9. An access management system, comprising: a first access management device, a second access management device and a third access management device; the first access management device is configured to: when detecting that a Mobile Node (MN) accessing the first access management device is offline, query a local Access Identity (AID) and Routing Identity (RID) mapping table, determine a correspondent node of the offline MN, send a mapping table entry update message carrying a mapping table entry corresponding to the MN in which the RID is set to be NULL to the second access management device and the third access management device, and set an RID in a mapping table entry corresponding to the MN to be NULL, which represents that the MN does not access any Access Switching Router (ASR) currently; and the second access management device is configured to: receive the mapping table entry update message sent by the first access management device, update the locally stored RID in the mapping table entry corresponding to the MN to be NULL according to the mapping table entry update message received, and when an RID in a mapping table entry corresponding to a destination AID of a locally received message is NULL, discard the locally received message; and the third access management device is configured to: receive the mapping table entry update message sent by the first access management device, update the locally stored RID in the mapping table entry corresponding to the MN to be NULL according to the mapping table entry update message received, and when a message for requesting the mapping table entry corresponding to the MN is received, send a response message, and carry the mapping table entry corresponding to the MN in the response message, wherein, the RID recorded in the mapping table entry is NULL.
9. An access management system, comprising: a first access management device, a second access management device and a third access management device; the first access management device is configured to: when detecting that a Mobile Node (MN) accessing the first access management device is offline, query a local Access Identity (AID) and Routing Identity (RID) mapping table, determine a correspondent node of the offline MN, send a mapping table entry update message carrying a mapping table entry corresponding to the MN in which the RID is set to be NULL to the second access management device and the third access management device, and set an RID in a mapping table entry corresponding to the MN to be NULL, which represents that the MN does not access any Access Switching Router (ASR) currently; and the second access management device is configured to: receive the mapping table entry update message sent by the first access management device, update the locally stored RID in the mapping table entry corresponding to the MN to be NULL according to the mapping table entry update message received, and when an RID in a mapping table entry corresponding to a destination AID of a locally received message is NULL, discard the locally received message; and the third access management device is configured to: receive the mapping table entry update message sent by the first access management device, update the locally stored RID in the mapping table entry corresponding to the MN to be NULL according to the mapping table entry update message received, and when a message for requesting the mapping table entry corresponding to the MN is received, send a response message, and carry the mapping table entry corresponding to the MN in the response message, wherein, the RID recorded in the mapping table entry is NULL. 12. The access management system according to claim 9 , wherein, the third access management device is further configured to: receive a mapping table entry update message sent by an ASR which the MN reaccesses, wherein, a mapping table entry recording a mapping relationship between an AID of the MN and an RID of the reaccessed ASR is included in the mapping table entry update message; update the mapping table entry corresponding to the MN to the mapping table entry in the mapping table entry update message, and inform the second access management device of a new mapping table entry corresponding to the MN.