I am running the following network to test a redistribution routing loop:
OSPF on router 1 and 2 is taking RIP routes and redistributing them into the OSPF domain.
R1#show run | s router
router ospf 1
router-id 1.1.1.1
log-adjacency-changes
redistribute rip subnets
network 172.16.13.1 0.0.0.0 area 0
router rip
version 2
network 172.50.0.0
default-metric 2
no auto-summary
R2#show run | s router
router ospf 1
router-id 2.2.2.2
log-adjacency-changes
redistribute rip subnets
network 172.16.23.1 0.0.0.0 area 0
router rip
version 2
network 172.50.0.0
default-metric 2
no auto-summary
From my understanding, both Router1 and 2 should create Type 5 LSAs with an administrative distance of 110 for all of the RIP networks.
When R1 looks at what to put in the routing table it sees that the AD to reach any given RIP network is better through R2 because this route will have an AD of 110 - as opposed to the route it learns directly from RIP which has an AD of 120.
R2 does the same thing with respect to R1.
Thus, if I ping a RIP subnet form R3, R1 and R2 should endlessly pass the between each other... voila! a loop.
BUT THAT ISN'T HAPPENING.
If I look at the LSDB I notice that only one of the two ASBRs is advertising the Type 5 LSAs for the RIP networks.
If I shutdown the OSPF interface on the router that is doing the advertising, the other takes over. When I bring the interface back up.... there is a breif period where the LSDB shows BOTH advertising the Type 5 LSAs. However this only lasts for a second and then the newly connected router's Type 5 LSAs disappear from the LSDB. It is as if the routers are aware that this behavour will cause a loop and are opting for only one of the two to advertise Type 5 LSAs.
My output from Router 3 here shows two quick views (within 1 second of each other) of the LSDB just after the interface has been restored:
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 4 0x80000006 0x0016E1 2
2.2.2.2 2.2.2.2 1138 0x80000006 0x001FBC 2
3.3.3.3 3.3.3.3 1137 0x80000003 0x0078C1 8
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
10.41.0.0 1.1.1.1 5 0x80000001 0x005910 0
10.41.0.0 2.2.2.2 1168 0x80000001 0x003B2A 0
10.42.0.0 1.1.1.1 5 0x80000001 0x004D1B 0
10.42.0.0 2.2.2.2 1168 0x80000001 0x002F35 0
10.43.0.0 1.1.1.1 5 0x80000001 0x004126 0
10.43.0.0 2.2.2.2 1168 0x80000001 0x002340 0
10.44.0.0 1.1.1.1 5 0x80000001 0x003531 0
10.44.0.0 2.2.2.2 1168 0x80000001 0x00174B 0
172.50.14.0 1.1.1.1 119 0x80000002 0x00FBB5 0
172.50.24.0 1.1.1.1 7 0x80000001 0x008F19 0
172.50.24.0 2.2.2.2 1297 0x80000001 0x007133 0
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 14 0x80000006 0x0016E1 2
2.2.2.2 2.2.2.2 1148 0x80000006 0x001FBC 2
3.3.3.3 3.3.3.3 1147 0x80000003 0x0078C1 8
Type-5 AS External Link States
Link ID ADV Router Age Seq# Checksum Tag
10.41.0.0 2.2.2.2 1179 0x80000001 0x003B2A 0
10.42.0.0 2.2.2.2 1179 0x80000001 0x002F35 0
10.43.0.0 2.2.2.2 1179 0x80000001 0x002340 0
10.44.0.0 2.2.2.2 1179 0x80000001 0x00174B 0
172.50.14.0 1.1.1.1 128 0x80000002 0x00FBB5 0
172.50.24.0 2.2.2.2 1305 0x80000001 0x007133 0
Can anyone explain why this is happening? I'm studying for CCNP ROUTE but nothing I have read explains ths behaviour.
Thanks in advance.
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