OSPF (Open Shortest Path First) is a router protocol used to find the best path for packets as they pass through a set of connected networks. OSPF is designated by the Internet Engineering Task Force (IETF) as one of several Interior Gateway Protocols (IGPs) — that is, protocols aimed at traffic moving around within a larger autonomous system network like a single enterprise’s network, which may in turn be made up of many separate local area networks linked through routers.
++OSPF Packet Type++
-HELLO= discover neighbor, build neighbor adjacency, maintain
-DBD= check LSDB between 2 router is the same, packet DBD is sammary LSDB
-LSR= Linkstate Request, Requests specific link-state records from an OSPF neighbor
-LSU= Linkstate Update, Sends specific link-state records that were requested. This packet is like an envelope with multiple LSAs in it.
-LSAck=Linkstate Acknolegment, OSPF is a reliable protocol so we have a packet to acknowledge the others.
++Step Become Neighbor
-DOWN= no neighbor forund
-TWO-WAY= response received packet with unicast include ospf neighbor and see own router ID found
-EXCHANGE= check and summary DBD of summary LSDB
-LOAD= after received DBD of sum LSDB it will do couple thing: sent LSR, LSU,
-FULL= LSack
We only use a DR/BDR on a multi-access network. There is no need to do this on a pointto-point link like a PPP connection.
OSPF network types:
- Non-Broadcast (NBMA)
- Point-to-multipoint
- Point-to-multipoint non-broadcast
- Broadcast
- Point-to-Point
• Multi-access means we have to select a DR and BDR.
• Non-broadcast means that OSPF expects us to configure neighbors ourselves.
• Broadcast and non-broadcast are compatible since they both require an election of DR/BDR. Set the hello timer to the same values and you are ready to go.
• Point-to-multipoint, point-to-multipoint non-broadcast and point-to-point are compatible since none of them requires a DR/BDR. Set the hello timer to the same values and off you are!
LSA Types is 11
- LSA 1 – O, Router LSA contains all Link IDs – network, generated by every router and is local to the area
- LSA 2 – O, Network LSA contains all routers attached to the segment, generated by DR and is local to the area
- LSA 3 – O IA, Network Summary LSA describes network from another area, generated by ABR and is propagated between areas
- LSA 4 – O IA, Summary ASB Link States, generated by the ABR and is propagated between areas
- LSA 5 – O E1, O E2, External Link States, generated by ASBR and is propagated between areas
- LSA 7 – O N1, O N2, NSSA External Link States, generated by ASBR into NSSA area and is propagated into area 0 as E1 or E2
Stub Area – area <area> stub
Allows propagation of LSA type 1,2 and 3 additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)
Blocks propagation of LSA type 4 and 5
- Stub area can not be a transit area for Virtual link, but a GRE tunnel can be used instead
- Stub area can not have an ASBR
- The Backbone area can not be configured as a Stub area
- No LSA type 4 and 5 (E1 or E2) is allowed in a Stub area, but the routers in the Stub Area can connect to the External routes via the default route that is injected into the area by the ABR
- Every router and the ABR of that area should have “area <area> stub” command
- By default, the cost of the default route is 1
- The cost of the default route can be changed by “area <area> default-cost <cost>”
Totally Stubby – area <area> stub no-summary
Allows propagation of LSA type 1 and 2 additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)
Blocks propagation of LSA type 3,4 and 5
- This area has the same features as standard Stub Area
- Only ABR has to be configured with the “no-summary” keyword in area definition
- Intra area routers in the Totally Stubby area should have “area <area> stub” command only
Not-So-Stubby – area <area> nssa
Allows propagation of LSA 1, 2, 3 and 7
Blocks propagation of LSA type 3,4 and 5, no default route
- All Intra Area routers in the Not-So-Stubby area should have “area <area> nssa” configured
- Not-So-Stubby area can not be a transit area for Virtual link, but a GRE tunnel can be used instead
- No LSA type 3,4 and 5 (E1 or E2) is allowed in a Not-So-Stubby area, but external routes can be injected into area and appear as LSA type 7, then translated on the ABR to LSA type 5
- Not-So-Stubby area can have an ASBR
- The backbone area can not be configured as a Not-So-Stubby area
Totally Stubby Not-So-Stubby – area <area> nssa no-summary
Allows propagation of LSA 1 and 2 additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)
Blocks propagation of LSA type 3,4 and 5
- This area has the same features as standard Not-So-Stubby the only one difference is that default route as Intra Area LSA format (O*IA 0.0.0.0/0) is produced
Not-So-Stubby – area <area> nssa default-information-originate
Allows propagation of LSA type 1, 2, 3 and 7 additionally with default route as LSA type 7 Eternal 2 LSA format (O*N2 0.0.0.0/0)
Blocks propagation of LSA type 4 and 5
- This area has the same features as standard Not-So-Stubby the only one difference is that default route as LSA type 7 Eternal 2 LSA format (O*N2 0.0.0.0/0) is produced
Totally Stubby Not-So-Stubby – area <area> nssa no-summary no-redistribution
Allows propagation of LSA 1 and 2 additionally with default route as Intra Area LSA format (O*IA 0.0.0.0/0)
Blocks propagation of LSA type 3,4 and 5
- This area has the same features as standard Not-So-Stubby the only one difference is that prefixes that have been redistributed on the ABR will not be propagated into area
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