11.4 OSPF在帧中继网络中的配置
原理概述
OSPF将网络分为4种不同的类型，即Point-to-Point、Broadcast、NBMA 及Point-to-MultiPoint，不同网络类型下OSPF的工作制不一样。比如在Broadcast网络中，OSPF能够直接建立邻居邻接关系；在NBMA网络中默认必须手工指定邻居等。在实际网络中，可通过配置接口的网络类型来强制改变默认的接口的网络类型。在帧中继的环境中，OSPF默认的网络类型是NBMA。
实验目的
- 掌握OSPF在帧中继网络中的配置方法
- 理解Hub-Spoke组网架构
- 掌握在帧中继网络中排除OSPF故障的方法
场景
某公司网络使用OSPF协议，该公司由一个总部和两个分支机构组成。R1为总部路由器，R2和R3分别是两个分支机构的出口路由。两个分支机构都是通过租用运营商的帧中继虚电路来与总部通信的。但为了节省成本，两个分支机构之间没有直接互联的虚电路，即典型的Hub-Spoke组网架构，R1称为Hub端设备，R2、R3为Spoke端设备。
实验编址
| 设备 | 接口 | IP地址 | 子网掩码 | 默认网关 | DLCI |
| R1 | Loopback 0 | 10.1.1.1 | 255.255.255.255 | N/A | N/A |
| Serial 1/0/0 | 10.0.123.1 | 255.255.255.0 | N/A | 102/103 | |
| R2 | Loopback 0 | 10.1.2.2 | 255.255.255.255 | N/A | N/A |
| Serial 1/0/0 | 10.0.123.2 | 255.255.255.0 | N/A | 201 | |
| R3 | Loopback 0 | 10.1.3.3 | 255.255.255.255 | N/A | N/A |
| Serial 1/0/0 | 10.0.123.3 | 255.255.255.0 | N/A | 301 |
实验拓扑
实验步骤
1.基本配置
在公司总部路由器R1和两个分部的路由器R2、R3上配置帧中继接口，关闭帧中继逆向地址解析功能。
首先根据实验编址表进行相应的基本IP地址配置，并配置帧中继静态地址映射。环回接口掩码为32位，用来模拟公司总部和分部的主机。注意：将R1设置位DR，调整其DR优先级为100.
[R1]int loopback 0
[R1-LoopBack0]ip add 10.1.1.1 32
[R1-LoopBack0]int s1/0/0
[R1-Serial1/0/0]ip add 10.0.123.1 24
[R1-Serial1/0/0]link-protocol fr
Warning: The encapsulation protocol of the link will be changed. Continue? [Y/N]:y
[R1-Serial1/0/0]undo fr inarp
[R1-Serial1/0/0]fr map ip 10.0.123.2 102
[R1-Serial1/0/0]fr map ip 10.0.123.3 103
[R1-Serial1/0/0]ospf dr-priority 100
[R2]int loopback 0
[R2-LoopBack0]ip add 10.1.2.2 32
[R2-LoopBack0]int s1/0/0
[R2-Serial1/0/0]ip add 10.0.123.2 24
[R2-Serial1/0/0]link-protocol fr
Warning: The encapsulation protocol of the link will be changed. Continue? [Y/N]:y
[R2-Serial1/0/0]undo fr inarp
[R2-Serial1/0/0]fr map ip 10.0.123.1 201
[R3]int loopback 0
[R3-LoopBack0]ip add 10.1.3.3 32
[R3-LoopBack0]int s1/0/0
[R3-Serial1/0/0]ip add 10.0.123.3 24
[R3-Serial1/0/0]link-protocol fr
Warning: The encapsulation protocol of the link will be changed. Continue? [Y/N]:y
[R3-Serial1/0/0]undo fr inarp
[R3-Serial1/0/0]fr map ip 10.0.123.1 301
配置完成后，检查帧中继的虚电路状态和映射表。
PVC statistics for interface Serial1/0/0 (DTE, physical UP)
DLCI = 102, USAGE = LOCAL (00000100), Serial1/0/0
create time = 2021/11/17 16:05:05, status = ACTIVE
InARP = Disable, PVC-GROUP = NONE
in packets = 0, in bytes = 0
out packets = 0, out bytes = 0
DLCI = 103, USAGE = LOCAL (00000100), Serial1/0/0
create time = 2021/11/17 16:05:05, status = ACTIVE
InARP = Disable, PVC-GROUP = NONE
in packets = 0, in bytes = 0
out packets = 0, out bytes = 0
可以观察到，PVC处于ACTIVE状态表示正常。
Map Statistics for interface Serial1/0/0 (DTE)
