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假装网络工程师11——ospf路径选取详解

时间:2020-02-01 00:41:09      阅读:120      评论:0      收藏:0      [点我收藏+]

标签:详细说明   wal   ctrl   splay   两种   with   引入   外部   roc   

一、背景介绍

提到路由,就一定会涉及选路,ospf与其他路由协议一样,同样存在选路,除了对比cost(metric)值,ospf协议还会对比表项,并且表项的优先级高于cost值,本文详细说明ospf协议的路径选取原则。

二、实验拓扑

技术图片
本次实验拓扑如上图所示,R2,R3环回接口模拟外部网络,通过import-route导入,每条路径的cost如标注所示

三、ospf选路详解

1.cost值比较

此时将基础配置按照上图配置好,外部路由直接使用import-route direct导入,未设置接口cost值时,此时在R1上看到去往192.168.0.0/24网段的路由如下所示:

[R1]display ip routing-table 
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 11       Routes : 12       

Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

       12.0.0.0/24  Direct  0    0           D   12.0.0.1        GigabitEthernet
0/0/0
       12.0.0.1/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
     12.0.0.255/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
       13.0.0.0/24  Direct  0    0           D   13.0.0.1        GigabitEthernet
0/0/1
       13.0.0.1/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/1
     13.0.0.255/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/1
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0
127.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0
    192.168.0.0/24  O_ASE   150  1           D   13.0.0.3        GigabitEthernet
0/0/1
                    O_ASE   150  1           D   12.0.0.2        GigabitEthernet
0/0/0
255.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0

通过路由表能够看到,R1认为13.0.0.3与12.0.0.2是等价路由,都可以去往192.168.0.0/24,且cost为1(环回接口cost默认为1),但如果只是将R2的g0/0/0与R3的g0/0/1端口开销值进行更,R1去往192.168.0.0/24还是负载均衡

[R2-GigabitEthernet0/0/0]display this 
[V200R003C00]
#
interface GigabitEthernet0/0/0
 ip address 12.0.0.2 255.255.255.0 
 ospf cost 100
#
return
[R3-GigabitEthernet0/0/1]display this 
[V200R003C00]
#
interface GigabitEthernet0/0/1
 ip address 13.0.0.3 255.255.255.0 
 ospf cost 10
#
return

只有将R1上的g0/0/0与g0/0/1端口开销修改后才能看到选路后的效果,此时R1路由表去往192.168.0.0/24网段只会保留去往R3的条目

[R1]dis ip routing-table 
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 11       Routes : 11       

Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

       12.0.0.0/24  Direct  0    0           D   12.0.0.1        GigabitEthernet
0/0/0
       12.0.0.1/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
     12.0.0.255/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
       13.0.0.0/24  Direct  0    0           D   13.0.0.1        GigabitEthernet
0/0/1
       13.0.0.1/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/1
     13.0.0.255/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/1
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0
127.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0
    192.168.0.0/24  O_ASE   150  1           D   13.0.0.3        GigabitEthernet
0/0/1
255.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0

原因就是R2与R3跟R1都是直连,cost为0,g0/0/1的端口开销为10,小于g0/0/0的100,所以优选从R3去往192.168.0.0/24网段。由此可以得知路径开销计算的方法:本地端口开销+到达目的地址经过所有网段的开销,以R2上12.0.0.0/24去往R3上13.0.0.0/24为例
技术图片
此时开销为R2本地g0/0/0接口开销(100)加R1上13.0.0.0/24网段端口(g0/0/1)开销(10),所以此时总的开销为110

<R2>display ip routing-table 
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 11       Routes : 11       

Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

       12.0.0.0/24  Direct  0    0           D   12.0.0.2        GigabitEthernet
0/0/0
       12.0.0.2/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
     12.0.0.255/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
       13.0.0.0/24  OSPF    10   110         D   12.0.0.1        GigabitEthernet
0/0/0
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0
127.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0
    192.168.0.0/24  Direct  0    0           D   192.168.0.1     LoopBack0
    192.168.0.1/32  Direct  0    0           D   127.0.0.1       LoopBack0
  192.168.0.255/32  Direct  0    0           D   127.0.0.1       LoopBack0
255.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0

2.外部表项类型1与类型2

上述在将外部路由使用import-route引入时,没有指定类型,此时默认值2,外部表项类型有两种:分为类型1与类型2

[R3-ospf-1]import-route direct ?
  cost          Set cost
  route-policy  Route policy
  tag           Specify route tag
  type          Metric type of the imported external routes
  <cr>          Please press ENTER to execute command 
[R3-ospf-1]import-route direct type ?
  INTEGER<1-2>  Type value

其中类型2为开销值不累加,即外部端口开销值为多少,引入ospf后,在ospf域内始终为多少,上文中等价路由的cost值为1,原因就是开销不累加,只按照环回接口本身的开销值计算,如果在导入时将R2的外部表项类型改为1,此时尽管R3去往192.168.0.0/24网段的开销为101,小于R2的开销1,但在R1的路由表中,存放的依然是R2的路由

<R1>display ip routing-table 
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 11       Routes : 11       

Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

       12.0.0.0/24  Direct  0    0           D   12.0.0.1        GigabitEthernet
0/0/0
       12.0.0.1/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
     12.0.0.255/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/0
       13.0.0.0/24  Direct  0    0           D   13.0.0.1        GigabitEthernet
0/0/1
       13.0.0.1/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/1
     13.0.0.255/32  Direct  0    0           D   127.0.0.1       GigabitEthernet
0/0/1
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0
127.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0
    192.168.0.0/24  O_ASE   150  101         D   12.0.0.2        GigabitEthernet
0/0/0
255.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0

