OSPF：MTU不一致探究实验

 

1.Router-ID大的设备拥有更大MTU

1.1实验拓扑

 

1.2实验需求

（1）按拓扑完成基本IP地址配置

（2）修改R1接口MTU值为1000byte

（3）开启OSPF邻接关系调试，运行OSPF，但拓扑指定Router-ID，观察实验现象

（4）解决出现的问题  www.2cto.com  
 

1.3实验步骤

（1）修改MTU

R1(config-if)#ip mtu ?

  <68-1500>  MTU (bytes)

 

R1(config-if)#ip mtu 1000

 

（2）调试现象

R1

*Mar  1 00:02:42.427:        DR: 2.2.2.2 (Id)   BDR: 2.2.2.2 (Id)

*Mar  1 00:02:42.431: OSPF: Send DBD to 2.2.2.2 on FastEthernet0/0 seq 0x21D opt 0x52 flag 0x7 len 32

确定了R2的接口角色后，向其发送DD消息，本地将其状态置为Exstart

*Mar  1 00:02:42.795: OSPF: Rcv DBD from 2.2.2.2 on FastEthernet0/0 seq 0x2405 opt 0x52 flag 0x7 len 32  mtu 1500 state EXSTART

*Mar  1 00:02:42.795: OSPF: Nbr 2.2.2.2 has larger interface MTU

 

来自R2的DD消息，其Interface MTU值大于本地，忽略该DD消息

 

*Mar  1 00:02:47.431: OSPF: Send DBD to 2.2.2.2 on FastEthernet0/0 seq 0x21D opt 0x52 flag 0x7 len 32

*Mar  1 00:02:47.431: OSPF: Retransmitting DBD to 2.2.2.2 on FastEthernet0/0 [1]

*Mar  1 00:02:47.771: OSPF: Rcv DBD from 2.2.2.2 on FastEthernet0/0 seq 0x2405 opt 0x52 flag 0x7 len 32  mtu 1500 state EXSTART

*Mar  1 00:02:47.775: OSPF: Nbr 2.2.2.2 has larger interface MTU

在重传计时器内未收到来自邻居的有效DD，重传该DD，然而接收到的DD消息的Interface MTU值依然大于本地接口值，再次忽略此消息  www.2cto.com  

此后该现象多次发生，R1本地不断重传该DD消息

 

R2

*Mar  1 00:02:42.247: OSPF: Rcv DBD from 1.1.1.1 on FastEthernet0/0 seq 0x21D opt 0x52 flag 0x7 len 32  mtu 1000 state 2WAY

*Mar  1 00:02:42.251: OSPF: Nbr 1.1.1.1 has smaller interface MTU

*Mar  1 00:02:42.251: OSPF: Nbr state is 2WAY

R2本地在未选举出接口角色时，就接收到DD消息，此时不会读取其中内容

 

*Mar  1 00:02:42.587:        DR: 2.2.2.2 (Id)   BDR: 1.1.1.1 (Id)

*Mar  1 00:02:42.587: OSPF: Send DBD to 1.1.1.1 on FastEthernet0/0 seq 0x2405 opt 0x52 flag 0x7 len 32

本地在确定R1接口角色后，向其发送DD消息，将R1状态置为Exstart

 

*Mar  1 00:02:47.255: OSPF: Rcv DBD from 1.1.1.1 on FastEthernet0/0 seq 0x21D opt 0x52 flag 0x7 len 32  mtu 1000 state EXSTART

*Mar  1 00:02:47.255: OSPF: Nbr 1.1.1.1 has smaller interface MTU

*Mar  1 00:02:47.259: OSPF: First DBD and we are not SLAVE

 

接收到来自R1的DD消息，实际上该消息已经是R1的重传消息了，该消息MTU值更小，但是有效，读取后确定we are not SLAVE

*Mar  1 00:02:47.587: OSPF: Send DBD to 1.1.1.1 on FastEthernet0/0 seq 0x2405 opt 0x52 flag 0x7 len 32

*Mar  1 00:02:47.587: OSPF: Retransmitting DBD to 1.1.1.1 on FastEthernet0/0 [1]

此时恰好之前发送的DD消息超时，本地重传该消息

   www.2cto.com  

（3）邻接关系

R1#sho ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           1   EXSTART/DR      00:00:38    31.31.12.2      FastEthernet0/0

 

R2#show ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface

1.1.1.1           1   EXSTART/BDR     00:00:38    31.31.12.1      FastEthernet0/0

