IS-IS网络协议 PSNP消息格式
IS-IS PSNP（Partial Sequence Number PDUs，部分时序报文）通过描述部分数据库中的LSP来同步各LSDB（Link-State DataBase），从而维护LSDB。PSNP只列举最近收到的一个或多个LSP的序号，它能够一次对多个LSP进行确认，当发现LSDB不同步时，也用PSNP来请求邻居发送新的LSP。
一、 L1/L2 PSNP格式报文格式
+--------------------------------------------+-------------------
| Intradomain Routing Protocol Discriminator | |
+--------------------------------------------+ |
| Length Indicator | |
+--------------------------------------------+ |
| Version/Protocol ID Extension | |
+--------------------------------------------+ |
| ID Length | |
+--------------------------------------------+ PDU Common
| R | R| R | PDU Type | Header
+--------------------------------------------+ |
| Version | |
+--------------------------------------------+ |
| Reserved | |
+--------------------------------------------+ |
| Maximum Area Address | |
+--------------------------------------------+------------------
| PDU Length |
+--------------------------------------------+
| Source ID |
+--------------------------------------------+
| Variable Length Fields |
+--------------------------------------------+
二、 L1/L2 PSNP格式报文格式各字段含义
| 字段 | 长度 | 含义 |
|---|---|---|
| Intradomain Routing Protocol Discriminator | 1字节 | 域内路由选择协议鉴别符，设置为0x83。 |
| Length Indicator | 1字节 | PDU头部的长度（包括通用头部和专用头部），以字节为单位。 |
| Version/Protocol ID Extension | 1字节 | 版本/协议标识扩展，设置为1（0x01）。 |
| ID Length | 1字节 | NSAP地址或NET中System ID区域的长度。值为0时，表示System ID区域的长度为6字节。值为255时，表示System ID区域为空（即长度为0）。 |
| R | 1比特 | 保留比特位，设置为0。 |
| PDU Type | 5比特 | PDU的类型。
|
| Version | 1字节 | IS-IS版本号，设置为1（0x01）。 |
| Reserved | 1字节 | 保留，设置为0 (0x00)。 |
| Maximum Area Address | 1字节 | 支持的最大区域个数。设置为1～254的整数，表示该IS-IS进程实际所允许的最大区域地址数；设置为0，表示该IS-IS进程最大只支持3个区域地址数。 |
| PDU Length | 2字节 | PDU的总长度，单位是字节。 |
| Source ID | ID Length + 1字节 | 发出PSNP报文的路由器的System ID。 |
| Variable Length Fields | 变长 |
由多个CLV（Code-Length-Value）三元组组成。CLV也称为TLV（Type-Length-Value）。不同PDU类型所包含的CLV是不同的，其中，Code值从1到10的CLV在ISO 10589中定义（有2类未在上表中列出），其他几种CLV在RFC 1195中定义。 |
三、 IS-IS PSNP消息报文示例
Frame 23: 84 bytes on wire (672 bits), 84 bytes captured (672 bits)
Arrival Time: Apr 4, 2005 01:16:54.557453999
Epoch Time: 1112548614.557453999 seconds
[Time delta from previous captured frame: 1.106162999 seconds]
[Time delta from previous displayed frame: 1.106162999 seconds]
[Time since reference or first frame: 28.869652999 seconds]
Frame Number: 23
Frame Length: 84 bytes (672 bits)
Capture Length: 84 bytes (672 bits)
[Frame is marked: False]
[Frame is ignored: False]
[Protocols in frame: eth:llc:osi:isis]
[Coloring Rule Name: Broadcast]
[Coloring Rule String: eth[0] & 1]
IEEE 802.3 Ethernet
Destination: ISIS-all-level-1-IS's (01:80:c2:00:00:14)
Address: ISIS-all-level-1-IS's (01:80:c2:00:00:14)
.... ...1 .... .... .... .... = IG bit: Group address (multicast/broadcast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Source: HuaweiTe_22:3c:dd (00:e0:fc:22:3c:dd)
Address: HuaweiTe_22:3c:dd (00:e0:fc:22:3c:dd)
.... ...0 .... .... .... .... = IG bit: Individual address (unicast)
.... ..0. .... .... .... .... = LG bit: Globally unique address (factory default)
Length: 70
Logical-Link Control
DSAP: ISO Network Layer (0xfe)
IG Bit: Individual
SSAP: ISO Network Layer (0xfe)
CR Bit: Command
Control field: U, func=UI (0x03)
000. 00.. = Command: Unnumbered Information (0x00)
.... ..11 = Frame type: Unnumbered frame (0x03)
ISO 10589 ISIS InTRA Domain Routeing Information Exchange Protocol
Intra Domain Routing Protocol Discriminator: ISIS (0x83)
PDU Header Length: 17
Version (==1): 1
System ID Length: 6
PDU Type : L1 PSNP (R:000)
Version2 (==1): 1
Reserved (==0): 0
Max.AREAs: (0==3): 3
ISO 10589 ISIS Partial Sequence Numbers Protocol Data Unit
PDU length: 67
Source-ID: 0030.0300.3003.00
LSP entries (48)
LSP-ID: 0040.0400.4004.00-01, Sequence: 0x00000000, Lifetime: 1158s, Checksum: 0xdf25
LSP-ID: : 0040.0400.4004.00-01
LSP Sequence Number : 0x00000000
Remaining Lifetime : 1158s
LSP checksum : 0xdf25
LSP-ID: 0050.0500.5005.00-00, Sequence: 0x00000000, Lifetime: 1147s, Checksum: 0x752d
LSP-ID: : 0050.0500.5005.00-00
LSP Sequence Number : 0x00000000
Remaining Lifetime : 1147s
LSP checksum : 0x752d
LSP-ID: 0050.0500.5005.01-00, Sequence: 0x00000000, Lifetime: 1144s, Checksum: 0x8c03
LSP-ID: : 0050.0500.5005.01-00
LSP Sequence Number : 0x00000000
Remaining Lifetime : 1144s
LSP checksum : 0x8c03
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