Why can IS-IS neighbors not be established

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This has the following possible causes:

-Point-to-multipoint (P2MP) interfaces do not support the Intermediate System to Intermediate System routing protocol (IS-IS).
-The MTU of the Ethernet Interface is smaller than the max-size of IS-IS LSP packets plus 3.
-The MTU of the point-to-point (P2P) interface is less than the length of the IS-IS LSP packet.
-The two ends have different parameter configurations.

Other related questions:
Why can i not establish the OSPF neighbor relationship over ATM or FR links
The possible cause of this is that the IP address of the neighboring router and designated router (DR) priority are not configured.

When configured with a link state protocol, why can the local device not establish neighbor relationship with the peer device
When the local device is configured with a link state protocol, first check the type of the interface on the peer device. -To set the network type for an OSPF interface, run the ospf network-type command. -To set the network type of an IS-IS interface to P2P, run the isis circuit-type p2p command. The network types of the interfaces on both ends of a link must be the same; otherwise, the neighbor relationship cannot be established.

Why can one switch view neighbor information but cannot the other switch view neighbor information after the two S series switches establish RIP neighbor relationship
Q: Why can one switch view neighbor information but cannot the other switch view neighbor information after the two switches establish RIP neighbor relationship? A: A switch enabled with RIP does not send the route of the interface directly connected to the neighbor. If both ends only have direct routes, only one switch can discover the neighbor. For example, before device A and device B establish RIP neighbor relationship, if device A sends the interface route to device B, device B will discover device A. Device B then does not advertise its interface route to device A. Consequently, device A cannot discover the neighbor device B. RIP does not stipulate that whether a switch needs to send the route of the interface directly connected to the neighbor to the neighbor. This issue occurs when Huawei's communications products establish RIP neighbor relationship.

Can an OSPF neighbor relationship be established between devices that are on different subnets
A neighbor relationship can be established between two routers that are not on the same subnet only when the devices are connected through point-to-point (P2P) links. On a Point-to-Multipoint (P2MP) network, you can determine whether adjacencies can be formed between neighbors that are not on the same subnet. In all other cases, the devices must be on the same subnet.

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