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By default, DRs are elected on point-to-point Ethernet links even though this is unnecessary (Ethernet is always considered a broadcast medium). Interfaces can be manually configured to operate in point-to-point mode to prevent this.
(B)DRs are not preemptable. New election will take place only when a current (B)DR goes offline or its OSPF process is administratively restarted.
Path CalculationAdjacencies within an area are advertised with type 1 and 2 LSAs, which are passed to the backbone by an ABR as type 3 summaries. These summaries are then injected into other areas from the backbone.
Virtual links are not encouraged and should be used only as a temporary fix.
Virtual links cannot use a stub area for transit.
Protocol Attributes
| Protocol Type | Link-state |
|---|---|
| Algorithm | Dijsktra |
| Metric (IOS) | Cost (bandwidth) |
| Administrative Distance | 110 |
| Specification | RFC 2328, RFC 2740 |
| Supported Protocols | IPv4 (OSPFv2), IPv6 (OSPFv3) |
| Transport | IP/89 |
| Authentication | None, plain text, MD5 |
| AllSPF Multicast Address | 224.0.0.5 |
| AllDR Multicast Address | 224.0.0.6 |
| Hello Timer | 10 seconds (broadcast), 30 seconds (nonbroadcast) |
Packet Types
- Hello - Used to establish communication with directly connected neighbors
- Database Descriptor (DBD) - Lists router IDs from which the router has an LSA and its current sequence number
- Link State Request (LSR) - Request for an LSA
- Link State Update (LSU) - Reply to an LSR with the requested information
- Link State Acknowledgment (LSAck) - Used to confirm receipt of link-state information
Adjacency Formation
Contents of an OSPF hello packet:- Router ID - 32-bit unique number (IP address)
- Hello/dead intervals - Timers
- Neighbor list - List of neighboring router IDs
- Area ID
- Priority - Used in the DR and BDR elections
- DR and BDR identifiers
- Authentication (if enabled)
- Stub Area Flag - On if this is a stub area
- Down
- Attempt - Used for manually configured neighbors on an NBMA link; unicast hellos sent to neighbor from which hellos have stopped being received
- Init - Hello packet received from neighbor, but without the recipient's router ID
- 2-Way - Bi-directional communication has been established
- Exstart - The DR and BDR have been elected, link-state exchange starting
- Exchange - Exchange of database descriptor (DBD) packets
- Loading - Exchange of link-state information
- Full - Full adjacency established
Designated Router Election
Neighbors on a broadcast segment elect a designated router (DR) and backup designated router (BDR), which peer with all other routers on the segment. All non-designated routers peer only with the DR and BDR.
Multicast destinations:224.0.0.5- All OSPF routers224.0.0.6- All designated OSPF routers (DR and BDR only)
By default, DRs are elected on point-to-point Ethernet links even though this is unnecessary (Ethernet is always considered a broadcast medium). Interfaces can be manually configured to operate in point-to-point mode to prevent this.
(B)DRs are not preemptable. New election will take place only when a current (B)DR goes offline or its OSPF process is administratively restarted.
OSPF Area Types
Areas are defined to logically segment a network and reduce routing table size and complexity. All routers in an area maintain an identical topological database.- Standard area
- Stub area - Will not accept external routes (type 5 LSAs); type 5 LSAs are replaced by a default route
- Totally stubby area - Will not accept LSAs of type 3, 4, or 5; routes are replaced by the ABR with a default route; Cisco proprietary
- Not-so-stubby area (NSSA) - Stub areas which contain one or more ASBRs; ASBRs in a NSSA generate type 7 LSAs which are then converted to type 5 by the ABR
Stub Areas
Normal Stub Area
- ABR replaces all external routes with a default route
- Must be configured on all routers in the area
Totally Stubby Area
- ABR replaces all inter-area and external routes with a default route
- Cisco proprietary
- Configured on ABRs; internal routers configured as normal stubs
Not-so-Stubby Area (NSSA)
- Is a stub or totally stubby area with an ASBR
- Advertises external routes with type 7 LSAs instead of the normal type 5
- NSSA routes appear in the routing table as type N1 or N2 (instead of E1 or E2)
- All routers within the area must be configured
OSPF Network Types
- Broadcast multiaccess
- Point-to-point
- Point-to-multipoint broadcast
- Point-to-multipoint nonbroadcast
- Nonbroadcast multiaccess (NBMA)
NBMA and point-to-multipoint are standards-compliant (RFC 2328), whereas point-to-multipoint nonbroadcast, broadcast, and point-to-point implementations are Cisco proprietary.
NBMA networks utilize DRs like broadcast networks, however neighbors must be manually defined instead of being automatically discovered.| NBMA | Point-to-multipoint broadcast | Point-to-multipoint nonbroadcast | Broadcast | Point-to-point | |
| DR/BDR | Yes | No | No | Yes | No |
| Identify neighbor? | Yes | No | Yes | No | No |
| Hello/dead timers | 30/120 | 30/120 | 30/120 | 10/40 | 10/40 |
| Standard | RFC | RFC | Cisco | Cisco | Cisco |
| Network supported | Full mesh | Any | Any | Full mesh | Point-to-point |
Router Roles
- Internal - Routers which have all interfaces in a single area
- Backbone routers - Routers with at least one interface in area 0
- Area Border Routers (ABRs) - Routers with at least two interfaces in different areas
- Autonomous System Boundary Routers (ASBR) - Routers which redistribute information from an external source
Link-State Advertisements (LSAs)
- Router link (type 1) - Lists a router's neighbors and its cost to each; flooded throughout the area
- Network link (type 2) - Advertisement by the DR containing all routers on the segment it is adjacent to; flooded throughout the area
- Network summary link (type 3) - ABRs generate this type of LSA to send between areas; it lists all prefixes available in an area
- AS external ASBR summary link (type 4) - Router link LSA for ASBRs
- External link (type 5) - Originated by an ASBR, contains a route external to OSPF
- NSSA external (type 7) - Equivalent to a type 5 LSA, but generated by an ASBR in a not-so-stubby area (NSSA); converted to a type 5 by the ABR
Path CalculationAdjacencies within an area are advertised with type 1 and 2 LSAs, which are passed to the backbone by an ABR as type 3 summaries. These summaries are then injected into other areas from the backbone.
OSPF will choose the path to a destination by the advertisement of the lowest LSA type, then by lowest cost.Internal Routes
The cost to another OSPF area is calculated as the smallest cost to an ABR plus the smallest cost across the backbone.External Routes
The cost of an external route can be evaluated two ways:- E1 = cost to ASBR + advertised external cost; internal routing influences path selection
- E2 (default) = only advertised external cost is considered; internal routing is not considered in path selection
Routing Table Codes
| LSA Type | Table Code | Description |
| 1 (Router) | O | Generated by all routers; lists neighbors and costs to them; propagated within an area |
| 2 (Network) | O | Generated by the DR on a multiaccess network; propagated within an area |
| 3 (Inter-area summary) | O IA | Advertises summaries from one area to another |
| 4 (ASBR Summary) | O IA | Advertises the location of an ASBR |
| 5 (External) | O E1 or O E2 | Advertises a route external to the AS |
Virtual Links
All OSPF areas must be connected to the backbone area (area 0) through an ABR. Virtual links must be implemented when an area cannot be directly connected to the backbone.Virtual links are not encouraged and should be used only as a temporary fix.
Virtual links cannot use a stub area for transit.
