Principle and configuration of IS-IS protocol

introduction

Like OSPF, IS-IS is also an IGP protocol based on link status and using the shortest path first algorithm for route calculation. IS-IS was originally a dynamic routing protocol designed by the international standards organization ISO for its connectionless network protocol CLNP.
In order to provide IP routing support, IETF has expanded and modified IS-IS in RFC1195 so that it can be used in TCP, IP and OSI environments at the same time. The revised IS-IS protocol is called integrated IS-IS . Due to the simplicity and scalability of IS-IS, it is currently widely deployed in large ISP networks.

1. ISIS Agreement

1.1 IS-IS application scenarios

1.2 IS-IS features

• IS-IS is often used for network construction within operators
• The feature of integrated IS-IS is to support CLNP network and IP network, which works at the network layer
• IS means router means intermediate system
• ES is the host system
• Unlike OSPF, in the IS-IS configuration process, NET can be configured with multiple, up to 3

1.3 IS-IS route calculation process

Establish neighbor relationship → synchronize LSDB database → perform SPF routing calculation

1.4 IS-IS address structure

• The NET address consists of three parts: (Area ID) + (System ID) + (SEL)
• The minimum NET address is 8 bytes, and the maximum is 20 bytes
• Area ID identifies the area in the routing domain
• System ID is used to uniquely identify the host and router in Quyu
• The role of SEL is similar to the "protocol identifier" of IP. The difference is that the transport layer protocol corresponds to different SELs. SEL is 00 on IP

1.5 Command to configure the NET address of the device

isis 1  
network-entity 49.0111.111.111.111.00      ## 配置地址
is-name AR1            ## 配置设备名称
interface G0/0/0        
ip add 172.16.10.1 255.255.255.0
isis enable 1                ## 将接口宣告进ISIS进程
dis isis peer                ## 查看ISIS邻居
dis isis lsdb                ## 查看ISIS数据库
dis isis int                 ## 查看本路由器上哪些接口宣告进ISIS进程
dis isis routing-table           ## 查看ISIS路由表
dis ip routing-table protocol isis

Two, IS-IS router

2.1 Three types of ISIS routers

2.1.1 Level-1 router

• Level-1 LSDB can only be created in the same area
• The establishment of Level-1 adjacency requires the same area ID

2.1.2 Level-2 router

• Able to create Level-2 LSDB in different regions
• Responsible for routing between areas, it can form an adjacency relationship with the same or different Level-2 routers or Level-1-2 routers in other areas;
• Maintain a Level-2 LSDB, which contains all the information of the IS-IS area;
• All Level-2 routers form a backbone network of routers and are responsible for communication between different areas. The Level-1-2 routers in the routing domain must be physically connected to ensure the continuity of the backbone network. There is no virtual link in IS-IS

2.1.3 Level-1-2 router

• Can create Level-1 and Level-2 LSDBs at the same time
• A router that belongs to both Level-1 and Level-2 is called a Level-1-2 router, and it can form a Level-1 neighbor relationship with Level-1 and Level-2 routers in the same area;
• It can also form a Level-2 neighbor relationship with Level-1 and Level-2 routers in other areas
• The Level-1 router must pass through the Level-1-2 router to connect to other areas (same area)

2.2 Summary

• The routing information generated in the same area is ISIS-Level-1, and the routing information generated in different areas is ISIS-Level-2;
• The LSDB of Chen Sheng in the same area will exist in the Level-1 table and the Level-2 table respectively, while the LSDB generated in different areas will only exist in the Level-2 table.

2.3 Establishment of ISIS neighbors

2.3.1 Same area number (Area ID)

2.3.2 Different area numbers (Area ID)

3. The carrier of link state information

3.1 LSP PDU-used to exchange link state information

• Real node LSP
• Pseudo node LSP: only exists in the broadcast link

3.2 SNP PDU-used to maintain the integrity and synchronization of the LSDB, and is summary information

• CSNP: used to synchronize LSP
• PSNP: used to request and confirm LSP

Protocol packets are divided into Level-1 and Level-2. The destination MAC of all protocols in the MA network is the group address

• The Level-1 address is: 0180-C200-0014
• The Level-2 address is: 0180-C200-0015

Four, routing algorithm

ISIS routing calculation cost: the default interface cost of the device is 10

4.1 SPF calculation process

• Single area LSDB synchronization completed
• Generate network topology map structure
• Generate the shortest path tree with this node as the root
• The cost value is the same across each node by default