The topology is as shown in the figure ↓

After the IP/protocol is configured, the routing of each device is shown in the figure ↓

 FW1 H>* 1.1.1.1/32 [0/0/1] is local address, loopback1 C>* 10.1.1.0/30 is directly connected, ethernet0/1 H>* 10.1.1.1/32 [0/0/1] is local address, ethernet0/1 C>* 20.1.1.0/30 is directly connected, ethernet0/2 H>* 20.1.1.1/32 [0/0/1] is local address, ethernet0/2 C>* 192.168.1.0/24 is directly connected, ethernet0/0 H>* 192.168.1.1/32 [0/0/1] is local address, ethernet0/0 RT1 C 10.1.1.0/30 is directly connected, GigaEthernet0/5 R 20.1.1.0/30 [120,1] via 10.1.1.1(on GigaEthernet0/5) C 30.1.1.0/30 is directly connected, GigaEthernet0/4 O 40.1.1.0/30 [110,2] via 50.1.1.2(on GigaEthernet0/3) [110,2] via 30.1.1.2(on GigaEthernet0/4) C 50.1.1.0/30 is directly connected, GigaEthernet0/3 R 192.168.1.0/24 [120,1] via 10.1.1.1(on GigaEthernet0/5) RT2 R 10.1.1.0/30 [120,1] via 20.1.1.1(on GigaEthernet0/5) C 20.1.1.0/30 is directly connected, GigaEthernet0/5 O 30.1.1.0/30 [110,2] via 50.1.1.1(on GigaEthernet0/3) [110,2] via 40.1.1.2(on GigaEthernet0/4) C 40.1.1.0/30 is directly connected, GigaEthernet0/4 C 50.1.1.0/30 is directly connected, GigaEthernet0/3 R 192.168.1.0/24 [120,1] via 20.1.1.1(on GigaEthernet0/5) RT3 C 30.1.1.0/30 is directly connected, GigaEthernet0/4 C 40.1.1.0/30 is directly connected, GigaEthernet0/3 O 50.1.1.0/30 [110,2] via 40.1.1.1(on GigaEthernet0/3) [110,2] via 30.1.1.1(on GigaEthernet0/4)

Check the routing table for two-way routing on RT1 and RT2

 RT1 C 10.1.1.0/30 is directly connected, GigaEthernet0/5 R 20.1.1.0/30 [120,1] via 10.1.1.1(on GigaEthernet0/5) C 30.1.1.0/30 is directly connected, GigaEthernet0/4 O 40.1.1.0/30 [110,2] via 50.1.1.2(on GigaEthernet0/3) [110,2] via 30.1.1.2(on GigaEthernet0/4) C 50.1.1.0/30 is directly connected, GigaEthernet0/3 R 192.168.1.0/24 [120,1] via 10.1.1.1(on GigaEthernet0/5) RT2 R 10.1.1.0/30 [120,1] via 20.1.1.1(on GigaEthernet0/5) C 20.1.1.0/30 is directly connected, GigaEthernet0/5 O 30.1.1.0/30 [110,2] via 50.1.1.1(on GigaEthernet0/3) [110,2] via 40.1.1.2(on GigaEthernet0/4) C 40.1.1.0/30 is directly connected, GigaEthernet0/4 C 50.1.1.0/30 is directly connected, GigaEthernet0/3 R 192.168.1.0/24 [120,1] via 20.1.1.1(on GigaEthernet0/5)

View LSDB tables of RT1 and RT2

 RT1 AREA: 0 Router Link States  Link ID         ADV Router      Age        Seq Num    Checksum Link Count  1.1.1.1         1.1.1.1         332        0x80000008 0x6df9   2     2.2.2.2         2.2.2.2         319        0x80000008 0xd475   2     3.3.3.3         3.3.3.3         1120       0x80000005 0x3935   2     Net Link States  Link ID         ADV Router      Age        Seq Num    Checksum  40.1.1.1        2.2.2.2         1160       0x80000003 0x1bda    30.1.1.1        1.1.1.1         1289       0x80000003 0x996e    50.1.1.1        1.1.1.1         1289       0x80000003 0x6295    ASE Link States  Link ID         ADV Router      Age        Seq Num    Checksum  Tag      20.1.1.0        1.1.1.1         331        0x80000001 0xbe7a    0x00000000 192.168.1.0     2.2.2.2         322        0x80000001 0x12cb    0x00000000  192.168.1.0     1.1.1.1         331        0x80000001 0x30b1    0x00000000 10.1.1.0        2.2.2.2         327        0x80000001 0x231c    0x00000000 RT2                    AREA: 0 Router Link States  Link ID         ADV Router      Age        Seq Num    Checksum Link Count  2.2.2.2         2.2.2.2         430        0x80000008 0xd475   2     1.1.1.1         1.1.1.1         445        0x80000008 0x6df9   2     3.3.3.3         3.3.3.3         1231       0x80000005 0x3935   2     Net Link States  Link ID         ADV Router      Age        Seq Num    Checksum  30.1.1.1        1.1.1.1         1402       0x80000003 0x996e    40.1.1.1        2.2.2.2         1270       0x80000003 0x1bda    50.1.1.1        1.1.1.1         1402       0x80000003 0x6295    ASE Link States  Link ID         ADV Router      Age        Seq Num    Checksum  10.1.1.0        2.2.2.2         438        0x80000001 0x231c 192.168.1.0     2.2.2.2         433        0x80000001 0x12cb   192.168.1.0     1.1.1.1         444        0x80000001 0x30b1 20.1.1.0        1.1.1.1         444        0x80000001 0xbe7a