两台设备，此时均卡在Exstart状态

 

（4）解决问题

R1(config-if)#ip ospf mtu-ignore

 

*Mar  1 00:09:12.615: %OSPF-5-ADJCHG: Process 1, Nbr 2.2.2.2 on FastEthernet0/0 from LOADING to FULL, Loading Done

 

 

2.Router-ID小的设备拥有更大MTU

2.1实验拓扑

 

2.2实验需求

（1）按拓扑完成基本IP地址配置

（2）修改R2接口MTU值为1000byte

（3）开启OSPF邻接关系调试，运行OSPF，但拓扑指定Router-ID，观察实验现象

2.3实验步骤  www.2cto.com  

（1）调试现象

R1

*Mar  1 00:01:21.015:        DR: 2.2.2.2 (Id)   BDR: 2.2.2.2 (Id)

*Mar  1 00:01:21.019: OSPF: Send DBD to 2.2.2.2 on FastEthernet0/0 seq 0x1E88 opt 0x52 flag 0x7 len 32

确定了接口角色后，就像对应邻居发送DD消息，本地将其状态置为Exstart

 

*Mar  1 00:01:21.795: OSPF: Rcv DBD from 2.2.2.2 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x7 len 32  mtu 1000 state EXSTART

*Mar  1 00:01:21.795: OSPF: Nbr 2.2.2.2 has smaller interface MTU

*Mar  1 00:01:21.795: OSPF: NBR Negotiation Done. We are the SLAVE

 

由于接收到的DD消息的MTU值更小，本地读取此消息，通过比较Router-ID，确定本地为Slave

*Mar  1 00:01:21.799: OSPF: Send DBD to 2.2.2.2 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x2 len 52

向邻居发送带有LSA头部信息的DD消息，此时已经将邻居的状态置为Exchange

 

*Mar  1 00:01:26.791: OSPF: Rcv DBD from 2.2.2.2 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x7 len 32  mtu 1000 state EXCHANGE  www.2cto.com  

*Mar  1 00:01:26.791: OSPF: Nbr 2.2.2.2 has smaller interface MTU

*Mar  1 00:01:26.795: OSPF: Send DBD to 2.2.2.2 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x2 len 52

此后收到来自邻居的重传DD消息，本地也不断发送带有LSA头部信息的DD消息作为回应

 

R2

*Mar  1 00:01:19.631: OSPF: Rcv DBD from 1.1.1.1 on FastEthernet0/0 seq 0x1E88 opt 0x52 flag 0x7 len 32  mtu 1500 state 2WAY

*Mar  1 00:01:19.635: OSPF: Nbr 1.1.1.1 has larger interface MTU

 

*Mar  1 00:01:20.379:        DR: 2.2.2.2 (Id)   BDR: 1.1.1.1 (Id)

*Mar  1 00:01:20.379: OSPF: Send DBD to 1.1.1.1 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x7 len 32

*Mar  1 00:01:20.455: OSPF: Rcv DBD from 1.1.1.1 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x2 len 52  mtu 1500 state EXSTART  www.2cto.com  

*Mar  1 00:01:20.455: OSPF: Nbr 1.1.1.1 has larger interface MTU

由于邻居DD消息具有更高的Interface MTU，因此忽略的消息

 

*Mar  1 00:01:25.379: OSPF: Send DBD to 1.1.1.1 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x7 len 32

*Mar  1 00:01:25.379: OSPF: Retransmitting DBD to 1.1.1.1 on FastEthernet0/0 [1]

*Mar  1 00:01:25.435: OSPF: Rcv DBD from 1.1.1.1 on FastEthernet0/0 seq 0x1C10 opt 0x52 flag 0x2 len 52  mtu 1500 state EXSTART

*Mar  1 00:01:25.435: OSPF: Nbr 1.1.1.1 has larger interface MTU

由于始终未收到有效的DD消息，本地将重传此前发送的DD  www.2cto.com  

 

（2）邻接关系

R1#show ip ospf neighbor

Neighbor ID     Pri   State           Dead Time   Address         Interface

2.2.2.2           1   EXCHANGE/DR     00:00:39    31.31.12.2      FastEthernet0/0

 

R2#show ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface

1.1.1.1           1   EXSTART/BDR     00:00:39    31.31.12.1      FastEthernet0/0

此时，邻接关系卡在了Exstart/Exchange状态