Router OSPF configuration 1

 

 

<OSPF (Open Shortest Path First)>

 

 

• OSPF is an IGP Protocol and is based on IP Pro 89.

• Use the SPF algorithm (Dijkstra algorithm) to calculate the optimal path.

• Quickly responds to network changes.

• Send regular updates at a low frequency (every 30 minutes), known as link status refresh.

• An update is triggered when the network changes.

• Supports equivalent load balancing.

 

 

Three tables maintained by OSPF:

1) Neighbor Table:

Ensure two-way communication between direct neighbors.

2) Topology Table:

LSDB (Link-State DataBase), all routers in the same region have the same LSDB.

3) Routing Table:

Apply the SPF algorithm to LSDB and select the optimal route entry to the route table.

 

 

S1 zsxd5h

 

 

 

 

OSPF region Division:

• OSPF uses hierarchical design and Area to separate routers.

The vrouters in the region Save the details of all links and vrouters in the region,

However, only the summary information of routers and links in other regions is saved.

 

 

Advantages of hierarchical design:

1. Reducing route table entries

2. the flooding of LSA stops at the network boundary to accelerate convergence.

3. limiting the impact of topology changes reduces network instability. Problems in one region do not affect other regions.

 

 

OSPF neighbor and Neighbor Relationship:

 

 

• The OSPF router establishes a neighbor relationship with its directly connected neighbor.

• The OSPF router only transmits the LSA to the router that establishes an adjacent relationship.

• A router can be in FULL State only when it is connected to a neighbor that has established an adjacent relationship.

• Route updates are transmitted only between vrouters in the FULL status.

• The P2P link can reach the FULL state.

• MA network, all routers and DR/BDR only reach the FULL status.

(Backup Designated Router)

The difference between neighbors and neighboring areas.

Neighbor --- must have a direct link

Adjacent --- 1. Must be a neighbor; 2. the database in the same region on both sides of the link must be synchronized (Status: FULL ).

 

Router-ID:

The router that uniquely identifies the OSPF domain.

Set the priority of Router-ID:

1) manually specify Route-ID x. x (can be arbitrary, but cannot be repeated)

2) Maximum Loopback IP

3) The largest interface IP address (ensure that the interface is active) higher active physical interface ip Address

 

 

We recommend that you use the loopback port and manually specified router-id because they are more stable.

 

 

DR/BDR election:

1) Comparison priority. The larger the value is, the better the value (the default value is 1. If it is set to 0, it indicates not participating in the election)

2) Compare the Router-ID.

 

 

• DRother send LSA to DR/BDR with 224.0.0.6

• DR sends LSA to DRother with 224.0.0.5

• Non-MA network (No DR/BDR), vrouters use 224.0.0.5

 

 

 

<DR/BDR> features

 

 

1) No preemption. When a DR is normal, a new router with a higher Priority than a DR cannot be preemptible to a DR.

2) When DR is normal, BDR only receives all information, and the tasks for forwarding LSA and synchronizing LSDB are completed by DR. When DR fails, BDR automatically becomes DR to complete the work of the original DR, and elect a new BDR.

3) DR is an interface concept. DR is elected for each CIDR block.

4) Select DR/BDR for different network segments

 

 

SPF algorithm:

1. All routers in a region have the same LSDB

2. Each router uses itself as the root of the tree during computing

3. routes with the lowest cost value to the target are the best paths.

4. The best route is put into the forwarding table

 

 

Timer:

• Hello Intervals: 10 S/30 S

• Dead Interval: 4 * Hello = 40 S

 

 

Hello packet sent to 224.0.0.5

 

 

The following two types of network hello time is 30 S

NON_BROADCAST

POINT_TO_MULTIPOINT

 

 

OSPF overhead value calculation:

• OSPF Cost = 108/BW (bps)

 

 

 

 

• Five types of OSPF packets:

1) Hello: discovers and establishes the adjacent relationship. There are also elections for DR and BDR !!!

2) DBD: contains the route summary information.

3) LSR: Requests the complete information of a specific route from another router.

4) LSU: used for LSA flooding and responding to the complete information of the LSR route. In OSPF, only LSU needs to be displayed for confirmation.

5) LSAck: Confirm LSU.

 

 

Prerequisites for OSPF neighbor creation:

1) Hello/Dead Intervals

2) Area ID

3) Authentication Password

4) Stub Area Mark

5) MTU

6) subnet mask (must be the same network segment)

 

 

OSPF state machine:

1. down state

2. init state

3. two-way state

4. exstart state

5. exchange state

6. loading state

7. full state

Bytes ---------------------------------------------------------------------------------------

<OSPF> basic operation commands

 

 

R1 (config) # router ospf 110 Note: Process number is cisco's private Technology

R1 (config-router) # network 1.1.1.0 0.0.255 area 0

 

 

R1 # show ip protocols

R1 # show ip ospf: view the router-id, process number, and number of domains.

R1 # show ip ospf interface

R1 # show ip ospf interface brief

R1 # show ip ospf neighbor view neighbor Information

 

 

R4 (config-if) # ip ospf hello-interval 9

(Dead automatic * 4)

 

 

R4 (config-if) # ip OS dead-interval 80

 

 

R4 (config-if) # ip ospf priority 10 modify priority

 

 

R1 # show ip OS database

 

 

Note: In OSPF, the routing mask of the loopback interface will change TO 32-bit available commands ip ospf network point-TO-POINT

-- If OSPF is not layered in a large network, the following problems may occur:

1. Each vro receives too many LSAs

2. route computing is performed frequently.

3. The route table is too large, and the vro memory is limited.

 

 

OSPF router type:

1. Internal routers-routers in a common area

2. Core router-vro in area 0

3. VBR In the ABR region-connect two vrouters in different regions

4. ASBR autonomous system virtual border router-a router connecting the OSPF domain to another Autonomous System

 

 

<LSA (Link-State Advertisement)>

 

 

Note:

1. Scope of dissemination

2. who generates

3. Content included

 

 

This article is from the "T76" blog