Basically, a flood war is declared if:
- we are the originating router and there is another OSPF router flushing our LSA again and again.
- we are the flushing router and we don’t agree that some LSA should be present in our LSDB. This can occur for LSA type-2 with link state ID equal to one of our interfaces or type-3/type-5 LSA with advertising router ID equal to our router ID.
Take the following topology as an example:
The network is stable right now. R3, which is the ABR, sends a summary (type-3) LSA into areas 0 and 1. The LSDB for all routers looks like this:
R1#show ip ospf database
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1046 0x80000003 0x0050EB 3
2.2.2.2 2.2.2.2 970 0x80000004 0x00E1F9 4
3.3.3.3 3.3.3.3 861 0x80000006 0x0090CF 2
44.44.44.44 44.44.44.44 890 0x80000002 0x00FC57 3
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.2.2 2.2.2.2 970 0x80000001 0x008589
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.3.0 3.3.3.3 856 0x80000001 0x0002E0
44.44.44.44 3.3.3.3 162 0x80000001 0x00CB73
R2#show ip ospf database
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1067 0x80000003 0x0050EB 3
2.2.2.2 2.2.2.2 989 0x80000004 0x00E1F9 4
3.3.3.3 3.3.3.3 878 0x80000006 0x0090CF 2
44.44.44.44 44.44.44.44 906 0x80000002 0x00FC57 3
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.2.2 2.2.2.2 988 0x80000001 0x008589
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.3.0 3.3.3.3 873 0x80000001 0x0002E0
44.44.44.44 3.3.3.3 169 0x80000001 0x00CB73
R3#show ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 1)
Router Link States (Area 0)
Link ID ADV Router Age Seq# Checksum Link count
1.1.1.1 1.1.1.1 1073 0x80000003 0x0050EB 3
2.2.2.2 2.2.2.2 995 0x80000004 0x00E1F9 4
3.3.3.3 3.3.3.3 881 0x80000006 0x0090CF 2
44.44.44.44 44.44.44.44 912 0x80000002 0x00FC57 3
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.2.2 2.2.2.2 995 0x80000001 0x008589
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.3.0 3.3.3.3 875 0x80000001 0x0002E0
44.44.44.44 3.3.3.3 172 0x80000001 0x00CB73
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
3.3.3.3 3.3.3.3 878 0x80000002 0x001E9C 2
44.44.44.44 44.44.44.44 174 0x80000003 0x00FA58 3
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
10.1.1.0 3.3.3.3 885 0x80000001 0x0022C1
10.1.2.0 3.3.3.3 885 0x80000001 0x00948E
11.11.11.11 3.3.3.3 885 0x80000001 0x00C9F8
22.22.22.22 3.3.3.3 885 0x80000001 0x004B8B
33.33.33.33 3.3.3.3 885 0x80000001 0x004566
R4#show ip ospf database
OSPF Router with ID (44.44.44.44) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
3.3.3.3 3.3.3.3 880 0x80000002 0x001E9C 2
44.44.44.44 44.44.44.44 176 0x80000003 0x00FA58 3
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
10.1.1.0 3.3.3.3 886 0x80000001 0x0022C1
10.1.2.0 3.3.3.3 886 0x80000001 0x00948E
11.11.11.11 3.3.3.3 886 0x80000001 0x00C9F8
22.22.22.22 3.3.3.3 886 0x80000001 0x004B8B
33.33.33.33 3.3.3.3 886 0x80000001 0x004566
Let’s take a closer look at the summary LSA going into area 0 - this should contain information about networks 10.1.3.0/24 and 44.44.44.44/32, with the advertising router ID set to 3.3.3.3.
R1#show ip ospf database summary adv-router 3.3.3.3
OSPF Router with ID (1.1.1.1) (Process ID 1)
Summary Net Link States (Area 0)
Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 953
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 10.1.3.0 (summary Network Number)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0x2E0
Length: 28
Network Mask: /24
MTID: 0 Metric: 64
Routing Bit Set on this LSA in topology Base with MTID 0
LS age: 259
Options: (No TOS-capability, DC, Upward)
LS Type: Summary Links(Network)
Link State ID: 44.44.44.44 (summary Network Number)
Advertising Router: 3.3.3.3
LS Seq Number: 80000001
Checksum: 0xCB73
Length: 28
Network Mask: /32
MTID: 0 Metric: 65
What would happen if I change the router ID of R3 to 1.1.1.1 now? The same summary LSA goes into area 0 with the same networks being advertised with the advertising router ID being set to 1.1.1.1 instead. When R1 receives this, it looks at the router ID and sees that it is its own router ID. If the router ID is the same, from R1s perspective, this implies that it has received a self-originated LSA (section 13.4 of RFC 2328). Does R1 have these networks in its OSPF LSDB? No. This means that the LSA is a stale LSA. When this happens, RFC dictates that we need to set max age for this LSA, increase the sequence number by one and flood it back out, which is what R1 does.