说明在ospf路径选取时:外部表项类型1>外部表项类型2>cost值

3.区域内与区域间

此时拓扑如下图所示,首先让所有路由器在同一区域内,按照标注建立好ospf邻居,并设置好端口开销值
技术图片
此时R1如果到24.0.0.0/24网段有2条路径:

  1. R1--R3--R4,此时开销值为120
  2. R1--R3--R2--R4,此时开销值为30

所以,R1会选取第2条路径放在自己的路由表里

[R1]tracert 24.0.0.4

 traceroute to  24.0.0.4(24.0.0.4), max hops: 30 ,packet length: 40,press CTRL_C
 to break 

 1 13.0.0.3 20 ms  20 ms  20 ms 

 2 23.0.0.2 30 ms  30 ms  40 ms 

 3 24.0.0.4 30 ms  30 ms  30 ms 

说明在同一区域中,路径取cost值小的,如果此时将拓扑变为如下所示:
技术图片
R2跟R3此时成为2个abr,还是以R1到24.0.0.0/24网段为例,尽管这时 R1--R3--R2--R4开销值仍为30

[R1]display ospf routing 

     OSPF Process 1 with Router ID 1.1.1.1
          Routing Tables 

 Routing for Network 
 Destination        Cost  Type       NextHop         AdvRouter       Area
 12.0.0.0/24        50    Stub       12.0.0.1        1.1.1.1         0.0.0.0
 13.0.0.0/24        10    Stub       13.0.0.1        1.1.1.1         0.0.0.0
 23.0.0.0/24        20    Stub       13.0.0.3        3.3.3.3         0.0.0.0
 24.0.0.0/24        30    Inter-area 13.0.0.3        2.2.2.2         0.0.0.0
 34.0.0.0/24        110   Inter-area 13.0.0.3        3.3.3.3         0.0.0.0

 Total Nets: 5  
 Intra Area: 3  Inter Area: 2  ASE: 0  NSSA: 0 

但此时他会选择 R1--R3--R4,原因就是从R1--R3后,他会认为R2是区域间路由(即上图中的Inter-area),尽管开销更低,他依然会选择同区域内的R4,虽然此时路径总开销为110

[R1]display ip routing-table 
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Routing Tables: Public
         Destinations : 15       Routes : 15       

Destination/Mask    Proto   Pre  Cost      Flags NextHop         Interface

       12.0.0.0/24  Direct  0    0           D   12.0.0.1        Serial1/0/0
       12.0.0.1/32  Direct  0    0           D   127.0.0.1       Serial1/0/0
       12.0.0.2/32  Direct  0    0           D   12.0.0.2        Serial1/0/0
     12.0.0.255/32  Direct  0    0           D   127.0.0.1       Serial1/0/0
       13.0.0.0/24  Direct  0    0           D   13.0.0.1        Serial2/0/1
       13.0.0.1/32  Direct  0    0           D   127.0.0.1       Serial2/0/1
       13.0.0.3/32  Direct  0    0           D   13.0.0.3        Serial2/0/1
     13.0.0.255/32  Direct  0    0           D   127.0.0.1       Serial2/0/1
       23.0.0.0/24  OSPF    10   20          D   13.0.0.3        Serial2/0/1
       24.0.0.0/24  OSPF    10   30          D   13.0.0.3        Serial2/0/1
       34.0.0.0/24  OSPF    10   110         D   13.0.0.3        Serial2/0/1
      127.0.0.0/8   Direct  0    0           D   127.0.0.1       InLoopBack0
      127.0.0.1/32  Direct  0    0           D   127.0.0.1       InLoopBack0
127.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0
255.255.255.255/32  Direct  0    0           D   127.0.0.1       InLoopBack0

这样就会产生一个问题:即流量从R1到R4的时候路径为R1--R3--R4

<R1>tracert -a 13.0.0.1 34.0.0.4

 traceroute to  34.0.0.4(34.0.0.4), max hops: 30 ,packet length: 40,press CTRL_C
 to break 

 1 13.0.0.3 80 ms  20 ms  20 ms 

 2 34.0.0.4 60 ms  40 ms  30 ms 

返回时路径为R4--R2--R3--R1

<R4>tracert -a 34.0.0.4 13.0.0.1

 traceroute to  13.0.0.1(13.0.0.1), max hops: 30 ,packet length: 40,press CTRL_C
 to break 

 1 24.0.0.2 30 ms  20 ms  10 ms 

 2 23.0.0.3 10 ms  10 ms  40 ms 

 3 13.0.0.1 20 ms  40 ms  20 ms 

造成来回路径不一致,如果R2,R3是2台安全设备,比如防火墙,在一些厂家(如华为)的策略中默认是不允许的,所以必须关闭原进原出的检查机制

undo firewall session link-state check  

四、总结

通过上述实验得知,ospf在进行路径选取时,优先级会按照以下方式进行(1优先级高于2):

  1. 表项:区域内>区域间>外部路由类型1>外部路由类型2
  2. cost值

假装网络工程师11——ospf路径选取详解

标签:详细说明   wal   ctrl   splay   两种   with   引入   外部   roc   

原文地址:https://blog.51cto.com/arkling/2468668

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