帧中继+重分发+管理距离（解决次优路由）

实验列表：

1、实验拓扑

2、查看配置完帧中继及rip 、ospf后各路由器的路由表

3、重分发的影响

4、修改管理距离

1）实验拓扑



2）各路由表

R2#sh ip rou

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default, U - per-user static route

o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

C 172.16.1.0 is directly connected, Serial0/1

R 172.16.3.0 [120/1] via 172.16.1.1, 00:00:05, Serial0/1

10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks

O 10.10.10.10/32 [110/65] via 10.0.3.1, 00:15:10, Serial0/3

C 10.0.3.0/24 is directly connected, Serial0/3

O 10.0.0.0/24 [110/128] via 10.0.3.1, 00:15:10, Serial0/3

O 10.0.1.0/24 [110/192] via 10.0.3.1, 00:15:10, Serial0/3

O 10.100.100.100/32 [110/129] via 10.0.3.1, 00:15:11, Serial0/3

O 10.200.200.200/32 [110/193] via 10.0.3.1, 00:15:11, Serial0/3

C 10.222.222.222/32 is directly connected, Loopback1

R 192.168.1.0/24 [120/1] via 172.16.1.1, 00:00:06, Serial0/1

R3(config-router)#do sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

R 172.16.1.0 [120/1] via 172.16.3.1, 00:00:26, Serial0/2

C 172.16.3.0 is directly connected, Serial0/2

10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

O 10.10.10.10/32 [110/129] via 10.0.1.1, 00:03:08, Serial0/0

O 10.0.3.0/24 [110/192] via 10.0.1.1, 00:03:08, Serial0/0

C 10.0.1.1/32 is directly connected, Serial0/0

O 10.0.0.0/24 [110/128] via 10.0.1.1, 00:03:08, Serial0/0

C 10.0.1.0/24 is directly connected, Serial0/0

O 10.100.100.100/32 [110/65] via 10.0.1.1, 00:03:10, Serial0/0

C 10.200.200.200/32 is directly connected, Loopback1

O 10.222.222.222/32 [110/193] via 10.0.1.1, 00:03:10, Serial0/0

R 192.168.1.0/24 [120/1] via 172.16.3.1, 00:00:01, Serial0/2

R7(config-router)#do sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

C 172.16.1.0 is directly connected, Serial0/0.1

C 172.16.3.0 is directly connected, Serial0/0.2

C 192.168.1.0/24 is directly connected, Ethernet1/0

5#sh ip rou

Gateway of last resort is not set

10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

O 10.10.10.10/32 [110/65] via 10.0.0.1, 00:16:37, Serial0/1

O 10.0.3.0/24 [110/128] via 10.0.0.1, 00:16:37, Serial0/1

C 10.0.1.2/32 is directly connected, Serial0/0

C 10.0.0.0/24 is directly connected, Serial0/1

C 10.0.1.0/24 is directly connected, Serial0/0

C 10.100.100.100/32 is directly connected, Loopback1

O 10.200.200.200/32 [110/65] via 10.0.1.2, 00:16:37, Serial0/0

O 10.222.222.222/32 [110/129] via 10.0.0.1, 00:16:39, Serial0/1

3）在R2上将rip重分发到ospf中的结果（单点重分发）

R2#sh ip rou

Codes: C - connected, S - static, R - RIP, M - mobile, B - BGP

D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area

N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2

E1 - OSPF external type 1, E2 - OSPF external type 2

i - IS-IS, su - IS-IS summary, L1 - IS-IS level-1, L2 - IS-IS level-2

ia - IS-IS inter area, * - candidate default, U - per-user static route

o - ODR, P - periodic downloaded static route

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

C 172.16.1.0 is directly connected, Serial0/1

R 172.16.3.0 [120/1] via 172.16.1.1, 00:00:23, Serial0/1

10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks

O 10.10.10.10/32 [110/65] via 10.0.3.1, 00:28:03, Serial0/3

C 10.0.3.0/24 is directly connected, Serial0/3

O 10.0.0.0/24 [110/128] via 10.0.3.1, 00:28:03, Serial0/3

O 10.0.1.0/24 [110/192] via 10.0.3.1, 00:28:03, Serial0/3

