Convergence

Understand

Recovery starts with detection, then LSA origination/flooding, SPF scheduling/calculation, route installation and forwarding programming. Local interface-down detection can be immediate; silent loss may wait for a dead timer or BFD. CPU load and forwarding hardware affect the observed outage.

SPF throttling batches changes during instability. Start, hold and maximum-wait values use milliseconds in this command family. LSA origination throttling is separate. Graceful restart needs helpers and forwarding preservation; it does not repair a broken physical link. Fast reroute needs a supported precomputed backup, not merely small timers.

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, R3: 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
interface GigabitEthernet2
 ip address 192.0.2.5 255.255.255.252
 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
interface GigabitEthernet2
 ip address 192.0.2.9 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

configure terminal
hostname R3
router ospf 1
 router-id 10.255.0.3
 passive-interface Loopback0
 exit
interface Loopback0
 ip address 10.255.0.3 255.255.255.255
 ip ospf 1 area 0
 ip ospf cost 1
 exit
interface GigabitEthernet1
 ip address 192.0.2.6 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
interface GigabitEthernet2
 ip address 192.0.2.10 255.255.255.252
 ip ospf network point-to-point
 ip ospf cost 10
 ip ospf 1 area 0
 no shutdown
 exit
end

Apply

The original triangle has direct cost 11 to R3’s loopback and backup cost 21 through R2. Record a forwarding measurement separately. These scheduling settings demonstrate a policy; they are not a universal production recommendation.

Each router

configure terminal
router ospf 1
 timers throttle spf 50 200 5000
end

R1 · fail the direct path

configure terminal
interface GigabitEthernet2
 shutdown
end

Verify and interpret

R1# show ip ospf neighbor
Neighbor ID     Pri State   Dead Time Address     Interface
10.255.0.2        0 FULL/-  00:00:37  192.0.2.2  GigabitEthernet1
R1# show ip route ospf | include 10.255.0.3
O 10.255.0.3/32 [110/21] via 192.0.2.2, GigabitEthernet1

The failed direct adjacency is gone. After convergence R2 supplies the backup; this excerpt contains no measured outage duration.

R1# show ip ospf | include SPF
 Initial SPF schedule delay 50 msecs
 Minimum hold time between two consecutive SPFs 200 msecs
 Maximum wait time between two consecutive SPFs 5000 msecs

Configured scheduler values are not a guarantee of 50-ms forwarding recovery.

R1 · restore

configure terminal
interface GigabitEthernet2
 no shutdown
end

R1# show ip route ospf | include 10.255.0.3
O 10.255.0.3/32 [110/11] via 192.0.2.6, GigabitEthernet2

The preferred direct path returns after adjacency and topology recovery. Use no timers throttle spf on all routers to restore platform scheduling defaults.

Compare administrative shutdown and silent loss only in a controlled lab. Collect timestamps and packet loss, including restoration, rather than inferring performance from timer configuration.

Technical references

OSPFv2 standard · Router configuration guide · SPF throttling