Now R3 gets a summary LSA with router ID set to 1.1.1.1, which again, is its own router ID. R3 assumes that it has received a self-originated LSA, however, the sequence number is higher than what it currently holds in the LSDB. This can potentially happen if a router reloads and gets its own LSA back (which would naturally have a higher sequence number than what the router has locally, since we start with sequence number 0x80000001). Also, since the LSA is present in the LSDB, it is not a stale LSA. Since the LSA received is a newer LSA (because of a higher sequence number), R3 increments the sequence number and re-originates the LSA (floods it back out again) as per RFC. This cycle will continue endlessly.
Let’s go ahead and make this change and verify that has been described above is indeed what happens.
R3(config)#router ospf 1
R3(config-router)#router-id 1.1.1.1
Reload or use "clear ip ospf process" command, for this to take effect
R3(config-router)#end
R3#clear ip ospf process
Reset ALL OSPF processes? [no]: y
After some time, we see the following log in R1:
%OSPF-4-FLOOD_WAR: Process 1 flushes LSA ID 10.1.3.0 type-3 adv-rtr 1.1.1.1 in area 0
and the following in R3:
%OSPF-4-FLOOD_WAR: Process 1 re-originates LSA ID 10.1.3.0 type-3 adv-rtr 1.1.1.1 in area 0
We would also see the LSA age constantly refreshing (outputs below are taken quickly, at a gap of around 1 second between each output):
R3#show ip ospf database
*snip*
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.3.0 1.1.1.1 8 0x80000029 0x00EDD4
44.44.44.44 1.1.1.1 8 0x80000029 0x00B767
*snip*
R3#show ip ospf database
*snip*
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.3.0 1.1.1.1 9 0x80000029 0x00EDD4
44.44.44.44 1.1.1.1 9 0x80000029 0x00B767
*snip*
R3#show ip ospf database
*snip*
Summary Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.1.3.0 1.1.1.1 0 0x8000002B 0x00E9D6
44.44.44.44 1.1.1.1 0 0x8000002B 0x00B369
*snip*
Another thing to note is the behavior observed on the middle router(s) – R2, in this case. Since there should now be two routers generating the same type-1 LSA, which one would R2 install? It would constantly oscillate between the two. This is because both R1 and R3, on receipt of a type-1 LSA with router ID of 1.1.1.1, would simply increment sequence number and flood it back out (in accordance with RFC 2328, section 13.4, Receiving Self-originated LSAs). R2, the router in the middle, would constantly get LSAs that have a higher sequence number than what it holds in the LSDB, and would thus install the newer LSA. This cycle would continue till the flood war persists.
Snapshots of R2s changing type-1 LSA is below:
R2#show ip ospf data router 1.1.1.1
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 0)
LS age: 9
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 80000053
Checksum: 0xAF3C
Length: 60
Number of Links: 3
Link connected to: a Stub Network
(Link ID) Network/subnet number: 11.11.11.11
(Link Data) Network Mask: 255.255.255.255
Number of MTID metrics: 0
TOS 0 Metrics: 1
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 2.2.2.2
(Link Data) Router Interface address: 10.1.1.1
Number of MTID metrics: 0
TOS 0 Metrics: 64
Link connected to: a Stub Network
(Link ID) Network/subnet number: 10.1.1.0
(Link Data) Network Mask: 255.255.255.0
Number of MTID metrics: 0
TOS 0 Metrics: 64
R2#show ip ospf data router 1.1.1.1
OSPF Router with ID (2.2.2.2) (Process ID 1)
Router Link States (Area 0)
LS age: 5
Options: (No TOS-capability, DC)
LS Type: Router Links
Link State ID: 1.1.1.1
Advertising Router: 1.1.1.1
LS Seq Number: 80000054
Checksum: 0x8C95
Length: 48
Area Border Router
Number of Links: 2
Link connected to: a Stub Network
(Link ID) Network/subnet number: 33.33.33.33
(Link Data) Network Mask: 255.255.255.255
Number of MTID metrics: 0
TOS 0 Metrics: 1
Link connected to: a Transit Network
(Link ID) Designated Router address: 10.1.2.2
(Link Data) Router Interface address: 10.1.2.3
Number of MTID metrics: 0
TOS 0 Metrics: 1
