On a P2P link, the establishment of a neighbor relationship is different from that of a broadcast link.
It is divided into two handshake mechanisms and a three-way handshake mechanism.
Two-way handshake
As long as the router receives the Hello packet from the peer,
it unilaterally announces that the neighbor is in the Up state and establishes a neighbor relationship.
Three-way handshake
This mode establishes a neighbor relationship by sending P2P IS-IS Hello PDUs three times,
similar to the establishment of a broadcast neighbor relationship.
The two-way handshake mechanism has obvious drawbacks.
If there are two or more links between the routers,
if the unidirectional state of the link to the peer is Down and the other link is Up in the same direction,
the adjacency can still be established between the routers.
The SPF uses the parameters on the link with the status UP when calculating.
This causes the router that has not detected the fault to still try to pass the link with the status Down when forwarding the message.
The three-way handshake mechanism solves the above problems in the unreliable point-to-point link.
In this way, the router only announces that the neighboring router is in the Up state when it knows
that the neighboring router also receives its packets, thus establishing a neighbor relationship.
IS-IS establishes neighbor relationships as follows:
Only neighboring routers at the same level are likely to become neighbors.
For Level-1 routers, the area numbers must be the same.
The network types of IS-IS interfaces on both ends of the link must be the same.
NOTE: By simulating an Ethernet interface as a P2P interface, you can establish a P2P link neighbor relationship.
The addresses of IS-IS interfaces on both ends of the link must be on the same network segment.
Since IS-IS is a protocol that runs directly on the data link layer, and the earliest design is used for CLNP,
the formation of IS-IS neighbor relationships is independent of the IP address. However, in the actual implementation,
since IS-IS is only run on the IP, it is necessary to check the IP address of the other party.
If the interface is configured with a secondary IP address,
the neighbors can be established as long as they have an IP (primary IP or secondary IP) on the same network segment.
The primary IP address does not have to be the same.
NOTE: If the addresses of the IS-IS interfaces on the two ends of the link are not in the same network segment,
the neighbor relationship can be established if the interface does not check the received Hello packets.
For a P2P interface, you can configure the interface to ignore the IP address check.
For an Ethernet interface, you need to simulate the Ethernet interface as a P2P interface.
Then you can configure the interface to ignore the IP address check.
