LSA and optimization of OSPF

One 、LSA

LSA（ Link status notification ）
Definition ：OSPF Protocols are carriers that carry different information in different network environments .

LSDB： Link state database ;

SPF： Shortest path first algorithm ;

Type：LSA The type of , stay OSPFV2 In the version , What needs to be mastered LSA The types are 6 Kind of .

LinkState ID（ Link state identifier ）： Used to mark a LSA Information , It's equivalent to a LSA The name of the message .

AdvRouter（ Notify router ）： Send this LSA Information device RID.
“LSA A triple ”：① Link state type ;② Link state ID;③ Notify router
The above three parameters can uniquely identify a LSA.

1、LSA Triple of

TYPE — type — It stands for LSA The type of
LinkState ID — Link state identifier — The function is to mark a LSA Information , It's equivalent to this LSA The name of .
AdvRouter — Notify router — Send this LSA Information router RID.

2、Age

LSA Aging time , Unit is s;
（1） When one LSA Information is generated by the router from 0 Start timing , Whole LSA The whole process of transmission in the network , The timing is uninterrupted .
（2） When new LSA On arrival , Will overwrite the old LSA Information .
（3） In general ,LSA The aging time of should be less than 1800s（OSPF Every time 1800s There will be a periodic update ）.
（4） When one LSA When the aging time of reaches the maximum aging time , Will be deemed invalid , Will be from local LSDB Delete .（Max age：3600s）

3、LSA The type of

Type - 1 LSA — Every device in the whole network will send 1 class LSA, And send only one .

LINK — Describe the connection of router interface , The connection information of an interface can pass through multiple channels LINK To describe

type — Link type , It is mainly through the network type of the interface to judge what kind of network our interface is connected to .

Type - 2 LSA — because 1 class LSA Can't completely express MA Complete information of the network , therefore , Need to introduce 2 class LSA Supplement the missing information .

2 class LSA It's a MA The Internet only has DR Send one message to the corresponding device . because ,2 class LSA All the information described in is public information , therefore , Sending multiple times will result in repeated updates .

All routing information must be available through topology information （1 Classes and 2 class LSA） Check calculation . — The so-called checking calculation means that you must pass 1 class

Type - 3 LSA — It transmits inter domain routing information , It should be noted that 3 class LSA Cost value carried in , It refers to the cost of the advertiser reaching the target network segment . The cost value of the route added with the table needs to use the cost value carried plus the cost value of the local arrival advertiser .

Type - 5 LSA

Metric — 5 class LSA It carries extraterritorial routing information , Import to by republishing OSPF In the network , Because different protocols have different metrics for overhead value , therefore , After route import , We will directly discard the overhead of the route in the original network , after , Give the route a specified value — seed mertic Seed measure .OSPF The default seed measure is 1, This value can be modified during republishing .

Two 、OSPF The optimization of the

OSPF The optimization purpose of is to reduce LSA Update quantity of

Summary — Reduce the of backbone area LSA Number
Make special areas — Reduce the of non backbone areas LSA Number

1、 Summary

OSPF Interface summary is not supported （ similar RIP Summary method of ）, because ,OSPF Routing information is transmitted only between regions , therefore ,OSPF The summary of is called Regional summary .

1、 Inter domain routing summary — Interdomain refers to OSPF Between regions , The essence is ABR Spread between areas on the device 3 class LSA To summarize .（ Be careful , Inter domain routing summary can only be used for ABR The device passes by itself 1 Classes and 2 class LSA The routing information is calculated and summarized , Instead of sending it to others 3 class LSA To summarize .）

2、 Extraterritorial routing summary — Its essence is in ASBR On , By republishing , For imported 5 class /7 class LSA Get into OSPF Domain time , To summarize .
[ospf process ]asbr-summary < Network segment > < Mask > — stay ASBR On the configuration , Summarize extraterritorial routes .
The cost value of the domain foreign exchange total network segment
If the cost value type is type 2, Then the initial measurement value of the summary network segment is equal to the maximum value in the measurement value of the detail network segment +1.
If the cost value type is type 1, Then the initial measurement value of the summary network segment is equal to the maximum value in the measurement value of the detail network segment .
[ospf Process number ]asbr-summary <> < Mask > cost < Cost value > — Modify the initial measurement of the summary network segment .

2、 Special area

OSPF There are two types of special areas , Four sub categories .

The first category of special areas ---- 1, It can't be the backbone area ;2, Virtual links cannot exist ;3, Can't exist ASBR

1） We call the area that meets the above conditions as The terminal area （STUB）

If , Configure an area as a terminal area , The effect is that the area will reject 4 Classes and 5 class LSA Information . however , Because there is still a need to access extradomain segments , therefore , After configuring the special area ,ABR The device will automatically send a message to the backbone 3 Class default .

[ospf Regional perspective ]stub — Configure the area to stu
Be careful , When configuring special areas , All equipment in the area must be configured in the same way , otherwise , It will lead to the failure of neighbor relationship establishment .

