Lab 3 Ticket 4 - I gave up on this

Hi experts,

Today is the second day I am pulling my hair out on this ticket, so I thought it's about time to ask for some help.

I got stuck at the very beginning, because I don't understand the logic behind the creation of the loop between R1, R3 and SW2 (prefixes coming from BB1, I focused on 212.18.1.0/24).

My initial analysis was that there won't be a loop, because:

- R2 redistributes RIP into OSPF, so LSA 5 is flooded all over the area. Since R3 is connected through R1, R1 must re-flood these LSAs to R3. I believe here aren't any timers involved and this happens really fast.

- R1 redistributes into RIP as well.

- R3 will receive the prefixes via both OSPF and RIP. Since OSPF has lower AD it should only install these routes. R3 then redistributes into RIP, but even if these routes will make to R1, it won't bother beacuse again, OSPF has lower AD. 

Later I started wondering if timing has something to do with this loop.

If R3 will receive the 212.18.1.0/24 prefix from SW2 _BEFORE_ R1 refloods LSA5, R3 might start redistributing this back into OSPF. It will originate its own LSA5, which will be eventually received by R1, who already selected path via R2. Why would R1 select path via R3?

I run several debugs on R1 and the results are confusing me even more. Below I pasted the debugs, I also uploaded them to pastebin and highlighted important lines, so probably it will be easier to read.

http://pastebin.com/bb4FNRnZ

*Mar  1 00:03:41.520: OSPF process partial spfQ LSA id
        212.18.1.0: mask 255.255.255.0, type 5 adv_rtr 150.1.3.3, age 1,
        seq 0x80000001 (Area dummy area)

      *Mar  1 00:03:41.520: OSPF: Start partial processing Type
        5 External LSA 212.18.1.0, mask 255.255.255.0, adv 150.1.2.2,
        age 106, seq 0x80000001, metric 20, metric-type 2, fw-addr
        0.0.0.0

      *Mar  1 00:03:41.520:    Add better path to LSA ID
        212.18.1.0, gateway 163.1.12.2, dist 20

      *Mar  1 00:03:41.520:    Add path: next-hop 163.1.12.2,
        interface Serial0/0

      *Mar  1 00:03:41.520: OSPF-RIB-LOCAL: Add
        212.18.1.0/255.255.255.0, area dummy area, type 16, dist 20,
        forward 64, tag 0x0, via 163.1.12.2 Serial0/0, route flags
        0x800, path flags 0x0, source 150.1.2.2, spf 29, list-type 3

      *Mar  1 00:03:41.524: OSPF: Route update succeeded for
        212.18.1.0/255.255.255.0, next-hop Serial0/0/163.1.12.2, metric
        20

      *Mar  1 00:03:41.524: OSPF: Start partial processing Type
        5 External LSA 212.18.1.0, mask 255.255.255.0, adv 150.1.3.3,
        age 1, seq 0x80000001, metric 20, metric-type 2, fw-addr 0.0.0.0

      *Mar  1 00:03:41.524:    Add better path to LSA ID
        212.18.1.0, gateway 163.1.13.3, dist 20

      *Mar  1 00:03:41.524:    Add path: next-hop 163.1.13.3,
        interface Serial0/1

      *Mar  1 00:03:41.524: OSPF-RIB-LOCAL: Add
        212.18.1.0/255.255.255.0, area dummy area, type 16, dist 20,
        forward 64, tag 0x0, via 163.1.13.3 Serial0/1, route flags
        0x800, path flags 0x0, source 150.1.3.3, spf 29, list-type 3

      *Mar  1 00:03:41.524: OSPF: Route update succeeded for
        212.18.1.0/255.255.255.0, next-hop Serial0/1/163.1.13.3, metric
        20

      *Mar  1 00:03:41.524: RT: add 212.18.1.0/24 via
        163.1.13.3, ospf metric [110/20]

      *Mar  1 00:03:41.524: RT: NET-RED 212.18.1.0/24

      *Mar  1 00:03:41.524: OSPF-RIB-GLOBAL: Network update
        succeeded 212.18.1.0/255.255.255.0, via 163.1.13.3 on Serial0/1,
        distance 20, flags 0x0, source 150.1.3.3, tag 0x0, type 16,
        return: 0

      *Mar  1 00:03:41.524: OSPF-RIB-LOCAL: Sync'ed
        212.18.1.0/255.255.255.0 type 16 - added 1 paths, deleted 0
        paths, change flag 0x4, spf 29, route instance 29


    *Mar  1 00:03:41.580: OSPF: Detect change in LSA type 5,
        LSID 212.18.1.0, from 150.1.2.2 area 1

      *Mar  1 00:03:41.580: OSPF: Schedule partial SPF - type 5
        id 212.18.1.0 adv rtr 150.1.2.2


      *Mar  1 00:03:41.584: OSPF process partial spfQ LSA id
        212.18.1.0: mask 255.255.255.0, type 5 adv_rtr 150.1.2.2, age
        3600, seq 0x80000002 (Area dummy area)

      *Mar  1 00:03:41.584: OSPF: Start partial processing Type
        5 External LSA 212.18.1.0, mask 255.255.255.0, adv 150.1.2.2,
        age 3600, seq 0x80000002, metric 16777215, metric-type 2,
        fw-addr 0.0.0.0

      *Mar  1 00:03:41.584: OSPF: Start partial processing Type
        5 External LSA 212.18.1.0, mask 255.255.255.0, adv 150.1.3.3,
        age 1, seq 0x80000001, metric 20, metric-type 2, fw-addr 0.0.0.0

      *Mar  1 00:03:41.584:    Add better path to LSA ID
        212.18.1.0, gateway 163.1.13.3, dist 20

      *Mar  1 00:03:41.584:    Add path: next-hop 163.1.13.3,
        interface Serial0/1

      *Mar  1 00:03:41.584: OSPF-RIB-LOCAL: Add
        212.18.1.0/255.255.255.0, area dummy area, type 16, dist 20,
        forward 64, tag 0x0, via 163.1.13.3 Serial0/1, route flags
        0x800, path flags 0x0, source 150.1.3.3, spf 35, list-type 3

      *Mar  1 00:03:41.584: OSPF: Route update succeeded for
        212.18.1.0/255.255.255.0, next-hop Serial0/1/163.1.13.3, metric
        20

      *Mar  1 00:03:41.584: RT: del 212.18.1.0 via 163.1.12.2,
        ospf metric [110/20]

      *Mar  1 00:03:41.584: RT: NET-RED 212.18.1.0/24

Now, at line 2 we see that R1 is processing LSA 5 from R2 and this route is put in the OSPF-RIB-LOCAL.

Next, at line 7 R1 is processing LSA 5 from R3, which have exactly the same metrics as the previous one.

It is also put in the OSPF-RIB-LOCAL and somehow makes to OSPF-RIB-GLOBAL. My question is why?

In lines 17 and 18 we see that OSPF detects some change in LSA5 type and runs parital SPF. I also don't quite understand why.

If you made to this point, thanks for reading :)