DLCI = 102, IP 10.0.123.2, Serial1/0/0
create time = 2021/11/17 16:05:05, status = ACTIVE
encapsulation = ietf, vlink = 1
DLCI = 103, IP 10.0.123.3, Serial1/0/0
create time = 2021/11/17 16:05:05, status = ACTIVE
encapsulation = ietf, vlink = 2
检查R1与R2，R1与R3之间的网络连通性。
PING 10.0.123.2: 56 data bytes, press CTRL_C to break
Reply from 10.0.123.2: bytes=56 Sequence=1 ttl=255 time=50 ms
Reply from 10.0.123.2: bytes=56 Sequence=2 ttl=255 time=20 ms
Reply from 10.0.123.2: bytes=56 Sequence=3 ttl=255 time=20 ms
Reply from 10.0.123.2: bytes=56 Sequence=4 ttl=255 time=20 ms
Reply from 10.0.123.2: bytes=56 Sequence=5 ttl=255 time=20 ms
PING 10.0.123.3: 56 data bytes, press CTRL_C to break
Reply from 10.0.123.3: bytes=56 Sequence=1 ttl=255 time=40 ms
Reply from 10.0.123.3: bytes=56 Sequence=2 ttl=255 time=20 ms
Reply from 10.0.123.3: bytes=56 Sequence=3 ttl=255 time=20 ms
Reply from 10.0.123.3: bytes=56 Sequence=4 ttl=255 time=20 ms
Reply from 10.0.123.3: bytes=56 Sequence=5 ttl=255 time=20 ms
此时通行正常。
2.在帧中继上搭建OSPF网络
在R1、R2和R3上配置OSPF协议。采用OSPF单区域配置，指定各自的环回接口地址为Router-ID，所有网段都属于区域0。
[R1]ospf 1 router-id 10.1.1.1
[R1-ospf-1]area 0
[R1-ospf-1-area-0.0.0.0]network 10.0.123.1 0.0.0.255
[R1-ospf-1-area-0.0.0.0]network 10.1.1.1 0.0.0.0
[R2]ospf 1 router-id 10.1.2.2
[R2-ospf-1]area 0
[R2-ospf-1-area-0.0.0.0]network 10.0.123.2 0.0.0.255
[R2-ospf-1-area-0.0.0.0]network 10.1.2.2 0.0.0.0
[R3]ospf 1 router-id 10.1.3.3
[R3-ospf-1]area 0
[R3-ospf-1-area-0.0.0.0]network 10.0.123.3 0.0.0.255
[R3-ospf-1-area-0.0.0.0]network 10.1.3.3 0.0.0.0
配置完成后，查看OSPF邻居建立情况。
OSPF Process 1 with Router ID 10.1.1.1
发现无法正常建立邻居，这是明显网络故障，现在需要立刻进行分析排除故障。排障时需注意遵从从底层逐步往上层排查的顺序，即先检查物理层线路是否正常，然后检查二层链路的连通性，再检查三层路由协议的运行情况，最后检查高层相关应用是否正常。
测试直连线路的连通性。
PING 10.0.123.2: 56 data bytes, press CTRL_C to break
Reply from 10.0.123.2: bytes=56 Sequence=1 ttl=255 time=30 ms
Reply from 10.0.123.2: bytes=56 Sequence=2 ttl=255 time=30 ms
Reply from 10.0.123.2: bytes=56 Sequence=3 ttl=255 time=30 ms
Reply from 10.0.123.2: bytes=56 Sequence=4 ttl=255 time=20 ms
Reply from 10.0.123.2: bytes=56 Sequence=5 ttl=255 time=20 ms
PING 10.0.123.3: 56 data bytes, press CTRL_C to break
Reply from 10.0.123.3: bytes=56 Sequence=1 ttl=255 time=30 ms
Reply from 10.0.123.3: bytes=56 Sequence=2 ttl=255 time=20 ms
Reply from 10.0.123.3: bytes=56 Sequence=3 ttl=255 time=20 ms
Reply from 10.0.123.3: bytes=56 Sequence=4 ttl=255 time=20 ms
Reply from 10.0.123.3: bytes=56 Sequence=5 ttl=255 time=20 ms
直连链路连通性没有问题。再查看三层路由协议，即相应接口是否被通告到OSPF进程中。
OSPF Process 1 with Router ID 10.1.1.1
