OSPF

OSPF (Open Shortest Path First) -

Standardized Link-State routing Protocol, designed to scale efficiently to support larger networks.

OSPF Area -

Heirarchial network design for OSPF

OSPF Characteristics -

Supports only IP routing

Routes have an AD of 110

Uses cost as its metric, which is computed based on the bandwidth of the link.

No Hop-count limit.

OSPF Tables -

Neighbor table

Topology table

Routing table

OSPF Neighbor Table -

Contains list of all neighboring routers

OSPF Topology Table -

Contains list of all possible routes to all known networks within an area

OSPF Routing Table -

Contains the best route for each known network

OSPF Router ID -

Can be Manually specified.

If not manually specified, the highest IP address configured on any Loopback interface on the router will be used.

If no loopback interface exists, the highest IP address configured on any Physical interface will be used.

OSPF Hello -

Sent out OSPF enabled interfaces every 10 seconds for broadcast and point-to-point interfaces, and 30 seconds for non-broadcast and p2p interfaces.

Also used as KEEPALIVES to allow routers to quickly discover if neighbor is down.

OSPF Dead Interval -

Indicates how long a router will wait without hearing any hellos before announcing the neighbor is "down"

Default for broadcast and p2p interfaces = 40 sec Non-broadcast and p2p = 120 sec

OSPF Neighbor established when all the following match: -

Area ID

Area Type (stub, NSSA, etc)

Prefix Subnet Mask

Hello Interval

Dead Interval

Network Type (broadcast, p2p, etc)

Authentication

OSPF Neighbor Table -

Constructed from OSPF Hello packets.

Includes: Router ID of each neighboring router.

Current "state" of each neighboring router interface directly connecting to each neighbor.

IP address of the remote interface of each neighbor.

OSPF Neighbor States -

Down Init 2-Way ExStart Exchange Loading Full

OSPF Neighbor state: Down -

Indicates that no Hellos have been heard from the neighboring router

OSPF Neighbor state: Init -

Indicates that a Hello packets has been heard from the neighbor, but 2-way communication has not yet been initialized.

OSPF Neighbor state: 2-Way -

Indicates that bidirectional communication has been established.

Communication is considered this once a router sees its own Router ID in its neighbor's Hello Packet.

DR/BDR are elected at this stage.

OSPF Neighbor state: ExStart -

Indicates that the routers are preparing to share link state information.

Master/slave relationships are formed between routers to determine who will begin the exchange.

OSPF Neighbor state: Exchange -

Indicates that the routers are exchanging Database Descriptors (DBD).

DBD contain a description of the router's Topology Database.

A router will examine a neighbor's DBD to determine if it has information to share.

OSPF Neighbor state: Loading -

Indicates that the routers are finally exchanging Link State Advertisements, containing information about all links connected to each router.

Routers are sharing topology tables.

OSPF Neighbor state: Full -

Indicates routers are fully synchronized. Topology table of all routers in the area should now be identical.

OSPF LSA Types -

Type 1 - Router

Type 2 - Network

Type 3 - Network Summary

Type 4 - ASBR Summary

Type 5 - External

OSPF LSA Type 1 -

Router LSA - Contains a list of all links local to the router, and the status and "cost" of those links.

Generated by all routers in OSPF and are flooded to all other routers within the local area.

OSPF LSA Type 2 -

Network LSA - Generated by all Designated Routers in OSPF, and contains a list of all routers attached to the DR.

OSPF LSA Type 3 -

Network Summary LSA - Generated by all ABRs in OSPF, contains a list of all destination networks within an area.

Sent between areas, to allow inter-area communication to occur.

OSPF LSA Type 4 -

ASBR Summary LSA - Generated by all ABRs in OSPF, contains a route to any ADBSs in the OSPF system.

Sent from an ABR into its local area, so that Internal routers know how to exit the AS.

OSPF LSA Type 5 -

External LSA - Generated by ASBRs in OSPF, contains routes to destination networks outside the local AS.

Can also take form of a default route to all network outside the local AS.

Are flooded to all areas in the OSPF system.