset to0. The default is 10 minutes.
max-lsa maximum-number[threshold-percentage] [warning-
only] [ignore-time minutes] [ignore-count count-number]
[reset-time minutes]
Router(config-router)#
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Changing Cost
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ip ospf cost interface-cost
Changing the Cost Metric
The cost, or metric, is an indication of the overhead tosend packets over an interface. Default = (100Mbps)/(bandwidth in Mbps).
auto-cost reference-bandwidth ref-bw
RouterA(config-if)#
RouterA(config-router)#
Overrides the default cost calculation. Values from 1 to65535 can be defined.
Sets the reference bandwidth to values other than 100Mbps (legal values range from 1 to 4,294,967 in Mbps).
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Propagating DefaultRoutes in NSSAs
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OSPFAuthentication
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Configuring Simple Password Authentication
ip ospf authentication-keypasswordRouter(config-if)#
Assign a password to be used with neighboring routers.
Router(config-if)#
ip ospf authentication [message-digest | null]
Specifies the authentication type for an interface(since IOS 12.0).
Router(config-router)#
area area-idauthentication [message-digest]
Specifies the authentication type for an area (was in IOSbefore 12.0).
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Example Simple Password AuthenticationConfiguration
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R2 Configuration for Simple PasswordAuthentication
interface Loopback0
ip address 10.2.2.2 255.255.255.0
interface Serial0/0/1
ip address 192.168.1.102 255.255.255.224
ip ospf authentication
ip ospf authentication-key plainpas
router ospf 10
log-adjacency-changesnetwork 10.2.2.2 0.0.0.0 area 0
network 192.168.1.0 0.0.0.255 area 0
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Verifying Simple Password Authentication
R1#sh ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.2.2.2 0 FULL/ - 00:00:32 192.168.1.102 Serial0/0/1
R1#show ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masksO 10.2.2.2/32 [110/782] via 192.168.1.102, 00:01:17, Serial0/0/1
C 10.1.1.0/24 is directly connected, Loopback0
192.168.1.0/27 is subnetted, 1 subnets
C 192.168.1.96 is directly connected, Serial0/0/1
R1#ping 10.2.2.2
Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/29/32 ms
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Configuring OSPF MD5 Authentication
Specifies the authentication type for an area (was in IOSbefore 12.0).
area area-idauthentication [message-digest]
Specifies the authentication type for an interface(since IOS 12.0).
ip ospf authentication [message-digest | null]
ip ospf message-digest-key key-idmd5 key
Router(config-if)#
Assign a key ID and key to be used with neighboring routers.
Router(config-if)#
Router(config-router)#
E l MD5 A th ti ti
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Example MD5 AuthenticationConfiguration
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R2 Configuration for MD5 Authenticaiton
interface Loopback0
ip address 10.2.2.2 255.255.255.0
interface Serial0/0/1
ip address 192.168.1.102 255.255.255.224
ip ospf authentication message-digest
ip ospf message-digest-key 1 md5 secretpass
router ospf 10
log-adjacency-changesnetwork 10.2.2.2 0.0.0.0 area 0
network 192.168.1.0 0.0.0.255 area 0
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Verifying MD5 Authentication
R1#sho ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.2.2.2 0 FULL/ - 00:00:31 192.168.1.102 Serial0/0/1
R1#show ip route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 2 subnets, 2 masksO 10.2.2.2/32 [110/782] via 192.168.1.102, 00:00:37, Serial0/0/1
C 10.1.1.0/24 is directly connected, Loopback0
192.168.1.0/27 is subnetted, 1 subnets
C 192.168.1.96 is directly connected, Serial0/0/1
R1#ping 10.2.2.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 28/28/32 ms
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Troubleshooting Simple PasswordAuthentication Problems
R1#
*Feb 17 18:54:01.238: OSPF: Rcv pkt from 192.168.1.102, Serial0/0/1 :
Mismatch Authentication Key - Clear Text
