Reachabiliy Diagnosis and Troubleshoot
- Check IPv4/IPv6 address of port and Port up/down status.
- ping each others physical interface
- Ping successful, check ping loopbacks(or advertised/ received prefixes)
- In case of Link State protocols, the link neiborship happens via link-local address(same IP address for IPv4 adjacencies whereas link-local FE:xx for IPv6 adjacencies), check for link-local address blocking.
- No duplicate router-ids or unavailability of router-id of neighboring router. When configuring ospf, manually mention router-id instead of letting router use it dynamically.
- All areas ( to be specific, all ospf processes) must be connected to Backbone area, Area0 because every Ospf process builts a different OSPF LSA Database, to see all the LSAs bifurcation clearly.
- In case of Sham-Links and Virtual-Links, check their up/down states.
- Network type(point-to-point, broadcast) mismatch.
- Prefixes advertisement depends on Area types: Stubby, etc
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