2019-07-30 ISIS Hello message

Record the Hello message of ISIS.
Hello messages are referred to as IIH (IS-to-IS Hello PDUs): used to establish and maintain ISIS neighbor relationships.
There are three types, Level-1 LAN IIH and Level-2 LAN IIH are used in broadcast networks. The name is clear. Level-1 LAN IIH is used by Level-1 routers, and Level-2 LAN IIH is used by Level-2 routers. . Used for P2P is called P2P IIH.
In broadcast network and P2P messages, the common fields are:

     Reserved/Circuit Type：8比特。高位的6比特保留，值为0。低位的2比特表示路由器的类型（00保留，01表示L1，10表示L2，11表示L1/2）。
     Source ID：发送Hello报文的路由器的System ID。
     Holding Time：保持时间。期间没有收到邻居发来的Hello报文，则中止已建立的邻居关系。
     PDU Length：PDU的总长度，以字节为单位。

The difference is that the
Hello message on the broadcast network has the following two fields:

Priority：选举DIS的优先级。
LAN ID：包括System ID和一字节的伪节点ID。

P2P IIH:
There is no Priority and LAN ID fields, and a Local Circuit ID field is added to indicate the local link ID.