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The difference of the DBD packet between a Huawei and a Cisco router

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As we konw, the DBD packets are exchanged between routers to transfer the LSDB. In the DBD packet, there is a MTU field which indicates the interface MTU, if the value mismatch between the routers, the neighbor relationship could not be established.

In the case of a Huawei router, if you guys have noticed, the MTU field in the DBD packet is 0, instead of the MTU vlaue of the interface. This is because the router doesn't set this field to the interface MTU value, just as the picture below shows:


In the case of a Cisco router, this MTU field is set to the interface MTU value by default: 

dbd - 副本

When the Huawei router connects to the Cisco router, if the interface MTU is modified to 1400, will the neighbor relationship be established successfully? The answer is yes.

However, if we enable the command ospf mtu-enable, the neighbor relationship will not be established successfully, because when enabling the command, the Huawei router will set the MTU field in the DBD packet to the interface MTU value, while the Cisco router enables the MTU inspection by default. So, the router will stuck in exstart.

By the way, we can disable the inspection function on the Cisco router by excuting the command ip ospf mtu-ignore. By doing this, the Cisco router will not check the MTU field. 

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