OSPF peers with same Router id but in different areas
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Hi Herediano,
The neighbor relationships between R2 and R1 and between R2 and R3 are normal, and intra-area and inter-area routes can be learned into the routing table. If R1 and R3 do not import external routes, the problem does not occur. OSPF uses LSA3 between areas. LSA3 is generated by the ABR in the area based on LSA1 and LSA2. adv is the router ID of the ABR. Inter-area routes are only attached to the ABR as leaf routes, no node with the same router ID exists in the SPT in the area. However, if routes are imported to devices with the same router ID, the problem occurs because the router ID of the ASBR must be known to all routers in the OSPF area. If the router ID of the ASBR is the same as that of the local device, the problem occurs. Assume that R1 imports a route to 10.10.10.0/24. R1 sends the LSA (adv=1.1.1.1, type=5, LS ID=1.1.1.1, seq=80000001) to R2. After receiving the LSA, R2 sends it to R3. After receiving the LSA, R3 finds that the advertiser is 1.1.1.1, but does not have the network segment, then, R3 sends an LSA5 (age=3600s, seq=80000001) to R2. After receiving the LSA5, R2 compares it with the LSA5 received previously. Because the seq and check sum are the same as those before, R2 preferentially selects the LSA5 with age=3600s, then, R1 forwards the LSA5 to R1. After receiving the LSA5, R1 finds that it has the network segment and sends a new LSA5 (seq=80000002). Such a repetitive situation may occur, causing route flapping. I hope it helps! |
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Hi Herediano,
The neighbor relationships between R2 and R1 and between R2 and R3 are normal, and intra-area and inter-area routes can be learned into the routing table. If R1 and R3 do not import external routes, the problem does not occur. OSPF uses LSA3 between areas. LSA3 is generated by the ABR in the area based on LSA1 and LSA2. adv is the router ID of the ABR. Inter-area routes are only attached to the ABR as leaf routes, no node with the same router ID exists in the SPT in the area. However, if routes are imported to devices with the same router ID, the problem occurs because the router ID of the ASBR must be known to all routers in the OSPF area. If the router ID of the ASBR is the same as that of the local device, the problem occurs. Assume that R1 imports a route to 10.10.10.0/24. R1 sends the LSA (adv=1.1.1.1, type=5, LS ID=1.1.1.1, seq=80000001) to R2. After receiving the LSA, R2 sends it to R3. After receiving the LSA, R3 finds that the advertiser is 1.1.1.1, but does not have the network segment, then, R3 sends an LSA5 (age=3600s, seq=80000001) to R2. After receiving the LSA5, R2 compares it with the LSA5 received previously. Because the seq and check sum are the same as those before, R2 preferentially selects the LSA5 with age=3600s, then, R1 forwards the LSA5 to R1. After receiving the LSA5, R1 finds that it has the network segment and sends a new LSA5 (seq=80000002). Such a repetitive situation may occur, causing route flapping. I hope it helps! |
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