OSPF Flashcards
OSPF Characteristics
Open standard link state protocol
Supports large networks
Very fast convergence time
Messages sent via multicast (224.0.0.5)
Uses Dijkstra’s Shortest Path First algorithm to determine the best path to learn networks
OSPF Operations
Discover neighbors
Form adjacencies
Flood Link State Database (LSDB)
Compute Shortest Path
Install best routes in routing table
Respond to network changes
OSPF Packet Types: Hello
A router will send out & listen for hello packets when OSPF is enabled on an interface, & form adjacencies with other OSPF routers on the link
Multicast to 224.0.0.5
OSPF Packet Types: DBD
DataBase Description:
Adjacent routers will tell each other the networks they know about with the DBD packet
OSPF Packet Types: LSR
Link State Request:
If a router is missing info about any of the networks in the received DBD, it will send the neighbor an LSR
OSPF Packet Types: LSA
Link State Advertisement:
A routing update
OSPF Packet Types: LSU
Link State Update:
Contains a list of LSA’s which should be updated, used during flooding
OSPF Packet Types: LSAck
Link State Acknowledgement:
Receiving routers acknowledge LSAs
Hello Packet Contents
Router ID: 32 bit unique ID for each OSPF router
Hello Interval: How often packets are sent (10 sec default)
Dead Interval: How long router waits to hear from a neighbor before declaring it out of service
Neighbors: List of adjacent OSPF routers that this router has received Hello packet(s) from
Area ID: The area configured for that interface
Router Priority: An 8 bit number used to select DR & BDR
DR & BDR IPv4 Address: If known
Authentication Flag: Authentication details if configured
Stub Area Flag: If the area is a stub area. Stub areas have a default route to their ABR rather than learning routes outside the area
OSPF Adjacency Matching Requirements
Must be in each other’s Neighbor list
Hello & Dead intervals
Area ID
IP Subnet
Authentication Flag
Stub Area Flag
MTU Size (Default 1500)
Command: Configure OSPF with Process ID of 1 for 10.0.0.0/24 and 10.1.0.0/24 networks in area 0
router ospf 1
network 10.0.0.0 0.0.0.255 area 0
network 10.1.0.0 0.0.0.255 area 0
OSPF Verification Commands
Show run | section ospf
Show ip protocols
Show ip ospf neighbor
Show ip route
Show ip ospf database
Show ip ospf int brief
OSPF Router ID
Used by OSPF routers to identify themselves
Takes form of an IP address
Defaults to being the highest IP of any loopback interfaces configured, or the highest other IP address if a loopback doesn’t exist
Can be manually specified
Command: Set OSPF Router ID to 2.2.2.2
router ospf 1
router-id 2.2.2.2
Must restart for changes to take place:
clear ip ospf process
Command: Set interface FastEthernet2/0 to Passive Interface
router ospf 1
passive-interface f2/0
Command: Set passive interface as default
Also, remove an interface from being passive
router ospf 1
passive-interface default
To remove this setting on an interface:
no passive-interface f0/0
Command: Default OSPF route injection
router ospf 1
default-information originate
OSPF Two Level Hierarchy
Transit Area (backbone/area 0): Does not generally contain end users
Regular Area (nonbackbone areas): Used to connect end users to the transit area
Small networks do not require hierarchical design & all routers can be area 0
OSPF Router Types: Backbone Routers
Routers which have all their OSPF interfaces in Area 0
OSPF Router Types: ABRs
Routers which have interfaces in multiple areas
Separates LSA flooding zones - Segregates the network
It becomes the primary point for area address summarization
Functions regularly as the source for default routes
Maintains the LSDB for each area which it’s connected to
Manual summarization (if not configured, all routes flooded everywhere)
OSPF Router Types: Normal Area Routers
Routers which have all their OSPF interfaces in a normal area
Maintain full LSDB of other routers & links in own area
Learn IA routes to other areas from their ABRs
OSPF Router Types: IA Routers
Inter-Area Routers:
Routers to other areas appear in routing table as IA routes
OSPF Router Types: ASBR Routers
Routers which redistribute into OSPF
OSPF Router Types: External Routes
Routes which are redistributed into OSPF appear as external routes
