OSPF anti ring

OSPF Routing control
priority
1、[r3-ospf-1]preference 50 —— modify OSPF Routing default priority , This command will only affect all types of fields as OSPF (OSPF Routing information inside the network ) Routing information for .

2、[r3-ospf-1]preference ase 100 —— Modify the default priority of routes imported outside the domain .

The above modification , Only modify the default priority of the corresponding type of route in the device local route table .

Cost value
COST = Reference bandwidth / Real bandwidth

1、 Change the overhead value by modifying the reference bandwidth

    [r3-ospf-1]bandwidth-reference 1000—— Modifying the reference bandwidth requires that all OSPF The devices in the network are all changed to the same .
     Because once the reference bandwidth is modified , All devices in the whole network have to be modified into the same , therefore , Its essence cannot control the choice of road . Its significance lies in that when the real bandwidth is large , The reference bandwidth can be increased , Avoid poor routing .

2、 The effect of modifying the cost value is achieved by modifying the real transmission rate of the interface

    [r3-GigabitEthernet0/0/0]undo negotiation auto —— Turn off i Close auto negotiation 
    [r3-GigabitEthernet0/0/0]speed 10 Be careful , The command takes effect after restarting the interface 

This method can indeed change the size of the cost value for the specified interface , Play the effect of controlling route selection . however , It will affect the real transmission rate of the interface , also , It can only be reduced , therefore , This scheme is not recommended when the route selection control is really carried out .
3、 Directly modify the cost value of the interface
[r3-GigabitEthernet0/0/0]ospf cost 1000— Pay attention to modifying the interface cost value : Because the cost value of the loopback interface is a defined value , So the result will not be changed by the outside world , namely 1,2 Neither method will affect the cost value of the loopback interface . But the first 3 This method can change the open value of the loopback interface .
Particular attention :OSPF The cost value of reaching the target network segment in , The cumulative value of the cost value of the flow interface in the direction of routing information transmission .
OSPF The appendix E
appendix E It mainly defines a solution in a special case .

appendix E The proposed solution masks a shorter normal entry , The longer the mask, the LS ID Use direct broadcast address to express .
OSPF The road selection principle
1、 Intra domain ： If there are multiple passes to reach the same target network segment 1 class ,2 class LSA Learned routing information , The cost value reaching the target network segment will be compared , Choose the one with low cost . If the cost value is the same , Then the load will be balanced .
2、 Inter domain ： If there are multiple passes to reach the same target network segment 3 class LSA Learned routing information , The cost value reaching the target network segment will be compared , Choose the one with low cost . If the cost value is the same , Then the load will be balanced .
3、 Outside the country
type 1: If the cost value type is type 1, Then the cost value of all devices in the domain to reach the target network segment outside the domain is equal to the cost value of local arrival notifier plus the seed measurement .
Modify the cost value type when republishing ：import-route rip 1 type 1
type 2: If the cost value type is type 2, Then the cost value of all devices in the domain to reach the target network segment outside the domain is equal to the seed metric .
Expand all at once 5 class LSA：display ospf lsdb ase
（1） type 1 The road selection principle : If there are multiple passes to reach the same target network segment 5 class /7 class LSA Learned routing information , And the cost value types are all types 1, The total cost will be compared ( Seed measure ＋ Accumulate the cost value along the way ), Give priority to those with low total cost . If the total cost is the same , Load balancing .
（2） type 2 The road selection principle ︰ If there are multiple passes to reach the same target network segment 5 class /7 class LSA Learned routing information , And the cost value types are all types 2, Then compare the seed metrics first , Give priority to those with small seed metrics ; If the seed measures the same , Then compare the accumulated cost value along the way , Give priority to those with small accumulated cost value along the way . If the accumulated cost along the way is the same , Load balancing .
（3） type 1 Always better than type 2. The purpose of this design is to facilitate interference and routing in some special cases .
4、 Intra domain and inter domain
If the same destination network segment exists, pass 1 Classes and 2 class LSA Learned information in the domain and through 3 class LSA Learned inter domain information , Will be preferred to pass 1 class 2 class LSA Learned , Even if the cost is more valuable .—— Intra domain is better than inter domain .
5、 Interregional and extraterritorial
If the same destination network segment exists, pass 3 class LSA Learned inter domain information and through 5 class /7 class LSA Learned foreign information . Will be preferred to pass 3 class LSA Learned , Even if the cost is more valuable .—— Inter domain is better than extradomain .
6、 In the Huawei system ,5 Classes and 7 class LSA The routing rules are the same .
OSPF My guard ring
1、 Inter domain anti ring
（1）OSPF Because routing information is also transmitted between regions , So there is a regional level segmentation mechanism . The routing information learned from which region will not be returned to this region .
（2） The region division of star topology requires itself to be an anti ring mechanism .
2、 In domain anti ring
OSPF Anti ring mechanism in the domain ——SPF Algorithm —— Shortest path first algorithm .
Reissue
1、 effect : In a network , If running multiple routing protocols or different processes of the same routing protocol ; The protocols cannot directly communicate with each other, including the independent forwarding and operation between processes ; Therefore, it is necessary to republish to realize the sharing of routes .
2、 Conditions :
（1） There must be ASBR equipment —— Connect two protocols or two processes at the same time ; You can learn the routing information on both sides at the same time , Then complete the route sharing .
（2） We must pay attention to three measures ——A The protocol and B The measurement calculation logic between protocols is illogical , Cannot be used directly . So I will A The agreement is published to B When the agreement ,ASBR Will not carry A The protocol measures to B agreement , It's sharing to B When the agreement , from ASBR The device adds a starting degree value to the route .
3、 The rules :
（1） take A The protocol is imported into B When the agreement , Need to be in ASBR Of B Configure republication on the protocol .
（2） take A The agreement is published to B When the agreement , Yes, it will ASBR All the people on the pass A Agreement learned and ASBR Announced on A All direct routes of the protocol are shared to B Agreement .
spot
Single point ： Between two protocols or two processes 1 individual ASBR equipment
Bipoint ： Between two protocols or two processes 2 individual ASBR equipment
Multipoint ： There are multiple... Between two protocols or two processes ASBR Equipment direction
A one-way ： Only will A The agreement is published to B In the agreement
two-way ：A/B All routes of the protocol are shared

4、 To configure

1、A->B:— A dynamic routing protocol is published to another ─ A dynamic routing protocol
2、 static state ->B: Import static routes into dynamic routing protocols
3、 Direct connection ->B: Import direct route into dynamic routing protocol

RIP
A->B:— A dynamic routing protocol is published to another ─ A dynamic routing protocol

[r2-rip-1]import-route ospf 1

Conclusion : Through republishing technology, the routing information of other protocols is imported into RIP in , The default seed measure assigned to it is 0.

How to modify the seed measure :
1, Modify the overall situation in the process :[ r2-rip-1]default-cost 2

2, Modify for this reissue ︰[r2-rip-1]import-route ospf 1cost 3
Be careful : When the above two commands are executed at the same time , Will be in accordance with paragraph 2 strip , Match more precise commands to execute .

static state ->B: Import static routes into dynamic routing protocols

[r2-rip-1]import-route static
Conclusion : During the republication process, the default route will not be republished to the dynamic routing protocol .

Direct connection ->B: Import direct route into dynamic routing protocol

    [r2-rip-1]import-route direct
     1、 except R1 Outside the direct network segment of , All direct connections will be imported 
     2、 if ASBR the A->B Re release and direct connection ->B Re release of , And pack twice   With the same routing information , Will give priority to learning direct connection ->B Re release information .