2） Complete peripheral area — Totally stub — The effect of complete peripheral region is based on the ordinary peripheral region , Further refuse to accept 3 class LSA, But you need to keep 3 Class default .
[ospf Regional perspective ]stub no-summary — Be careful , This command only needs to be in ABR Just configure it on the device .

The second category of special areas — 1, It can't be the backbone area ;2, Virtual links cannot exist ;3, There is ASBR

1） Areas that meet the above conditions can be configured as Incomplete ending area （NSSA）.
If , Configure a region as an incomplete terminal region , The effect is that the area will reject 4 Classes and 5 class LSA Information . because ,ASBR The device needs to pass the extraterritorial routing information through 5 class LSA Pass on to OSPF In the network , however ,NSSA Area reject 5 class LSA, therefore , Extraterritorial routing information in NSSA The area will pass through 7 class LSA To carry . after , Pass... Before passing to other areas ABR Equipment time , Will put 7 class LSA convert to 5 class LSA To pass . meanwhile , In order to ensure normal access to external network segments , therefore , It will also automatically generate a line pointing to the backbone area 7 Class default .
[ospf Regional perspective ]nssa
Special area marking
E position — General arrangement 1, Support... On behalf of the region 5 class LSA
N position — General arrangement 0, Only in NSSA Area time setting 1, Support... On behalf of the region 7 class LSA
P position — If P Location 1, On behalf of support 7 turn 5
7 class LSA The forwarding address in carries by default ASBR Loopback address of the device , If ASBR The device is not configured with loopback interface , Will use ASBR Physical interface address of the device .
2） Complete and incomplete peripheral regions （Totally NSSA） — complete NSSA Area — Further rejection on common incomplete peripheral areas 3 class LSA, And generate a 3 Class points to the backbone by default .
[ospf Regional perspective ]nssa no-summary
Be careful — After the configuration is complete NSSA After area , Will automatically generate a 3 Class default , however , ordinary NSSA The area will generate a 7 Class default , This results in two default messages in the database , Final , because 3 Class is better than 7 The principle of class , We will use 3 class LSA Generated default route .
summary — OSPF Network optimization , To avoid loop generation , We need to ensure that the default direction generated automatically is consistent with the default direction added manually .

3、OSPF Extended configuration

1、OSPF Manual certification of — Certification is in OSPF All data packets between neighbors carry passwords , The same password on both sides means that the identity is legal .
Interface authentication — Configure on the neighbor interface , If the passwords at both ends are inconsistent , You will not be able to establish a neighbor relationship .
[ Interface perspective ]ospf authentication-mode md5 cipher
Regional certification — In fact, essence and interface authentication are the same thing , Equivalent to simultaneously
[ Regional perspective ]authentication-mode md5 cipher
Virtual link authentication — Authenticate when establishing the link — In essence .
[ Regional perspective ]vlink-peer md5 cipher

2, Speed up convergence — Reduce timer time

modify hello Time
[ Interface perspective ]ospf timer hello
Be careful ：Hello Once the time is modified , The time of death will be automatically modified

waiting — Waiting for the timer

POLL — Polling timer — 120S — And the status is DOWN Your neighbors sent hello Packet interval . — This kind of scene will only appear in NBMA In the network .

Retransmit — Retransmission time — 5S — OSPF The data packet interaction needs to be confirmed , If after sending a message , No confirmation reply sent by the other party within the retransmission time , Will resend .

Transmit Delay — Transmission delay — 1S — Is attached to LSA Of aging time , To compensate LSA The time it takes for information to pass through the network .

3, Silent interface — If you configure an interface as a silent interface , Then this interface will only accept and not send OSPF Data packets of .

[ospf visual angle ]silent-interface g < Interface >

Be careful ：OSPF Silent interface in , It will inhibit the sending of multicast and unicast packets at the same time , therefore , Can not be like RIP And unicast neighbors in some special scenarios .

4, Default route
3 Class default — Such defaults can only be generated automatically in special areas .
Common endings , Complete endings , Completely NSSA
given — adopt 3 class LSA Default information passed , Marker bit OSPF, The default priority is 10.
5 Class default — It can be issued manually by command ：default-route-advertise
characteristic — adopt 5 class LSA Pass default information ,default-route-advertise The precondition for issuing is that the routing table on the boundary device itself has default information obtained by other means , after , Use this command to republish the default route to OSPF In the network . Marker bit O_ASE, The default priority is 150.
[ospf visual angle ]default-route-advertise always — If , There is no default information on the border router , Then you can add always Parameter to force the default .
7 Class default — One way is through special areas （NSSA） Automatically generate , Another generation method is to issue a command manually — [ Regional perspective ]nssa default-route-advertise（ Not commonly used ）
characteristic — adopt 7 class LSA Pass default information , Marked as O_NSSA, The default priority is 150.

5, Route filtering — The routing filtering here refers to the filtering for 3 class ,5 class ,7 class LSA Filtering information .