R2#
*Feb 17 18:53:13.050: OSPF: Rcv pkt from 192.168.1.101, Serial0/0/1 :
Mismatch Authentication Key - Clear Text
Simple authentication on R1 and R2, but differentpasswords:
R1#
*Feb 17 18:51:31.242: OSPF: Rcv pkt from 192.168.1.102, Serial0/0/1 :
Mismatch Authentication type. Input packet specified type 0, we use type 1
R2#
*Feb 17 18:50:43.046: OSPF: Rcv pkt from 192.168.1.101, Serial0/0/1 :
Mismatch Authentication type. Input packet specified type 1, we use type 0
Simple authentication on R1, no authentication on R2:
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Troubleshooting MD5 Authentication
R1#debug ip ospf adjOSPF adjacency events debugging is on
*Feb 17 17:14:06.530: OSPF: Send with youngest Key 1
*Feb 17 17:14:06.546: OSPF: 2 Way Communication to 10.2.2.2 on Serial0/0/1,
state 2WAY
*Feb 17 17:14:06.546: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0xB37 opt
0x52 flag 0x7 len 32
*Feb 17 17:14:06.546: OSPF: Send with youngest Key 1*Feb 17 17:14:06.562: OSPF: Rcv DBD from 10.2.2.2 on Serial0/0/1 seq 0x32F opt
0x52 flag 0x7 len 32 mtu 1500 state EXSTART
*Feb 17 17:14:06.562: OSPF: NBR Negotiation Done. We are the SLAVE
*Feb 17 17:14:06.562: OSPF: Send DBD to 10.2.2.2 on Serial0/0/1 seq 0x32F opt
0x52 flag 0x2 len 72
*Feb 17 17:14:06.562: OSPF: Send with youngest Key 1
R1#show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
10.2.2.2 0 FULL/ - 00:00:35 192.168.1.102 Serial0/0/1
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Summary
OSPF is an open-standard link-state routing protocol,offering quick convergence and the ability to scale largenetworks.
There are five OSPF packet types: hello, DBD, LSU,
LSR, and LSAck. Configuration of OSPF is a two-step process:
Enter OSPF configuration with the router ospf command.
Use the network command to describe which interfaces will run OSPF
in which area.
OSPF defines 3 types of networks: point-to-point,broadcast, and NBMA. On NBMA networks, OSPFmode options include nonbroadcast, broadcast, point-to-multipoint, point-to-multipoint nonbroadcast, and
oint-to- oint.
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Activity
Using this simple three router diagram and the lessons
presented in the module, setup an OSPF multiareaconfiguration including authentication.
You can also verify your connections by running theshow commands discussed in this module.
show ip ospf database, show ip ospf neighbor
Using the debugging command: debug ip ospfadj, verify OSPF operation.
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Self Check
LSA types fall into four categories, name them.
Identify each type of LSA within each category.
What are the three types of OSPF networks?
What authentication types are supported by OSPF?
What are the advantages of route summarization inOSPF?
7/31/2019 BSCI Module 3 OSPF Edited
68/70
2006 Cisco Systems, Inc. All rights reserved. Cisco PublicBSCI Module 3 68
Additional Links
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.html
http://www.cisco.com/en/US/tech/tk365/technologies_c
onfiguration_example09186a0080094069.shtml
http://www.cisco.com/en/US/products/ps6121/products_user_guide_chapter09186a00806a2f02.html
http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094069.shtmlhttp://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094069.shtmlhttp://www.cisco.com/en/US/products/ps6121/products_user_guide_chapter09186a00806a2f02.htmlhttp://www.cisco.com/en/US/products/ps6121/products_user_guide_chapter09186a00806a2f02.htmlhttp://www.cisco.com/en/US/products/ps6121/products_user_guide_chapter09186a00806a2f02.htmlhttp://www.cisco.com/en/US/products/ps6121/products_user_guide_chapter09186a00806a2f02.htmlhttp://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094069.shtmlhttp://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094069.shtmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.htmlhttp://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_configuration_guide_chapter09186a00800b3f2e.html7/31/2019 BSCI Module 3 OSPF Edited
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2006 Cisco Systems, Inc. All rights reserved. Cisco PublicBSCI Module 3 69
Q and A
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