Interfaces
Area: 0.0.0.0 (MPLS TE not enabled)
IP Address Type State Cost Pri DR BDR
10.1.1.1 P2P P-2-P 0 1 0.0.0.0 0.0.0.0
10.0.123.1 NBMA DR 48 100 10.0.123.1 0.0.0.0
观察到，所有接口已经被通告进入OSPF进程。
此时可以对R1的S1/0/0接口进行抓包分析，产看协议的运行情况。
发现R1始终没有向外发送OSPF数据包。这是由于OSPF在帧中继上默认的网络类型为NBMA，即非广播多路访问。这种网络类型的特定是不支持广播和组播的数据包，而OSPF协议默认是采用组播方式发送报文，所以设备的OSPF报文无法在帧中继链路上进行发送，导致没有成功建立邻居关系。
这时可采用peer命令手工指定OSPF邻居，才用单播方式发送报文。
[R1]ospf 1
[R1-ospf-1]area 0
[R1-ospf-1-area-0.0.0.0]peer 10.0.123.2
[R1-ospf-1]peer 10.0.123.3
[R2]ospf 1
[R2-ospf-1]area 0
[R2-ospf-1-area-0.0.0.0]peer 10.0.123.1
[R3]ospf 1
[R3-ospf-1]area 0
[R3-ospf-1-area-0.0.0.0]peer 10.0.123.1
配置完成后，再次检查OSPF的邻居关系状态。
OSPF Process 1 with Router ID 10.1.1.1
Peer Statistic Information
----------------------------------------------------------------------------
Area Id Interface Neighbor id State
0.0.0.0 Serial1/0/0 10.1.2.2 Full
0.0.0.0 Serial1/0/0 10.1.3.3 Full
----------------------------------------------------------------------------
可以观察到，这时R1与R2、R3都建立了完全的邻接关系。再查看R1、R2、R3的路由表。
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Public routing table : OSPF
Destinations : 2 Routes : 2
OSPF routing table status :
Destinations : 2 Routes : 2
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.1.2.2/32 OSPF 10 48 D 10.0.123.2 Serial1/0/0
10.1.3.3/32 OSPF 10 48 D 10.0.123.3 Serial1/0/0
OSPF routing table status :
Destinations : 0 Routes : 0
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Public routing table : OSPF
Destinations : 2 Routes : 2
OSPF routing table status :
Destinations : 2 Routes : 2
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.1.1.1/32 OSPF 10 48 D 10.0.123.1 Serial1/0/0
10.1.3.3/32 OSPF 10 48 D 10.0.123.3 Serial1/0/0
OSPF routing table status :
Destinations : 0 Routes : 0
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Public routing table : OSPF
Destinations : 2 Routes : 2
OSPF routing table status :
Destinations : 2 Routes : 2
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.1.1.1/32 OSPF 10 48 D 10.0.123.1 Serial1/0/0
10.1.2.2/32 OSPF 10 48 D 10.0.123.2 Serial1/0/0
OSPF routing table status :
Destinations : 0 Routes : 0
可以观察到此时的R1、R2、R3路由表中都互相接收到了各自环回口所在网段的路由条目。测试环回口之间的连通性。
PING 10.1.2.2: 56 data bytes, press CTRL_C to break
Reply from 10.1.2.2: bytes=56 Sequence=1 ttl=255 time=20 ms
Reply from 10.1.2.2: bytes=56 Sequence=2 ttl=255 time=10 ms
Reply from 10.1.2.2: bytes=56 Sequence=3 ttl=255 time=10 ms
Reply from 10.1.2.2: bytes=56 Sequence=4 ttl=255 time=10 ms
Reply from 10.1.2.2: bytes=56 Sequence=5 ttl=255 time=10 ms
......
