OSPF routing protocol study notes sharing

Types of OSPF packets

Hello: Used to establish and maintain neighbor relationships. It is sent once every 10 seconds and timed out after 30 seconds.

hello, area-id, authentication, stub

Passive-interface, MTU, and an ACL on the interface filter OSPF traffic

Five OSPF messages

1. HELLO

2. LSU

3. LSR

4. DBD: Directory

5. LSACK 

Three tables of OSPF 

Topology table (LSDB): the same LSDB in the same area

Neighbor table: also called neighbor state database

Routing table: the best path to the target network

Various types of LSA

Point-to-point broadcast

The reason is because LSDB is not synchronized

Point-to-point does not elect DR, no type 2 LSA

Broadcast has type 1 LSA type 2 LSA

LSA: 1 2 3

L1

Content: This router announces the link information to OSPF,

Who generated it: OSPF router

Scope of spread: Flooding in the area

L2

What routers are in this area

DR

Flooding in the area

L3

LSA in other regions

ABR

Flood to the entire AS, except for special areas

LSA: 4 5 6 7 8 9

Type 4 LSA

Advertise ASBR information

ABR is produced,

Flood to the entire AS, except for special areas

Category 5 LSA

External link information

ASBR generation

Flood to the entire AS, except for special areas

Type 6 LSA is used for MOSPF protocol and used for multicast

Category 7 LSA

External LSA information imported from NSSA area

ASBR generation

7 to 5 process, flooding in the NSSA area

Type 8 LSA, replacing Type 1 LSA in IPv6 network

Type 9 LSA, replace Type 2 LSA in IPv6 network

OSPF routing type

O: Routing in the area    Type 1 LSA Type 2 LSA

OIA: Inter-area Routing    Type 3 LSA

OE1 OE2: External routing    Type 5 LSA

ON1 ON2: Route outside NSSA area    Type 7 LSA

OSPF neighbor relationship

1. Down: In the initial state, no hello packets from neighbors have been received.

2. Attempt: only used in NBMA network, neighbors are valid.

3. Init: Received the neighbor's hello, marking the establishment of its own neighbor relationship table.

4.Two-way: I saw my router id in the neighbor's hello packet and selected DR BDR.

5. Exstart: elect the master and slave routers, the MTU should be consistent.

6. Exchange: send DBD messages.

7. Loading: Exchange LSA information and send LSR LSU LSACK.

8.Full: completely adjacent. Calculate the shortest path synchronously and load the routing table.

OSPF network type

1. Point-to-point: destination IP 224.0.0.5, a pair of routers form an adjacency relationship. Without DR, each sub-interface belongs to a different IP subnet.

2. Point-to-multipoint: destination IP 224.0.0.5, no DR, same IP subnet. PTP and PTMP cannot form an adjacency.

3. Point-to-multipoint non-broadcast: destination IP unicast, no DR, same IP subnet.

4. NBMA: destination IP unicast, select DR, the same IP subnet. Fully or partially interconnected.

5. Broadcast: elect DR, all routers send messages to 224.0.0.6, and then DR sends updates to 224.0.0.5, DR establishes adjacency relationship with all routers, and all DRohter routers converge to 2-way state. The same IP subnet, fully interconnected or partially interconnected.

Summary of features:

1. Whether to elect DR or whether to manually specify neighbors

2. Point family does not need to elect DR and BDR

3. Non-broadcast multiple access NBMA, non-broadcast are unicast updates

4. If this network type cannot deliver multicast, neighbors need to specify manually

5. Looking at the name of the network type, if there is no broadcast, it means that multicast cannot be delivered

Why do you need a virtual link?

The non-backbone area and the backbone area are required to be connected

Why are non-backbone areas and backbone areas connected?

Prevent loops

OSPF relies on SPF algorithm to ensure that there is no loop in an area,

LSDB of each area is synchronized

When to use virtual links?

Backbone areas are isolated by non-backbone areas area 0---area 1 --area 0

The backbone area and the non-backbone area are separated by the non-backbone area area 0---area 2---area3

Authentication method: regional authentication, interface authentication

Authentication type: Clear text authentication MD5 authentication

OSPF route summary type

Route summary between areas area 1 rang

Summary of external routes: summary-address

Configure GTSM under the routing process, which is enabled by default on all OSPF interfaces

Access prefix-list matches route entries

Route-map x permit 10

 Match ip address 10

router ospf process-id

  prefix-priority low route-map x

fast-reroute per-prefix enable prefix-priority low

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