When configured with a link state protocol, why can the local device not establish neighbor relationship with the peer device

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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.

Other related questions:
Problem and solution when the OSPF status is abnormal
To solve the problem that the OSPF status between the firewall and the peer device cannot reach the Full state, perform the following steps: 1. Check the OSPF status. Check whether the OSPF neighboring relationship can be established between the firewall and the peer device. 2. If no, check the security policy configuration. Check whether the security policy control function for unicast packets is enabled. That is, check whether the firewall packet-filter basic-protocol enable command is configured. If yes, run the undo firewall packet-filter basic-protocol enable command to disable the function. To establish an OSPF neighboring relationship, devices need to exchange DD packets. DD packets are OSPF unicast packets. By default, the forwarding of OSPF unicast packets is not controlled by security policies. However, if you run the firewall packet-filter basic-protocol enable command to enable the security policy control function for OSPF unicast packets, you need also to configure the corresponding security policy to allow the packets to be forwarded. For details, see OSPF can not step into full state caused by security policy deny.

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.

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.

Why can IS-IS neighbors not be established
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.

The state of an optical interface is not Up when it interconnects with the peer device
For the problem that the state of an optical interface is not Up when it interconnects with the peer device, see the troubleshooting guide. For details, access the URL in the right column.

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