Description

This article provides troubleshooting guidelines to follow when continuous ospf route flap is observed.

 

Topology:

[lo0.0/RID:1.1.1.1/32]        [lo0.0/RID:2.2.2.2/32]                [lo0.0/RID:3.3.3.3/32]

R1-----------10.10.10.0/24-----------R2------------20.20.20.0/24----------R3

                                                                                      |

                                                                                      |

                                                                             30.30.30.0/24

                                                                                      |

                                                                                      |

                                                                                     R4

 

Symptoms

R4 advertises the connected network 30.30.30.0/24 through OSPF. The route for this particular network was seen to be flapping in the R1.

 

The output of "show ospf route | match 30.30.30.0" showed the route installed and uninstalled constantly in the routing table.

user@R1> show ospf route |match 30.30.30.0 |refresh 2   

---(refreshed at 2024-01-15 10:54:21 UTC)--- <<<<<<<<<<< No route found in o/p

---(refreshed at 2024-01-15 10:54:23 UTC)---

30.30.30.0/24     Intra Network   IP           2 ge-0/0/0.0   10.10.10.12 <<<<<<<<<<< Route installed 

---(refreshed at 2024-01-15 10:54:25 UTC)---

30.30.30.0/24     Intra Network   IP           2 ge-0/0/0.0   10.10.10.12

---(refreshed at 2024-01-15 10:54:27 UTC)---

30.30.30.0/24     Intra Network   IP           2 ge-0/0/0.0   10.10.10.12

---(refreshed at 2024-01-15 10:54:29 UTC)---

30.30.30.0/24     Intra Network   IP           2 ge-0/0/0.0   10.10.10.12

---(refreshed at 2024-01-15 10:54:31 UTC)---

30.30.30.0/24     Intra Network   IP           2 ge-0/0/0.0   10.10.10.12

---(refreshed at 2024-01-15 10:54:33 UTC)---

30.30.30.0/24     Intra Network   IP           2 ge-0/0/0.0   10.10.10.12

---(refreshed at 2024-01-15 10:54:35 UTC)---

---(refreshed at 2024-01-15 10:54:37 UTC)--- <<<<<<<<<<< Route uninstalled again

---(refreshed at 2024-01-15 10:54:39 UTC)---

 

You will see "Full SPF runs" count incrementing.

user@R1> show ospf overview |match " Full SPF runs" |refresh 4 

---(refreshed at 2024-01-15 11:41:02 UTC)---

  Full SPF runs: 1659  

---(refreshed at 2024-01-15 11:41:06 UTC)---

  Full SPF runs: 1660

---(refreshed at 2024-01-15 11:41:10 UTC)---

  Full SPF runs: 1660

---(refreshed at 2024-01-15 11:41:14 UTC)---

  Full SPF runs: 1663

---(refreshed at 2024-01-15 11:41:18 UTC)---

  Full SPF runs: 1664

---(refreshed at 2024-01-15 11:41:22 UTC)---

  Full SPF runs: 1666

---(refreshed at 2024-01-15 11:41:26 UTC)---

  Full SPF runs: 1669

---(refreshed at 2024-01-15 11:41:30 UTC)---

  Full SPF runs: 1672

 

Solution

Check how OSPF neighbors are formed in the R2, by command "show ospf neighbor":

 

user@R2> show ospf neighbor

Address         Interface             State    ID              Pri Dead

10.10.10.11     xe-0/0/0.0            Full     1.1.1.1         128   38

20.20.20.12     xe-0/0/1.0            Full     3.3.3.3         128   30

30.30.30.12     xe-0/0/2.0            Full     3.3.3.3         128   38

 

Looking at the o/p we can see 2 neighbors R3 and R4 have same RID 3.3.3.3 . This will cause the Intra-SPF to be run continuously on the device and hence flap the routes learned from those neighbors.

 

This issue is caused by using the same router-id on multiple devices, which are forming neighbor/adjacencies with this R2. In OSPF, no two neighbors should have same neighbor router-id. The solution is to configure a unique router-id per neighbor.

Modification History

2024-01-15 : Article Created

Related Information

Verifying and Monitoring OSPF Configuration