O 10.100.100.100/32 [110/129] via 10.0.3.1, 00:28:04, Serial0/3

O 10.200.200.200/32 [110/193] via 10.0.3.1, 00:28:04, Serial0/3

C 10.222.222.222/32 is directly connected, Loopback1

R 192.168.1.0/24 [120/1] via 172.16.1.1, 00:00:24, Serial0/1

R3(config-router)#do sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

O E2 172.16.1.0 [110/100] via 10.0.1.1, 00:29:04, Serial0/0

C 172.16.3.0 is directly connected, Serial0/2

10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

O 10.10.10.10/32 [110/129] via 10.0.1.1, 00:29:04, Serial0/0

O 10.0.3.0/24 [110/192] via 10.0.1.1, 00:29:04, Serial0/0

C 10.0.1.1/32 is directly connected, Serial0/0

O 10.0.0.0/24 [110/128] via 10.0.1.1, 00:29:04, Serial0/0

C 10.0.1.0/24 is directly connected, Serial0/0

O 10.100.100.100/32 [110/65] via 10.0.1.1, 00:29:05, Serial0/0

C 10.200.200.200/32 is directly connected, Loopback1

O 10.222.222.222/32 [110/193] via 10.0.1.1, 00:29:05, Serial0/0

O E2 192.168.1.0/24 [110/100] via 10.0.1.1, 00:29:05, Serial0/0

R3(config-router)#

R5#sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

O E2 172.16.1.0 [110/100] via 10.0.0.1, 00:31:01, Serial0/1

O E2 172.16.3.0 [110/100] via 10.0.0.1, 00:31:01, Serial0/1

10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

O 10.10.10.10/32 [110/65] via 10.0.0.1, 00:31:01, Serial0/1

O 10.0.3.0/24 [110/128] via 10.0.0.1, 00:31:01, Serial0/1

C 10.0.1.2/32 is directly connected, Serial0/0

C 10.0.0.0/24 is directly connected, Serial0/1

C 10.0.1.0/24 is directly connected, Serial0/0

C 10.100.100.100/32 is directly connected, Loopback1

O 10.200.200.200/32 [110/65] via 10.0.1.2, 00:31:02, Serial0/0

O 10.222.222.222/32 [110/129] via 10.0.0.1, 00:31:02, Serial0/1

O E2 192.168.1.0/24 [110/100] via 10.0.0.1, 00:31:02, Serial0/1

从R3路由表中的红色文字可以知道，R3到达192.168.1.0网段没有选择最佳路由(R3-R7），而是选择了次优路由（R3-R5-R4-R2-R7)，即在R3上本地rip失效。为什么会出现这种情况呢？当外域路由重分发到ospf后，外域路由的管理距离变为110，与此同时ospf更新lsa链路拓扑，这时对于R3而言出现了两条运往192.168.1.0的路径，根据路由器选择最佳路由的原则，最终把ospf外域路由（ospf外域路由管理距离110<rip管理距离120）放入路由表中，这就是重分发后对边界路由器（只有边界路由才出现这种情况）选择最佳路由的影响。

4）修改管理距离

在有些情况下（比如重分发后路由），路由器相信的路由协议提供的路由更糟，但其管理距离更小，因此选择次优路由。一种确保某种路由协议提供的路由被选中的方法是，给其他路由协议提供的路由协议指定较大的管理距离。本例中为了解决路由重分发后不影响边界路由器选择最佳路由，可在出现选择次优路由边界路由器（不是作重分发的路由器）上修改外来路由的管理距离（注意修改管理距离只对本身路由器生效，不会传到域内其他路由器，域内其他路由器对外来的路由管理距离依然不变（本例中为110））

4.1、举例：修改重分发路由器的外来路由管理距离不能影响域内其他路由器的外来路由管理距离。但如果是在路由协议的redistribute命令中修改管理距离或打标签之类（如将rip重分发到ospf中命令redistribute rip route-map
xxx subnets，route-map中可以使用set修改管理距离与打标签之类等 ）就会全局ospf协议生效，在此章不做讨论。

修改重分发路由器的域外路由管理距离R2(config-router)#distance ospf external 130 \\将域外路由管理值修改为130

结果

R3#sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

O E2 172.16.1.0 [110/100] via 10.0.1.1, 01:20:38, Serial0/0

C 172.16.3.0 is directly connected, Serial0/2

10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

O 10.10.10.10/32 [110/129] via 10.0.1.1, 01:20:38, Serial0/0

O 10.0.3.0/24 [110/192] via 10.0.1.1, 01:20:38, Serial0/0

C 10.0.1.1/32 is directly connected, Serial0/0

O 10.0.0.0/24 [110/128] via 10.0.1.1, 01:20:38, Serial0/0

C 10.0.1.0/24 is directly connected, Serial0/0

O 10.100.100.100/32 [110/65] via 10.0.1.1, 01:20:40, Serial0/0

C 10.200.200.200/32 is directly connected, Loopback1

O 10.222.222.222/32 [110/193] via 10.0.1.1, 01:20:40, Serial0/0

O E2 192.168.1.0/24 [110/100] via 10.0.1.1, 01:20:40, Serial0/0 \\边界路由器R3还是选择了次优路由（管理距离依然是110)，表明修改重分发路由器管理距离不起作用。

