[科普中国]-非广播式多路访问

简介

OSPF协议的全称是Open Shortest Path First，即开放式最短路径优先，它是一个内部网关协议，用于在单一自治系统内决策路由。区别于距离矢量协议(RIP)，OSPF具有支持大型网络、路由收敛快、占用网络资源少等优点，在目前应用的路由协议中占有相当重要的地位。根据路由器所连接的物理网络不同，OSPF将网络划分为四种类型：广播多路访问型（Broadcast Multi Access）、非广播多路访问型（None Broadcast Multi Access，NBMA）、点到点型（Point-to-Point）、点到多点型（Point-to-MultiPoint）1。

NBMA网络模型为了更详细地对NBMA网络中出现的问题进行分析，我们首先建立一个简单的NBMA网络模型（Fram Relay为例），作为研究DR选举的网络环境。如图所示，该网络模型为一个帧中继类型的网络，R0、R1、R2为网络主干，利用R01、R12、R22模拟业务子网，该网络拓扑中各设备的接口地址按照表1~表2所示进行配置。以上网络模型通过帧中继互联完成后，R0、R1、R2三台路由器均可相互通信，由于没有配置静态路由或相关路由协议，R01、R12、R22暂无法相互通信。

例子本次实验是NBMA网络上，广播模式下的OSPF，网络拓扑是全互联的，接下来还有部分互联的

R1配置：

interface Loopback1

ip address 1.1.1.1 255.255.255.0

ip ospf network point-to-point //OSPF将通告环回接口的实际子网掩码如不配这句将通告成/32

!

interface Serial1/2

ip address 10.0.1.1 255.255.255.0

encapsulation frame-relay //封装成帧中继

ip ospf network broadcast //设置成广播模式,广播模式下自动建立邻居关系

serial restart-delay 0

!

router ospf 1

log-adjacency-changes

network 1.1.1.0 0.0.0.255 area 0

network 10.0.1.0 0.0.0.255 area 0

!

R2配置：

interface Loopback1

ip address 2.2.2.2 255.255.255.0

ip ospf network point-to-point

!

interface Serial1/2

ip address 10.0.1.2 255.255.255.0

encapsulation frame-relay

ip ospf network broadcast

serial restart-delay 0

!

router ospf 1

log-adjacency-changes

network 2.2.2.0 0.0.0.255 area 0

network 10.0.1.0 0.0.0.255 area 0

!

R3配置：

interface Loopback1

ip address 3.3.3.3 255.255.255.0

ip ospf network point-to-point

!

interface Serial1/2

ip address 10.0.1.3 255.255.255.0

encapsulation frame-relay

ip ospf network broadcast

serial restart-delay 0

!

router ospf 1

log-adjacency-changes

network 3.3.3.0 0.0.0.255 area 0

network 10.0.1.0 0.0.0.255 area 0

!

各个路由器路由表：

R1(config-router)#do show ip route

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

1.0.0.0/24 is subnetted, 1 subnets

C 1.1.1.0 is directly connected, Loopback1

2.0.0.0/24 is subnetted, 1 subnets

O 2.2.2.0 [110/65] via 10.0.1.2, 00:14:05, Serial1/2

3.0.0.0/24 is subnetted, 1 subnets

O 3.3.3.0 [110/65] via 10.0.1.3, 00:14:05, Serial1/2

10.0.0.0/24 is subnetted, 1 subnets

C 10.0.1.0 is directly connected, Serial1/2

Router(config-if)#do show ip route

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

1.0.0.0/24 is subnetted, 1 subnets

O 1.1.1.0 [110/65] via 10.0.1.1, 00:31:00, Serial1/2

2.0.0.0/24 is subnetted, 1 subnets

C 2.2.2.0 is directly connected, Loopback1

3.0.0.0/24 is subnetted, 1 subnets

O 3.3.3.0 [110/65] via 10.0.1.3, 00:31:00, Serial1/2

10.0.0.0/24 is subnetted, 1 subnets

C 10.0.1.0 is directly connected, Serial1/2

R3(config-if)#do show ip route

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

1.0.0.0/24 is subnetted, 1 subnets

O 1.1.1.0 [110/65] via 10.0.1.1, 00:15:40, Serial1/2

2.0.0.0/24 is subnetted, 1 subnets

O 2.2.2.0 [110/65] via 10.0.1.2, 00:15:40, Serial1/2

3.0.0.0/24 is subnetted, 1 subnets

C 3.3.3.0 is directly connected, Loopback1

10.0.0.0/24 is subnetted, 1 subnets

C 10.0.1.0 is directly connected, Serial1/2

各个路由器邻居关系：

R1(config-router)#do show ip ospf ne

Neighbor ID Pri State Dead Time Address Interface

2.2.2.2 1 FULL/DROTHER 00:00:35 10.0.1.2 Serial1/2

3.3.3.3 1 FULL/DR 00:00:37 10.0.1.3 Serial1/2

R2(config-if)#do show ip ospf ne

Neighbor ID Pri State Dead Time Address Interface

1.1.1.1 1 FULL/BDR 00:00:38 10.0.1.1 Serial1/2

3.3.3.3 1 FULL/DR 00:00:39 10.0.1.3 Serial1/2

R3(config-if)#do show ip ospf ne

Neighbor ID Pri State Dead Time Address Interface

1.1.1.1 1 FULL/BDR 00:00:35 10.0.1.1 Serial1/2

2.2.2.2 1 FULL/DROTHER 00:00:35 10.0.1.2 Serial1/2

R3是DR，R1是BDR，R2是DROTHER