OSPF

Link-state IGP, Dijkstra SPF, AD 110, IP protocol 89. Cost = 100,000 Kbps ÷ link speed.

// neighbor adjacency states

1 Down

2 Attempt

3 Init

4 2-Way

5 Exstart

6 Exchange

7 Loading

8 Full

// LSA types

Type	Name	Description
1	Router	Neighbors + cost; flooded within the area
2	Network	From the DR; routers on a segment; within the area
3	Network Summary	From an ABR; advertised between areas
4	ASBR Summary	From an ABR; locates an ASBR
5	External	From an ASBR; routes external to OSPF
7	NSSA External	ASBR in an NSSA; ABR converts to type 5

// network types

Type	DR?	Hello/Dead	Topology
Broadcast	Yes	10 / 40	Any
Non-broadcast (NBMA)	Yes	30 / 120	Full mesh
Point-to-Point	No	10 / 40	P2P
Point-to-Multipoint	No	30 / 120	Any

// packet types

Hello	Discover & maintain neighbors
DBD	Database Description — LSDB summary
LSR	Link-State Request
LSU	Link-State Update (carries LSAs)
LSAck	Acknowledge an LSU

// area types

Standard	Default area type
Stub	Type 5 LSAs → default route
Totally Stubby	Type 3, 4 & 5 → default route
NSSA	Stub with an ASBR; type 7 → type 5 at ABR

// router types

Internal	All interfaces in one area
Backbone	An interface in area 0
ABR	Connects two or more areas
ASBR	Connects to other routing domains

// facts

Cost = 100,000 Kbps ÷ link speed (reference bandwidth tunable)
AllSPFRouters 224.0.0.5 · AllDRouters 224.0.0.6 · IP proto 89
DR/BDR priority 0–255 (default 1); 0 never elected; no preemption
External E1 = ASBR cost + external · E2 (default) = external only
Auth: plaintext or MD5 · AD 110 · standard RFC 2328

configuration · cisco ios

router ospf 100
 router-id 1.1.1.1
 network 192.168.0.0 0.0.0.255 area 0
 network 10.0.0.0 0.0.0.3 area 0
 area 1 stub
 passive-interface default
!
interface GigabitEthernet0/0
 ip ospf 100 area 0
 ip ospf priority 100
 ip ospf authentication message-digest
 ip ospf message-digest-key 1 md5 S3cret

verify

show ip ospf neighbor
show ip ospf interface brief
show ip ospf database
show ip route ospf