验证OSPF中对外部路由路由的选择规则



OSPF在选择外部路由的时候，遵循的原则是：

[1]O E1优于O E2

[2]在同样的情况下，Cost越小越优先

[3]在cost相同的情况下，选择到达ASBR最优的路径


下面来通过实验来验证该规则:
[案例1]当外部路由都是O E1的情况

基本配置：(注意ASBR的配置)
R1:
interface Loopback0

ip address 1.1.1.1 255.255.255.0

ip ospf network point-to-point
!
interface Serial1/0

ip address 12.1.1.1 255.255.255.0
!
interface Serial2/0

ip address 13.1.1.1 255.255.255.0
!
router ospf 1
redistribute rip subnets metric-type 1

network 12.1.1.0 0.0.0.255 area 0
!
router rip

version 2

network 13.0.0.0

distance 109(因为RIP的AD大于OSPF,所以重发布到OSPF中，会发生路由抖动,故将其改小，详解看等级三实验手册1.3)

no auto-summary
!
R2:
interface Loopback0

ip address 2.2.2.2 255.255.255.0

ip ospf network point-to-point
!
interface Serial1/0

ip address 24.1.1.2 255.255.255.0
!
interface Serial2/0

ip address 12.1.1.2 255.255.255.0
!
router ospf 1
network 12.1.1.0 0.0.0.255 area 0

network 24.1.1.0 0.0.0.255 area 0
!


R3:
interface Loopback0

ip address 3.3.3.3 255.255.255.0

ip ospf network point-to-point
!
interface Serial1/0

ip address 13.1.1.3 255.255.255.0
!
interface Serial2/0

ip address 35.1.1.3 255.255.255.0
!
router rip

version 2

network 3.0.0.0

network 13.0.0.0

network 35.0.0.0

no auto-summary
!

R4:
interface Loopback0

ip address 4.4.4.4 255.255.255.0

ip ospf network point-to-point
!
interface Serial1/0

ip address 45.1.1.4 255.255.255.0
!
interface Serial2/0

ip address 24.1.1.4 255.255.255.0
!
router ospf 1
network 24.1.1.0 0.0.0.255 area 0

network 45.1.1.0 0.0.0.255 area 1
!


R5:
interface Loopback0

ip address 5.5.5.5 255.255.255.0

ip ospf network point-to-point
!
interface Serial1/0

ip address 35.1.1.5 255.255.255.0
!
interface Serial2/0

ip address 45.1.1.5 255.255.255.0
!
router ospf 1
redistribute rip subnets metric-type 1

network 45.1.1.0 0.0.0.255 area 1
!

router rip

version 2

network 35.0.0.0

distance 109

no auto-summary
!
监视和测试配置:


查看R2的路由表:
R2#sh ip rou

35.0.0.0/24 is subnetted, 1 subnets
O E1
35.1.1.0 [110/84] via 12.1.1.1, 00:00:25, Serial2/0

2.0.0.0/24 is subnetted, 1 subnets
C
2.2.2.0 is directly connected, Loopback0

3.0.0.0/24 is subnetted, 1 subnets
O E2
3.3.3.0 [110/84] via 12.1.1.1, 00:00:25, Serial2/0

24.0.0.0/24 is subnetted, 1 subnets
C
24.1.1.0 is directly connected, Serial1/0

12.0.0.0/24 is subnetted, 1 subnets
C
12.1.1.0 is directly connected, Serial2/0

13.0.0.0/24 is subnetted, 1 subnets
O E2
13.1.1.0 [110/84] via 12.1.1.1, 00:00:25, Serial2/0

45.0.0.0/24 is subnetted, 1 subnets
O IA
45.1.1.0 [110/128] via 24.1.1.4, 00:00:26, Serial1/0
查看R2的ospf数据库

