IS-IS Router Protocol entry-level classic

In our networking process, we usually connect two routers. So how will the Protocol Regulation be implemented here? Now let's talk about this problem, that IS, the IS-IS routing protocol. A router IS an intermediate system (IS), a host IS an end system (ES), the Protocol that runs between a host and a router IS called a ES-IS, the protocol between the router and the router IS-IS.

IS-IS a hierarchical connection status routing protocol based on the DECnet PhaseV routing algorithm. IS-IS can be operated on different subnets, including broadcast LAN, WAN, and point-to-point links. IS-IS a connection status protocol. In fact, it IS very similar to OSPF. It also uses the Hello protocol to find adjacent nodes and uses a communication protocol to send link information. The IS-IS Message uses the serial number, but it IS only a simple addition counter. When the counter reaches the maximum value, a is-IS router has no other choice. Only one error can be forged to trigger a refresh of all old information. However, because the serial number is 3-2 bits long, there is a large space for serial number before reaching the maximum value, so this is not a problem. However, there are at least two technical problems: IS-IS uses a small measurement value of 6 bits), severely limiting the information that can be converted with it; in addition, the connection status is only 8 bits long, and the number of records that can be advertised by routers is only 256.

A non-technical problem IS that the IS-IS routing protocol IS constrained by OSI, making it slower than OSPF in the past. But the current IS-IS Integrated in non-OSI (RFC). With a lot of extensions, it makes it easier to support new requirements than OSPF, such as IPV6 or TE, and is easier to implement.

The IS-IS Routing Protocol divides network routes into Level1 and Level2. The vro in Level1 only knows the routing information of the AREA in which it is located, and the vro in LEVEL2 knows the routing information to other AREAS. That is to say, all L1 routers form the LEVEL1 AREAS, while all L2 routers form the BACKBONE of the network to transmit routing information between LEVEL1 AREAS. As shown in.

ROUTER1 and ROUTER4 are the routers of LEVEL1, and ROUTER2 and ROUTER3 are the routers of LEVEL1/2.

The router of L1 only knows the route of the local AREA. For example, ROUTER1 knows the route to ROUTER2 but does not know the route to AREA2. Similarly, ROUTER4 only knows the route in AREA2, you only know the route to network ROUTER3, but you do not know how to go to AREA1.

ROUTER2 and ROUTER3 of LEVEL1/2 form a backbone network. They know the routing information of the region and broadcast the routing information of this AREA to all the routers of L1/2, that is, all L1/2 routers know the routing information of the full Autonomous Region. In, if ROUTER1 receives the packet destined for ROUTER4 and finds that ROUTER1 does not have the route information in its route table, it sends the packet to ROUTER2 on the L1/2 VBR, ROUTER2 knows the route information of the full autonomous domain, that is, the route information to router ROUTER4, which sends the packet to router3.

Because L1/2 vrouters are many fewer than L1 vrouters. Therefore, the IS-IS routing protocol can quickly converge the route information of the network.