Memory hook: Neighbor first, route second, packet path last.
Must remember
Read a route entry's prefix, source code, next hop, outgoing interface, administrative distance and metric. A connected route requires the associated interface to be operational. Longest prefix determines forwarding; protocol preference resolves competing sources for the same destination prefix.
Static route forms include network, default, host and floating routes for IPv4/IPv6. Specify a reachable next hop or suitable exit-interface/next-hop combination. IPv6 link-local next hops require interface context because the same link-local address can exist on different links.
For single-area OSPFv2, know router ID, area, hello/dead timers, network type, interface participation, cost and passive interfaces. Adjacent routers must agree on critical parameters; subnet/mask, authentication and MTU mismatches can prevent the expected adjacency or full exchange. A passive interface can advertise its connected network while suppressing neighbor formation there.
On appropriate multiaccess networks, OSPF elects a designated router and backup to reduce adjacency complexity. Priority influences eligibility/selection, with router ID used as a tie-breaker; priority zero is ineligible. Elections are not simply repeated every time a higher-ID router appears. Point-to-point OSPF links do not need DR/BDR election.
Use show ip ospf neighbor, show ip ospf interface, show ip protocols, show ip route and targeted pings/traces. A full adjacency does not guarantee a desired route is advertised, selected or allowed through an ACL. Check both ends and the return route.
First-hop redundancy presents a shared virtual gateway. Understand active/standby roles and virtual addresses independently of OSPF, which exchanges routing information. A gateway failover protocol and dynamic routing solve related but different problems.
Choose under exam pressure
| Requirement | Choice and reason |
|---|---|
| OSPF neighbor stuck before full adjacency | Compare interface/network parameters and relevant logs. |
| Advertise a LAN without forming neighbors there | An appropriate passive-interface configuration. |
| Backup static route | Higher administrative distance than the intended primary source. |
Traps
- OSPF router ID is not necessarily an address currently assigned to a forwarding interface.
- A full neighbor relationship is not proof of end-to-end application access.
Active recall
1. Which command shows OSPF neighbor state?
show ip ospf neighbor.
2. What does OSPF priority zero mean on a DR-election network?
The router is not eligible to become DR or BDR there.
3. Does a point-to-point link elect DR/BDR?
No, not for the point-to-point OSPF network type.
4. Why specify an interface with an IPv6 link-local next hop?
The address is scoped to a link and otherwise ambiguous.
5. OSPF versus first-hop redundancy?
OSPF exchanges routes; first-hop redundancy preserves a virtual gateway for hosts.