OSPF

Administrative distance - 110

Technial overview of OSPF
1. OSPF send HELLO packet to multicast address 224.0.0.5 (all OSPF routers)
2. Neighbor evaluates packet. May or may not form neighbor
3. Link-state data exchange (DBD)
4. Net info. Recorded and Flooded.
5. When database complete, SPF Algorigthm runs.
6. Valid routes enter the routing table

OSPF Hello Packet
contains the followings
1. Router ID
2. Area ID (x)
3. Address of originate terface
4. Authentication information (x)
5. Hello Interval (x)
6. Dead Interval (x)
7. Priority
8. DR/BDR IDs
9. Neighbor Router IDs
 (x) = must match

OSPF Network Types
- Point to Point
There is no DR/BDR selection. Updates sent to 224.0.0.5.
- Broadcast
DR/BDR selection. They listen to 224.0.0.6
- NBMA network
Frame relay, ATM, Virtual circuti, Hub & spoke,
All neighbors are statically defined and unicast is used to communicate instead of multicast. DR selection-DR must have full connectivity to all neighbors.
- Point to multipoint
Very similar to NBMA but there is no DR election and messages will be multicasted.It treats its own virtual circuit as point to point(from HUB view)connection even though it's configured on multipoint interface.

http://www.youtube.com/watch?v=tSzJoxcep5o

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Building OSPF neighbors and neighbor states

OSPF Areas, Router Types and LSAs

Reason to create normal area is for route summarization.
Unlike EIGRP, OSPF does not support route summarization anywhere in the network excpet at Area boundary routers(ABR) and Autonomous system boundary routers(ASBR).

STUB area blocks external route advertisements but still accept LSA type 3.
Totally stubby area blocks external route advertisements and ads from other areas.It only knows about routes in its area.

ABRs for Stub and Stubby areas inject default routes into these areas.
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LSA Type 1 - Router LSA. current networks that the router is connected to.
LSA Type 2 - DR advertising on behalf of a network. DR/BDR to other internal routers
LSA Type 3 - inter area routes injected by ABR. Routes from other areas
LSA Type 4 - Info about ASBR. Injected by ABR connected to other areas
LSA Type 5 - external LSA/routes injected by ASBR.
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OSPF Link State database, Redistribution, Summarization

In stub area, default route is added automatically.

When redistributing Eigrp to ospf, must include 'subnets' command to redistribute classless subnets. If not, only classful networks are redistributed.
When redistributing to OSPF, E1 will add cost while E2 is a fixed cost in OSPF Areas.

To see OSPF link state database info -sh ip ospf database 

Summarization can be only done at/by ABR (hmm ASBR as well).
To summarize LSA Type 3 Area 1 route to inject to Area 0 at ABR - area 1 range 172.16.0.0 255.255.0.0
To summarize LSA Type 5 Area 1 route to inject to Area 0 at ASBR - summary-address 172.18.0.0 255.255.0.0