Timers

Understand

Hello interval sets the periodic discovery cadence. Dead interval is the time without a valid Hello before declaring a neighbor lost. On common router broadcast and point-to-point defaults these are 10/40 seconds; NBMA and point-to-multipoint commonly use 30/120. Verify the actual interface instead of assuming defaults.

Hello and dead intervals must match across a link. Faster Hellos consume more CPU and packet capacity and can flap under congestion. Interface-down signals or BFD may detect a failure before the dead timer. Detection is followed by LSA flooding, SPF and forwarding installation; a small timer alone does not guarantee a particular restoration time.

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

Begin at the pair baseline with default timers. Change only R1 to 1/4 seconds to create a mismatch, then align R2. Existing adjacency may persist briefly until expiry.

R1 · deliberate mismatch

configure terminal
interface GigabitEthernet1
 ip ospf hello-interval 1
 ip ospf dead-interval 4
end

R1# show ip ospf interface GigabitEthernet1 | include intervals
  Timer intervals configured, Hello 1, Dead 4, Wait 4, Retransmit 5
R2# show ip ospf interface GigabitEthernet1 | include intervals
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5

Compare both ends; one-sided timer tuning prevents a stable adjacency.

R2 · align timers

configure terminal
interface GigabitEthernet1
 ip ospf hello-interval 1
 ip ospf dead-interval 4
end

Verify and interpret

R1# show ip ospf neighbor
Neighbor ID     Pri State   Dead Time Address     Interface
10.255.0.2        0 FULL/-  00:00:03  192.0.2.2  GigabitEthernet1

The live countdown is below the configured 4-second dead interval. Measure a silent loss separately from an administrative shutdown.

Restore the baseline on both routers with the commands below; do not leave only one end at 1/4.

R1 and R2 · independently

configure terminal
interface GigabitEthernet1
 no ip ospf hello-interval
 no ip ospf dead-interval
end

Subsecond fast-Hello modes and BFD have platform-specific prerequisites. Test them under realistic load before adopting them.

Technical references

OSPFv2 standard · Router configuration guide