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OSPF(Open Shortest Path First) (LSA Types (Router LSA (Type 1), Network…
OSPF(Open Shortest Path First)
Tables
1.Neighbor
Router# show ip ospf neighbor
Neighbor ID Pri State Dead Time Address Interface
7.7.7.7 1 FULL/ - :00:36 150.50.17.2 Serial0
6.6.6.6 1 FULL/DR 00:00:11 150.50.18.1 Ethernet0
The Router ID of the remote neighbor.
• The OSPF priority of the remote neighbor (used for DR/BDR
elections).
• The current neighbor state.
• The dead interval timer.
• The connecting IP address of the remote neighbor.
• The local interface connecting to the remote neighbor.
2.Topology
Router# show ip ospf database
OSPF Router with ID (9.9.9.9) (Process ID 10)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
7.7.7.7 7.7.7.7 329 0x80000007 0x42A0 2
8.8.8.8 8.8.8.8 291 0x80000007 0x9FFC 1
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
192.168.12.0 7.7.7.7 103 0x80000005 0x13E4
192.168.34.0 7.7.7.7 105 0x80000003 0x345A
The actual link (or route).
• The advertising Router ID.
• The link-state age timer.
• The sequence number and checksum for each entry.
3.Routing
OSPF areas
4 Types
Totally not so stubby area
No Type 3,5 LSA, ASBR Allowed
Stub areas
No Type 5 External LSA, No ASBR Allowed
Single exit point ,Does not allow external routing
Totally Stubby areas
No Type 3,5 LSA, No ASBR Allowed
Not So Stubby areas (NSSA)
No Type 5 External LSA, ASBR Allowed
Advantages
LSA flooding to a minimum,
LSDB smaller,
less SPF calculations
and a smaller routing table
Router Types
Internal Routers
Area Border Routers (ABRs)
Backbone Routers
Autonomous System Border Routers (ASBRs)
Multicast Addresses
All OSPF routers
224.0.0.5
Traffic from DRs and BDRs to other routers
All Designated Routers
224.0.0.6
Traffic to DRs and BDRs
LSA Types
Router LSA (Type 1)
Network LSA (Type 2)
Network Summary LSA (Type 3)
ASBR Summary LSA (Type 4)
External LSA (Type 5)
Multicast OSPF (MOSPF) LSA ( Type 6)
Not-so-stubby area (NSSA) External LSAs (Type 7)
External attribute LSA for BGP (Type 8)
Charecterstics
Administrative Distance
110
Algorithm
Dijkstra Shortest Path First
Protocol support
IP routing
Metric
Cost
Computed based on the
bandwidth of the link
Backbone Area
Area 0
Time
Update
Hello Interval
Every 30 Seconds
10 seconds (Broadcast)
30 seconds(Non Broadcast)
Dead Interval
Non Broadcast
point-to-multipoint
120 Seconds
Broadcast
Point to point
40 Seconds
default
Dead interval timer is four times the
Hello interval
Configuration
Router(config-if)# ip ospf hello-interval 15
Router(config-if)# ip ospf dead-interval 60
DR and BDR elections
OSPF priority/ Router Id
1.Highest Priority
2.Highest Router ID
Router Id
The Router ID can be manually specified
If not manually specified, highest IP address -Loopback interface
If not manually specified, highest IP address -Physical interface
Router(config-if)# ip ospf priority 125
Default priority on Cisco routers is 1
A priority of 0 means you will never be elected as DR or BDR.
Description
DR
Designated Router (DR)
BDR
Backup Designated Router (BDR)
DR/BDR election is per multi-access segment…not per area!
Neighbor States (7)
ExStart
Indicates that the routers are preparing to share link state
information. Master/slave relationships are formed
2-Way
Indicates that bidirectional communication has been established Designated and Backup Designated Routers are elected at
this stage
Init
Indicates a Hello packet has been heard from the neighbor, but twoway communication has not yet been initialized.
Down
Indicates that no Hellos have been heard from the neighboring
router
Exchange
Indicates that the routers are exchanging Database Descriptors
(DBDs).
Loading
Indicates the routers are finally exchanging Link State
Advertisements, containing information about all links connected to each router. Essentially, routers are sharing their topology tables with each other.
Full
Indicates that the routers are fully synchronized
Full/DR – indicating that the neighbor is a Designated Router (DR)
Full/BDR – indicating that the neighbor is a Backup Designated
Router (BDR)
Full/DROther – indicating that the neighbor is neither the DR or
BDR
OSPF Configuration
RouterA(config)# router ospf 1
RouterA(config-router)# router-id 1.1.1.1
RouterA(config-router)# network 172.16.0.0 0.0.255.255 area 1
RouterA(config-router)# network 172.17.0.0 0.0.255.255 area 0
Link-State Packets (LSPs)
5 Types
Link-State Request (LSR)
Database Description (DBD or DD)
Hello
Link-State Update (LSU)
Link-State Acknowledgement (LSAck):
OSPF Neighborship Requirement
Area ID
Authentication
Hello and Dead Intervals
Stub Flag
MTU Size
Troubleshooting
show ip protocols
show ip ospf
show ip ospf interface
show ip ospf neighbor
show ip ospf database
debug ip ospf packet
debug ip ospf hello
debug ip ospf adj
OSPF Network Types
Broadcast Multi-Access
indicates a topology where broadcast occurs
Point-to-Point
indicates a topology where two routers are directly
connected.
Point-to-Multipoint
indicates a topology where one interface can connect
to multiple destinations.
Non-broadcast Multi-access Network (NBMA)
indicates a topology by where one interface can connect to multiple destinations;however,broadcasts cannot be sent across a NBMA network
