summarize OSPF features in a nutshell

One, multicast address and protocol number
Multicast address: 224.0.0.5 (all OSPF router) and 224.0.0.6 (all DR router)
Protocol number: IP Protocol No. 89th

Ii. Types of LSA

1. Router LSA: Each router sends a Class 1 LSA that contains the details of the link and status
2. Network LSA:DR, including all routers and subnet masks on the link
3. Network Rollup LSA: Routing information between regions generated by the ABR
4. ASBR rollup LSA: route to abbr generated by ABR
5. As external LSA: external route generated by ASBR
6. External route generated by the external Lsa:nssa region of the NSSA zone, 7 ext. 5 after ABR

Third, how to prevent the ring

1. The anti-ring inside the area is calculated using the SPF algorithm and does not cause loops
   2. Inter-zone anti-ring: Similar horizontal segmentation, that is, all non-backbone areas can only be connected to the backbone area, the backbone region only accepts non-backbone areas of the 1 LSA and 2 LSA, not accept non-backbone zone notification of the 3 class LSA.

Iv. How to summarize routes

1. The zone cannot be summarized because it is a route that is computed through the LSA
2. The inter-area aggregation is performed on the ABR, under the region abr-summary

V. How to summarize external routes
Summarize on ASBR, asbr-summary

Vi. how routes are filtered

1. Filter-policy Import: Operating under the routing process, any router can operate, affecting its own routing table, does not affect the downstream router. The downstream router is affected only by operating on the ABR. (OSPF attribute: for Class 1 LSA and Class 2 Lsa,abr that do not enter the local routing table, no 3 classes of LSA are generated for it to advertise to other zones)
2. Filter-policy Export: Operates under the routing process, and on ASBR, filters the imported external routes.
3. Filter Export: Operates in the region, and on the ABR, filters the 3 LSA that leave the area.
4. Filter import: Operates in the area, and on the ABR, filters the 3 types of OSA that enter the area.

Seven, DR: multi-access Dr, that is, broadcast multi-access and non-broadcast multi-access all need to elect the DR, election rules: The priority is higher priority, the same priority is the same as the rid of the greater the higher the priority. And Dr's election is non-preemptive, so the result is that in the multi-access network, the first router to start the OSPF process is Dr. In multi-access, Dr-other is the adjacency state of two-way, only the full adjacency between DR, BDR and Dr-other.

Eight, the data package:

1. Hello: Establish and maintain a neighbor relationship, send 1 times every 10 seconds in the broadcast and point network, the expiration interval is 40S, in NBMA and P2MP networks, 30/120 seconds.
2. DD: Database Description message, LSA summary information.
3. LSR: Link status request to request LSA to peer
4. LSU: Link status update with one or more LSA
5. Lsack: Used to confirm the received LSA

Nine, neighbor relationship state machine

1. Down: Initial state
2. Init: The neighbor's Hello package is received, but the neighbor field in the Hello packet does not have the router-id of this router
3. Two-way: Receive the neighbor's Hello packet, and see the Router-id of this router in the Hello package
4. Exstart: The initial state of information exchange, determine the master-slave relationship, who first initiate the exchange, determine the DD message sequence number.
5. Exchange: Information exchange status, the interaction of DD messages
6. Loading: Information loading status, exchanging LSA information via LSR, LSU
7. Full: Fully contiguous state

X. ROUTER-ID: Manual designation is the highest priority, if not manually specified, the active loopback address with the largest IP is elected, otherwise, the IP of the active physical interface with the largest IP is elected. When configured manually, it can be configured globally or in the OSPF process.

Xi. conditions for establishing a neighbour:

1. The IP address of both interfaces in the chain is the same network, the subnet mask
2. Same MTU
3. Certified by
4. In the same area
5. In the case of special areas, such as stubs and Nssa, all routers in this area need to be configured.
6. Hello and dead Time consistent
7. Consistent network type

12: OSPF Network type

1. Broadcast Type network: The election of the DR, the link layer protocol is Ethernet, the default is broadcast type network.
2. Dot-to-point type networks: No DR, link-layer protocol is a point-to-point sub-interface for PPP, HDLC, and Fr.
3. NBMA type network: When the link layer protocol is FR and ATM, the default is NBMA network, manually designate the neighbor, elect the Dr.
4. Point-to-multipoint type networks: can be seen as multiple point-to-points networks.

summarize OSPF features in a nutshell