LSAs and flooding

Understand

An LSA is a database record; an LSU is a packet carrying one or more records. Database-description packets summarize records, requests identify missing records and acknowledgments support reliable flooding. Retransmission lists track outstanding delivery to adjacent neighbors.

OSPFv2 LSA identity combines type, link-state ID and advertising router. Sequence number, checksum and age select the newer instance. Ordinary self-originated records refresh around 1800 seconds; MaxAge is 3600 seconds and is used for flushing. Refresh is not the same as expiration. Exact acknowledgment and duplicate handling depend on flooding context; a duplicate does not automatically require SPF.

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 1.

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
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 1
 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 1
 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 1
 no shutdown
 exit
end

Apply

The border baseline has R1–R2 in area 0 and R2–R3 in area 1. Add a passive /24 on R1, then trace its area-0 router-LSA and R2’s Type 3 into area 1.

R1

configure terminal
interface Loopback10
 ip address 10.10.10.1 255.255.255.0
 ip ospf network point-to-point
 ip ospf cost 1
 ip ospf 1 area 0
 exit
router ospf 1
 passive-interface Loopback10
end

Verify and interpret

R1# show ip ospf database router 10.255.0.1
  LS Type: Router Links
  Link State ID: 10.255.0.1
  Advertising Router: 10.255.0.1
  Link connected to: a Stub Network
   Network/subnet number: 10.10.10.0
   Network Mask: 255.255.255.0
   Metric: 1

R3# show ip ospf database summary 10.10.10.0
  LS Type: Summary Links(Network)
  Link State ID: 10.10.10.0
  Advertising Router: 10.255.0.2
  Network Mask: /24
  Metric: 11

R2 originates its own summary LSA; it does not flood R1’s area-0 router-LSA unchanged into area 1.

R3# show ip route ospf | include 10.10.10
O IA 10.10.10.0/24 [110/21] via 192.0.2.5, GigabitEthernet1

R3 adds its cost 10 to the ABR’s advertised 11. Compare LSDB and routing entries rather than treating them as interchangeable.

Shut R1 Loopback10, compare the changed router-LSA and disappearing Type 3, then restore it. A change inside one area can alter inter-area reachability, even though the detailed Type 1 topology remains area-local.

Technical references

OSPFv2 standard · Router configuration guide