R1#sh ip os da
                 Type-5 AS External Link States
Link ID         ADV Router      Age         Seq#       Checksum Tag
3.3.3.0         1.1.1.1         237         0x80000001 0x00D93A 0
3.3.3.0         5.5.5.5         239         0x80000001 0x0061A2 0
13.1.1.0        1.1.1.1         509         0x80000001 0x008588 0
13.1.1.0        5.5.5.5         239         0x80000001 0x000DF0 0
35.1.1.0        1.1.1.1         238         0x80000001 0x006691 0
35.1.1.0        5.5.5.5         381         0x80000001 0x00EDF9 0
可以看到R2从两个ASBR都收到了LSA 5的通告，但是R2选择了从R1走。ASBR通告外部路由默认都是以metric值20通告出去的，但是内部接受的路由器看到是 O E1的外部路由，会加上自己到ASBR的cost值来算出新的metric值，这样实际上在我们来看只要选择到ASBR cost值小的路由器就可以了。

[案例2]当外部路由都是O E2的情况
在ASBR R1和R5上更改重发布命令
R1(config)#router ospf 1
R1(config-router)#redistribute rip metric-type 2 subnets
R5(config)#router ospf 1
R5(config-router)# redistribute rip metric-type 2 subnets
(注意这里直接改为 redistribute rip  subnets是没有效果的，因为前面已经将类型改为E1,这里没有指明类型，是不会改为O E2的；这样写只有原来没有作过重发布，类型默认才会为O E2)


监视和测试配置


R2#sh ip rou

35.0.0.0/24 is subnetted, 1 subnets
O E2
35.1.1.0 [110/20] via 12.1.1.1, 00:00:25, Serial2/0

2.0.0.0/24 is subnetted, 1 subnets
C
2.2.2.0 is directly connected, Loopback0

3.0.0.0/24 is subnetted, 1 subnets
O E2
3.3.3.0 [110/20] via 12.1.1.1, 00:00:25, Serial2/0

24.0.0.0/24 is subnetted, 1 subnets
C
24.1.1.0 is directly connected, Serial1/0

12.0.0.0/24 is subnetted, 1 subnets
C
12.1.1.0 is directly connected, Serial2/0

13.0.0.0/24 is subnetted, 1 subnets
O E2
13.1.1.0 [110/20] via 12.1.1.1, 00:00:25, Serial2/0

45.0.0.0/24 is subnetted, 1 subnets
O IA
45.1.1.0 [110/128] via 24.1.1.4, 00:00:26, Serial1/0

察看R2的OSPF数据库
R2#sh ip os da

OSPF Router with ID (2.2.2.2) (Process ID 1)


Type-5 AS External Link States

Link ID
ADV Router
Age
Seq#
Checksum Tag
3.3.3.0
1.1.1.1
1540
0x80000001 0x005D36 0
3.3.3.0
5.5.5.5
1551
0x80000001 0x00E49E 0
13.1.1.0
1.1.1.1

1540
0x80000001 0x000984 0
13.1.1.0
5.5.5.5
1551
0x80000001 0x0090EC 0
35.1.1.0
1.1.1.1
1540
0x80000001 0x00E98D 0
35.1.1.0
5.5.5.5
1551
0x80000001 0x0071F5 0
    可以看到R2对于每一个外部条目都从两个边界路由器分别学到了,让我们看一下它通过两个ASBR学到3.3.3.0这个路由条目的metric值是多少
R2#sh ip os da ex 3.3.3.0

OSPF Router with ID (2.2.2.2) (Process ID 1)


Type-5 AS External Link States


Routing Bit Set on this LSA

LS age: 1595

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 3.3.3.0 (External Network Number )

Advertising Router: 1.1.1.1

LS Seq Number: 80000001

Checksum: 0x5D36

Length: 36

Network Mask: /24

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 0


LS age: 1606

Options: (No TOS-capability, DC)

LS Type: AS External Link

Link State ID: 3.3.3.0 (External Network Number )

Advertising Router: 5.5.5.5

LS Seq Number: 80000001

Checksum: 0xE49E

Length: 36

Network Mask: /24

Metric Type: 2 (Larger than any link state path)

TOS: 0

Metric: 20

Forward Address: 0.0.0.0

External Route Tag: 0
   可以看到都是20，度量值一样，但是R2并没有作负载均衡，而是选择了从R1到达3.3.3.0网段,由于R2到ASBR R1的cost是64,到另一个ASBR R5的cost是128，所以选择R1是没有疑义的，现在我们将R2到R1的metric值改大，看看R2是否还选择R1
R2(config)#int s2/0
R2(config-if)#ip ospf cost 200
R2(config-if)#end
重启OSPF进程看结果
R2#clear ip os pro
Reset ALL OSPF processes? [no]: y


