Neighbor states and requirements

Understand

Normal progress on this lab is Down → Init → 2-Way → ExStart → Exchange → Loading → Full. Some states are brief or skipped in sampled output. Router IDs must differ. Area and hello/dead values must agree; area options and authentication must be compatible. On broadcast/NBMA links the subnet mask is checked; point-to-point has different mask handling. A network-type mismatch may form a misleading partial adjacency, so configure compatible types explicitly.

Lab setup

Use a fresh lab for this lesson; changes from other lessons are not cumulative. All outputs are illustrative excerpts, not recorded router tests. Transit cost is explicitly 10 and Loopback0 cost is 1. Interface names may vary by image.

Loopback0 is 10.255.0.<router number>/32; area assignments: R1: area 0, R2: area 0.

configure terminal
hostname R1
router ospf 1
 router-id 10.255.0.1
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.1 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.1 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R2
router ospf 1
 router-id 10.255.0.2
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.2 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.2 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

Apply

Use the pair baseline. Inspect before modifying anything. The following is an isolated fault: make R2 passive on the transit interface, wait for R1’s dead timer to expire, then remove that fault.

R2 · inject fault

configure terminal
router ospf 1
 passive-interface GigabitEthernet1
end

R1# show ip ospf neighbor
[No entries; explanatory annotation]
R2# show ip ospf interface GigabitEthernet1
  No Hellos (Passive interface)

An empty neighbor table follows expiry; passive operation prevents neighbor formation.

R2 · repair

configure terminal
router ospf 1
 no passive-interface GigabitEthernet1
end

Verify and interpret

R1# show ip ospf neighbor
Neighbor ID     Pri State   Dead Time Address     Interface
10.255.0.2        0 FULL/-  00:00:36  192.0.2.2  GigabitEthernet1

Full confirms restoration. If stuck earlier, use the state table to select evidence rather than repeatedly resetting the process.

Hellos discover and maintain neighbors; DBDs summarize LSAs; LSRs request missing LSAs; LSUs carry LSAs; acknowledgments support reliable flooding. A Hello can pass while larger DBD/LSU packets fail, which is why basic reachability alone is insufficient.

Technical references

OSPFv2 standard · Router configuration guide