修改管理R4域外管理距离

R4(config-router)#distance ospf ex 140

R4#sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

O E2 172.16.1.0 [140/100] via 10.0.3.2, 00:00:11, Serial0/3

O E2 172.16.3.0 [140/100] via 10.0.3.2, 00:00:11, Serial0/3

10.0.0.0/8 is variably subnetted, 7 subnets, 2 masks

C 10.10.10.10/32 is directly connected, Loopback1

C 10.0.3.0/24 is directly connected, Serial0/3

C 10.0.0.0/24 is directly connected, Serial0/1

O 10.0.1.0/24 [110/128] via 10.0.0.2, 00:00:11, Serial0/1

O 10.100.100.100/32 [110/65] via 10.0.0.2, 00:00:13, Serial0/1

O 10.200.200.200/32 [110/129] via 10.0.0.2, 00:00:13, Serial0/1

O 10.222.222.222/32 [110/65] via 10.0.3.2, 00:00:13, Serial0/3

O E2 192.168.1.0/24 [140/100] via 10.0.3.2, 00:00:13, Serial0/3 \\说明本身域外路由管理距离值生效

而R5#sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

O E2 172.16.1.0 [110/100] via 10.0.0.1, 01:33:35, Serial0/1

O E2 172.16.3.0 [110/100] via 10.0.0.1, 01:33:35, Serial0/1

10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

O 10.10.10.10/32 [110/65] via 10.0.0.1, 01:33:35, Serial0/1

O 10.0.3.0/24 [110/128] via 10.0.0.1, 01:33:35, Serial0/1

C 10.0.1.2/32 is directly connected, Serial0/0

C 10.0.0.0/24 is directly connected, Serial0/1

C 10.0.1.0/24 is directly connected, Serial0/0

C 10.100.100.100/32 is directly connected, Loopback1

O 10.200.200.200/32 [110/65] via 10.0.1.2, 01:33:36, Serial0/0

O 10.222.222.222/32 [110/129] via 10.0.0.1, 01:33:36, Serial0/1

O E2 192.168.1.0/24 [110/100] via 10.0.0.1, 01:33:36, Serial0/1 \\无论R2、R4域外管理距离怎么变，都没有影响到R5。

4.2、修改选择次优路由的路由器R3。

方法1：R3(config-router)#distance ospf external 125 (也可以使用distance 125 10.0.0.0 0.255.255.255或者distance 125 0.0.0.0 255.255.255.255
access_list，而10.0.0.0或access_list里边的网段可以分开写，但要包含域内所有路由器的网段）

方法2：路由映射表

router ospf 1

log-adjacency-changes

network 10.0.1.0 0.0.0.255 area 0

network 10.200.200.200 0.0.0.0 area 0

distance 125 0.0.0.0 255.255.255.255
access_list

!

ip prefix-list net_10 seq 5 permit 10.0.0.0/8 le 32

!

route-map access_list permit 10

match ip address net_10

!

其结果一样

R3#sh ip rou

Gateway of last resort is not set

172.16.0.0/24 is subnetted, 2 subnets

R 172.16.1.0 [120/1] via 172.16.3.1, 00:00:06, Serial0/2

C 172.16.3.0 is directly connected, Serial0/2

10.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

O 10.10.10.10/32 [110/129] via 10.0.1.1, 00:00:50, Serial0/0

O 10.0.3.0/24 [110/192] via 10.0.1.1, 00:00:50, Serial0/0

C 10.0.1.1/32 is directly connected, Serial0/0

O 10.0.0.0/24 [110/128] via 10.0.1.1, 00:00:50, Serial0/0

C 10.0.1.0/24 is directly connected, Serial0/0

O 10.100.100.100/32 [110/65] via 10.0.1.1, 00:00:52, Serial0/0

C 10.200.200.200/32 is directly connected, Loopback1

O 10.222.222.222/32 [110/193] via 10.0.1.1, 00:00:52, Serial0/0

R 192.168.1.0/24 [120/1] via 172.16.3.1, 00:00:07, Serial0/2 \\rip管理距离120<ospf域外管理距离125，终于选择最优路由。

排查没有选择最优路由方法：命令traceroute和debug