PING 10.1.3.3: 56 data bytes, press CTRL_C to break
Reply from 10.1.3.3: bytes=56 Sequence=1 ttl=255 time=30 ms
Reply from 10.1.3.3: bytes=56 Sequence=2 ttl=255 time=20 ms
Reply from 10.1.3.3: bytes=56 Sequence=3 ttl=255 time=20 ms
Reply from 10.1.3.3: bytes=56 Sequence=4 ttl=255 time=20 ms
Reply from 10.1.3.3: bytes=56 Sequence=5 ttl=255 time=20 ms
此时，R1与R2，R1与R3之间的环回口通信正常。再次测试R2与R3环回口之间的通行情况。
PING 10.1.3.3: 56 data bytes, press CTRL_C to break
Request time out
Request time out
Request time out
Request time out
Request time out
发现R2无法连通R3的环回口，再次进行排障。物理链路和二层链路的连通性测试省略。首先查看R2的OSPF路由条目，观察到去往10.1.3.3的网段下一跳地址是10.0.123.3。
Route Flags: R - relay, D - download to fib
------------------------------------------------------------------------------
Public routing table : OSPF
Destinations : 2 Routes : 2
OSPF routing table status :
Destinations : 2 Routes : 2
Destination/Mask Proto Pre Cost Flags NextHop Interface
10.1.1.1/32 OSPF 10 48 D 10.0.123.1 Serial1/0/0
10.1.3.3/32 OSPF 10 48 D 10.0.123.3 Serial1/0/0
OSPF routing table status :
Destinations : 0 Routes : 0
然后在R2上查看帧中继映射关系。
Map Statistics for interface Serial1/0/0 (DTE)
DLCI = 201, IP 10.0.123.1, Serial1/0/0
create time = 2021/11/17 16:05:09, status = ACTIVE
encapsulation = ietf, vlink = 1
可以观察到，此时没有关于10.0.123.3的映射，如果R2要发送数据包至下一跳10.0.123.3，但无法知晓该从哪条PVC上进行发送和封装，可以使用PVC复用技术解决此问题。这时需要在R2的S1/0/0接口下添加一条帧中继静态映射，通过R1与R2的PVC去往10.0.123.3。
[R2]int s1/0/0
[R2-Serial1/0/0]fr map ip 10.0.123.3 201
在R3上也添加关于10.0.123.2的相关映射。
[R3]int s1/0/0
[R3-Serial1/0/0]fr map ip 10.0.123.2 301
配置完成后，再在R2上查看帧中继映射关系。
Map Statistics for interface Serial1/0/0 (DTE)
DLCI = 201, IP 10.0.123.1, Serial1/0/0
create time = 2021/11/17 16:05:09, status = ACTIVE
encapsulation = ietf, vlink = 1
DLCI = 201, IP 10.0.123.3, Serial1/0/0
create time = 2021/11/17 16:56:50, status = ACTIVE
encapsulation = ietf, vlink = 2
此时已经添加上相关映射，再次测试R2与R3环回口的连通性。
PING 10.1.3.3: 56 data bytes, press CTRL_C to break
Reply from 10.1.3.3: bytes=56 Sequence=1 ttl=254 time=30 ms
Reply from 10.1.3.3: bytes=56 Sequence=2 ttl=254 time=30 ms
Reply from 10.1.3.3: bytes=56 Sequence=3 ttl=254 time=30 ms
Reply from 10.1.3.3: bytes=56 Sequence=4 ttl=254 time=30 ms
Reply from 10.1.3.3: bytes=56 Sequence=5 ttl=254 time=20 ms
此时通信正常。