R2#sh ip rou
Gateway of last resort is not set


35.0.0.0/24 is subnetted, 1 subnets
O E2
35.1.1.0 [110/20] via 24.1.1.4, 00:00:20, Serial1/0

2.0.0.0/24 is subnetted, 1 subnets
C
2.2.2.0 is directly connected, Loopback0

3.0.0.0/24 is subnetted, 1 subnets
O E2
3.3.3.0 [110/20] via 24.1.1.4, 00:00:20, Serial1/0

24.0.0.0/24 is subnetted, 1 subnets
C
24.1.1.0 is directly connected, Serial1/0

12.0.0.0/24 is subnetted, 1 subnets
C
12.1.1.0 is directly connected, Serial2/0

13.0.0.0/24 is subnetted, 1 subnets
O E2
13.1.1.0 [110/20] via 24.1.1.4, 00:00:20, Serial1/0

45.0.0.0/24 is subnetted, 1 subnets
O IA
45.1.1.0 [110/128] via 24.1.1.4, 00:00:20, Serial1/0
       可以看到R2此时选择了R4作为下一条，也就是接收了R5通告的外部路由条目
[案例3]E1和E2路由共存的情况
       下面我们将R1通告的外部路由类型改为 O E1，来验证O E1比O E2优先
R1(config-router)#redistribute rip subnets metric-type 1
在R2上重启OSPF进程看结果
R2#clear ip os pro
Reset ALL OSPF processes? [no]: y


R2#sh ip rou
Gateway of last resort is not set

R2#sh ip rou
Gateway of last resort is not set


35.0.0.0/24 is subnetted, 1 subnets
O E1
35.1.1.0 [110/220] via 12.1.1.1, 00:00:01, Serial2/0

2.0.0.0/24 is subnetted, 1 subnets
C
2.2.2.0 is directly connected, Loopback0

3.0.0.0/24 is subnetted, 1 subnets
O E1
3.3.3.0 [110/220] via 12.1.1.1, 00:00:01, Serial2/0

24.0.0.0/24 is subnetted, 1 subnets
C
24.1.1.0 is directly connected, Serial1/0

12.0.0.0/24 is subnetted, 1 subnets
C
12.1.1.0 is directly connected, Serial2/0

13.0.0.0/24 is subnetted, 1 subnets
O E1
13.1.1.0 [110/220] via 12.1.1.1, 00:00:01, Serial2/0

45.0.0.0/24 is subnetted, 1 subnets
O IA
45.1.1.0 [110/128] via 24.1.1.4, 00:00:02, Serial1/0
可以看到R2又一次选择了R1通告的外部路由，此时虽然到R5的cost小，但是没有考虑，
可见优先比较O E1和O E2是首要条件
最后总结一下：(说法不同于开始的规则，只是更简单)
(1)OSPF中外部路由类型不同，优先选择O E1路由
(2)OSPF中外部路由类型相同，看到ASBR的cost值,越小越优先
[注:实际实验效果是这样，也就是我们看拓扑图，可以这样分析；但是路由器实际处理过程还是本贴开始的那个规则更符合实际情况]

重要问题补充说明:
在案例一和案例二中,外部路由类型相同,此时路由器看到ASBR的cost值决定路径,实际上这是OSPF内部选路的问题,从哪 里到达ASBR;而在OSPF内部选路时,第一个原则就是“O路由”>“O IA路由”,而案例二的后半步,在更改了到同一区域ASBR的cost值以后,可以发现走了从O IA路由学习到的ASBR,可见在对ASBR的选路，访问外部路由时，这个原则无效.

谢谢楼主给了验证版,以后遇到这个问题就可以自己解决了

是挺不错的哦，这样的版本用起来是比叫方便的呢？

看起来有点复杂！不过实用起来感觉蛮不错的！

学习了～OSPF中外部路由类型相同，看到ASBR的cost值,越小越优先

看不懂呀，能不能弄个简单点的呢，这么难懂呢~~~~~