的读书笔记.docx

第一部分
ISIS 的 PDU包头结构 1
目
录
-IS common PDU header1
-IS LAN Hello PDU2
-IS Point-to-Point Hello PDUevel 1 link-state PDU (18)
_ Level 2 link-state PDU (20)
_ Level 1 Complete SNP (24)
_ Level 2 Complete SNP (25)
_ Level 1 Partial SNP (26)
_ Level 2 Partial SNP (27)
Version (1 octet) This field is set to a constant value of 0x01, the current IS-IS version.
Reserved (1 octet) This field is set to a constant value of 0x00 and is ignored on receipt.
Maximum Area Addresses (1 octet) This field is set to a constant value of 0x00. It informs other systems how many area addresses are supported by the local router. A value of 0 means that no more than three area addresses are assigned to this router.
2. IS-IS LAN Hello PDU
We’ ve previously stated that IS-IS routers exchange IS-IS Hello (IIH) PDUs to establish an adjacency. While
the purpose of the Hello PDU is the same, there are three different formats the router can use. One is for
point-to-point links, and the two others are for broadcast links —one each for Level 1 and Level 2. Recall
from the “ IS-IS Levels ” section earlier that L1 routers must share the same area address to form an
adjacency, while L2 routers do not have this limitation. The separate LAN Hello PDUs simply tell the
receiving router to check or ignore this information.
L1 LAN Hello PDUs are multicast to the “ All L1 ISs ” address of 01:80:c2:00:00:14. L2 routers share a
separate multicast address “ All L2 ISs 2:00:00:15. Both LAN Hello PDUs share a common packet
format, as shown in Figure .
The IS-IS LAN Hello PDU consists of the following fields:
Circuit Type (1 octet) The first 6 bits are set to 0. The remaining bits designate the level at which the
interface is operating: L1 (0x01), L2 (0x02), or L1/L2 (0x03). PDUs with a value of 0x00 in this field are
ignored.
Source ID (6 octets) This field designates the sender of the IIH. The field is set to the 6-byte system ID of