OSPF Flashcards
OSPF Charasteristics
OSPF (open shortest path first)
• dynamic IGP
• not distance vector
• Link-State algorithm
Neighbor table:
• Also known as the Adjacency database
• Contains list of recognized neighbors
Topology table:
• Typically referred to as LSDB (Link State DataBase)
• Contains all routers and their attached links in the area or network
• Identical LSDB for all routers within an area
Routing table:
• Commonly named a Forwarding database
• Contains list of best paths to destinations
OSPF Neighbours
“Hello” packets sent periodically on all OSPF- enabled interfaces
• become “neighbors”
• establishes that link can carry data
• used to determine if neighbor is up
• Adjacencies (virtual point-to-point links)
formed between some neighbors
• Broadcast* HELLO on network segment
• Receive ACK
• Establishes 2-way communication
• Repeat periodically
OSPF Adjacencies
–The number of adjacencies on the LAN segment can be large
–Exchange of significant amount of routing information between each router par
–Router could become ’overloaded’ with these exchange as well as links
Solution : lection of the ’main’ router on the segment (simular to hub-and- spoke topology)
• OSPF routers in a network which need DR (Designated router) and
BDR (Backup designated router) do not share routing information
directly with all each other’s.
• To minimize the routing information exchange, they select one router
as designated router (DR) and one other router as backup designated
router (BDR). Remaining routers are known as DROTHERs.
• All DROTHERs share routing information with DR.
• DR will share this In case DR is down, BDR will immediately take place the DR and would elect new BDR for itself.
DR and BDR Election Process
• OSPF uses priority value to select DR and BDR.
• OSPF router with the highest priority becomes DR.
• Router with second highest priority becomes BDR.
• If there is a tie, router with the highest Router ID will be chosen.
• If we priority is set to 0, it will never become DR or BDR.
Router ID
• A router ID is a 32-bit IP address that uniquely identifies a router in
an Autonomous System (AS). A router ID can be generated as follows:
• Manually configured
• Configured by the protocol
• Automatically selected
What would happen if router ID is not configured
If router ID is not configured (command), the IP address of the first
interface to come online is used. This is usually the loopback
interface. Otherwise, the first hardware interface with an IP address
is used.
OSPF Topology
• Once an adjacency is established, each router sends ’topology’ information to all neighbors
• Topology information is packaged in a “link state announcement (LSA)”
• Announcements are sent ONCE, and only updated if there’s a change (or every 30 minutes)
• Each router receives LSAs, adds them into its database, and passes the information along to its neighbors
• Each router builds identical link-state database
• Runs SPF algorithm on the database to build SPF tree
• Forwarding table built from SPF tree
When change occurs:
• Broadcast change
• All routers run SPF algorithm
• Install output into forwarding table
Link-state routers recognize more information about the network than their distance vector counterparts.
• Each router has a full picture of the topology.
• Consequently, link-state routers tend to make more accurate decisions.
SPF Calculation
Routers find the best paths to destinations by applying Dijkstra’s SPF algorithm to the link-state database as follows:
• Every router in an area has the identical link-state database.
• Each router in the area places itself into the root of the tree that is built.
• The best path is calculated with respect to the lowest total cost of links to a specific destination.
• Best routes are put into the forwarding database (routing table).
OSPF Packet Types
1.Hello
2.Database Description
3.Link-State Request
4.Link-State Update
5.Link-State Acknowledgment
Neighborship: The Hello Packet
Entry must match on neighboring routers:
1. hello and dead intervals
2.area id
3.authentification password
4.Stub area flag
Establishing Bidirectional Communication
1.DOWN STATE
sending hello msg =Area ID, hello interval, hold down timer, stub flag and MTU
2.INIT STATE
Neighborship building process starts A multicasts first hello packet so other
routers in network can learn about it. This hello packet contains Router ID
and some essential values
3.TWO-WAY STATE
If essential configuration values match,R2 will add R1 in neighbor table and
reply with its hello packet. As R2 knows the exact address of R1, it will use unicast for reply. Beside RID and configuration values, this packet also contains the R2’s neighbor table data.
Discovering the Network Routes
Routers who decided to build adjacency will form a master / slave relationship. In each adjacency router who has higher RID will become master and other will become slave.. It is used to decide the Router who will start exchange process. Always Master starts exchange process.
Adding the Link-State Entries
In this state actual routing information is exchanged. Routers exchange LSAs from LSR list. Routers will use LSU (Link state update) to exchange the LSAs. Each LSA contains routing information about a particular link. Routers also maintain a retransmission list to make sure that every sent LSA is acknowledged.
LSA Sequence Numbering
• Each LSA in the LSDB maintains a sequence number.
• OSPF floods each LSA every 30 minutes to maintain proper database synchronization. Each time the LSA is flooded, the sequence number is incremented by one.
OSPF Network Types
The three types of networks defined by OSPF are:
• Point-to-point: A network that joins a single pair of routers.
• Broadcast: A multiaccess broadcast network, such as Ethernet.
• Nonbroadcast multiaccess (also called NBMA): A network that interconnects more than two routers but that has no broadcast capability.
• Frame Relay, ATM, and X.25 are examples of NBMA networks.
• Five modes of OSPF operation are available for NBMA networks.
