IS-IS小常识
CLNP相当于IP数据报，是一种用于传输数据的数据报，也有自己的CLNP Header, Payload..
CLNS通过CLNP提供网络层到传输层的服务
CLNS地址
Area ID SEL三部分组成
ISIS是在CLNS环境中提供CLNP路由的路由协议
NSAP是在OSI网络层地址的实现（表示）
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IDP | DSP |
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AFI|IDI|High| Order|DSP|System ID|NSEL|
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? NSAP: Network Service Access Point
? An NSAP has an address that consists
of 3 parts (1-12个字节)
Variable length area-address
6 Byte system ID 标识终端地址
Byte n-selector (indicating transport layer)（1个字节）
Total length between 8 and 20 bytes
? NET: Network Entity Title （表示网络实体自己的地址Selector=0）
? Multiple NETs are like secondary IP addresses;
only use them when merging or splitting areas
Example 1:
47.0001.aaaa.bbbb.cccc.00
Area = 47.0001, SysID = aaaa.bbbb.cccc, NSel = 00
Example 2:
39.0f01.0002.0000.0c00.1111.00
Area = 39.0f01.0002, SysID = 0000.0c00.1111, NSel = 00
? Example 3:
49.0002.0000.0000.0007.00
Area = 49.0002, SysID = 0000.0000.0007, Nsel = 00
IP地址和NET的映射
1.The LOOPBACK IP Address: 192.168.3.25
2.The AREA the Router Under Is: 49.0001
3.IP Address Conversion Process to System ID:
4.192.168.3.25
5.192.168.003.025
6.1921.6800.3025
7.49.0001.1921.6800.3025
Areas and Backbone Routers
? IS-IS has a 2 layer hierarchy
The backbone (Level 2)
The areas (Level 1)
? An IS can be
Level 1 router (intra-area routing)
Level 2 router (inter-area routing)
Level 1-2 router (intra and inter-area routing)
? For each level (1 and 2) a DIS will be elected
on LANs
Level 1 router
Has neighbors only on the same area
Has the Level 1 LSDB with all routing information for the area
Use the closest Level 2 router to exit the area
This may result in sub-optimal routing
? Level 2 router
May have neighbors in other areas
Has a Level 2 LSDB with all information about inter-area routing
? Level 1?2 router
May have neighbors on any area
Has two LSDBs:
Level 1 for the intra-area routing
Level 2 for the inter-area routing
If the router has adjacencies to other areas, it will inform the Level 1 routers (intra-area) it is a potential exit point for the area.
Backbone must be L2 contiguous
IS-IS PDUs
? IS-IS packets are encapsulated directly in a data-link frame
? There is no CLNS or IP header
Hello PDUs (IIH, ISH, ESH)
LSP
Non-pseudonode LSP
Pseudonode LSPs
CSNP
PSNP