At this time, it can be found that RT1/RT2 transmits the routing information 192.168.1.0/24 to RT2/RT1 through route introduction. Because the priority of ospf route introduction is 150, the routing information transmitted through RIP protocol will be selected, and the RIP priority is 120. There is no loop in the topology at this time.

Add a device running ospf ↓

After completing the ospf, import the L0 of LSW1 into the ospf

View the routing table of RT1, FW1 and RT2 ↓

 RT1 R 1.1.1.1/32 [120,2] via 10.1.1.1(on GigaEthernet0/5) C 10.1.1.0/30 is directly connected, GigaEthernet0/5 R 20.1.1.0/30 [120,1] via 10.1.1.1(on GigaEthernet0/5) C 30.1.1.0/30 is directly connected, GigaEthernet0/4 O 40.1.1.0/30 [110,2] via 50.1.1.2(on GigaEthernet0/3) [110,2] via 30.1.1.2(on GigaEthernet0/4) C 50.1.1.0/30 is directly connected, GigaEthernet0/3 O 60.1.1.0/30 [110,2] via 50.1.1.2(on GigaEthernet0/3) R 192.168.1.0/24 [120,1] via 10.1.1.1(on GigaEthernet0/5) FW1 R>* 1.1.1.1/32 [120/2/1] via 20.1.1.2, ethernet0/2, 00:02:09 C>* 10.1.1.0/30 is directly connected, ethernet0/1 H>* 10.1.1.1/32 [0/0/1] is local address, ethernet0/1 C>* 20.1.1.0/30 is directly connected, ethernet0/2 H>* 20.1.1.1/32 [0/0/1] is local address, ethernet0/2 R>* 30.1.1.0/30 [120/2/1] via 20.1.1.2, ethernet0/2, 00:26:54 R>* 40.1.1.0/30 [120/2/1] via 10.1.1.2, ethernet0/1, 00:27:21 R>* 60.1.1.0/30 [120/2/1] via 10.1.1.2, ethernet0/1, 00:05:20 C>* 192.168.1.0/24 is directly connected, ethernet0/0 H>* 192.168.1.1/32 [0/0/1] is local address, ethernet0/0 RT2 R 1.1.1.1/32 [120,2] via 20.1.1.1(on GigaEthernet0/5) R 10.1.1.0/30 [120,1] via 20.1.1.1(on GigaEthernet0/5) C 20.1.1.0/30 is directly connected, GigaEthernet0/5 O 30.1.1.0/30 [110,2] via 50.1.1.1(on GigaEthernet0/3) [110,2] via 40.1.1.2(on GigaEthernet0/4) C 40.1.1.0/30 is directly connected, GigaEthernet0/4 C 50.1.1.0/30 is directly connected, GigaEthernet0/3 C 60.1.1.0/30 is directly connected, GigaEthernet0/6 R 192.168.1.0/24 [120,1] via 20.1.1.1(on GigaEthernet0/5)

At this point, the loop can be found. Because LSW1 sends the route to RT2 (priority 150), RT2 to RT1 (priority 150), FW1 (priority 120 for routing), RT1 to FW1 (priority 120), and FW1 to RT2 (priority 120), the next hop from RT2 to 1.1.1.1 is FW1.

Solution ↓

• Modify the ospf external route priority on RT2 to x (x<120)

 router ospf 1 distance ospf external 100

• Use ACL/Prefix list on RT2 to match 1.1.1.1/32 routes, and use the filter tool in the rip view to filter

 ip prefix-list 1 seq 10 deny 1.1.1.1/32 ip prefix-list 1 seq 20 permit any ---------------------or--------------------- ip access-list standard 1 deny 1.1.1.1 255.255.255.255 permit any  router rip  filter gigaEthernet 0/5 in access-list/prefix-list 1