Establishment of IS-IS neighbor relationships.
Two routers running IS-IS must first establish a neighbor relationship before implementing the routing function of the protocol packets.
IS-IS neighbors are established differently on different types of networks.
Establishment of broadcast link neighbor relationships.
Figure 1 shows the process of establishing a neighbor relationship in a broadcast link by taking a Level-2 router as an example.
The neighbors are established between Level-1 routers.
Figure 1 Broadcast link neighbor relationship establishment process.
1. Router A broadcasts Level-2 LAN IIH. There is no neighbor identifier in this packet.
2. After receiving the packet, Router B identifies the neighbor status of Router A and Router A as Initial.
Router B then replies to Level 2 LAN IIH to Router A. This packet identifies Router A as the neighbor of Router B.
3. After receiving the packet, Router A identifies the neighbor status of Router B as Up.
RouterA then sends a Level-2 LAN IIH to RouterB that identifies RouterB as the neighbor of RouterA.
4. After receiving the packet, Router B identifies the neighbor status of Router A and Router A as Up.
In this way, the two routers successfully establish a neighbor relationship.
Because it is a broadcast network, you need to elect a DIS. Therefore, after the neighbor relationship is established,
the router waits for the interval between two Hello packets and then performs the election of the DIS.
The Hello message contains the Priority field. The one with the highest priority value will be elected as the DIS of the broadcast network.
If the priorities are the same, the interface with the larger MAC address is elected as the DIS.
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