SPF Running Constantly-Cause: Duplicate Router ID

SPF Running Constantly ‚ Cause: Duplicate Router ID

This is also a common problem in OSPF. When two routers have identical router IDs, confusion results in the OSPF topology database, and the route keeps getting added and deleted. The most common symptom of this problem is that the LS Age field always has a small value.

This problem usually is generated by a cut and paste of a router configuration into another router. This results in two routers with identical router IDs. Figure 9-103 shows a network setup in which R2 and R3 have duplicate router IDs of 192.168.1.129.

Figure 9-103. OSPF Network with Duplicate Router IDs

Figure 9-104 shows the flowchart to follow to solve this problem.

Figure 9-104. Problem-Resolution Flowchart

Debugs and Verification

When there is a duplicate router ID, it causes SPF frequently, and the SPF counter keeps incrementing unless the problem is fixed. Example 9-273 shows that SPF in area 0 ran 2446 times, which is a large number.

Example 9-273 Determining How Often SPF Is Running

 R1#  show ip ospf  Routing Process "ospf 1" with ID 192.168.2.129  Supports only single TOS(TOS0) routes  It is an area border  SPF schedule delay 5 secs, Hold time between two SPFs 10 secs  Minimum LSA interval 5 secs. Minimum LSA arrival 1 secs  Number of external LSA 8. Checksum Sum 0x48C3E  Number of DCbitless external LSA 0  Number of DoNotAge external LSA 0  Number of areas in this router is 4. 1 normal 0 stub 0 nssa     Area BACKBONE(0)         Number of interfaces in this area is 1         Area has no authentication  SPF algorithm executed 2446 times  

The next step is to turn on debug ip ospf monitor. This debug shows exactly which LSA to chase. Example 9-274 shows the output of debug ip ospf monitor, which shows that a router with a router ID of 192.168.1.129 is the problem. The output also shows that it's a router LSA.

Example 9-274 debug ip ospf monitor Output Pinpoints the Router Causing This Problem

 R1#  debug ip ospf monitor  OSPF: Schedule SPF in area 0.0.0.0  Change in LS ID 192.168.1.129, LSA type R,  OSPF: schedule SPF: spf_time 1620348064ms wait_interval 10s 

Example 9-275 shows the output of the router LSA in question. There are two instances of this output taken 15 seconds apart. The first output shows that the number of links in this router is one; the second output shows that the number of links on this router is three. This is a discrepancy because of a duplicate router ID. This means that there must be another router with the same router ID causing the number of links to change every 15 seconds. Also, the LS Age field is always less than 10 seconds.

The first output in this example is the router LSA of R2; the second output is the router LSA of R3.

Example 9-275 Determining the Discrepancy in the Router LSA

  R1    #show ip ospf database router 192.168.1.129   OSPF Router with ID (192.168.2.129) (Process ID 1)                 Router Link States (Area 0.0.0.0)  LS age: 9  Options: (No TOS-capability, DC)   LS Type: Router Links   Link State ID: 192.168.1.129   Advertising Router: 192.168.1.129  LS Seq Number: 80067682  Checksum: 0xC456   Length: 36  Number of Links: 1  Link connected to: a Transit Network      (Link ID) Designated Router address: 192.168.254.14      (Link Data) Router Interface address: 192.168.254.14       Number of TOS metrics: 0        TOS 0 Metrics: 10  R1    #show ip ospf database router 192.168.1.129   OSPF Router with ID (192.168.2.129) (Process ID 1)                 Router Link States (Area 0.0.0.0)  LS age: 7  Options: (No TOS-capability, DC)   LS Type: Router Links   Link State ID: 192.168.1.129   Advertising Router: 192.168.1.129  LS Seq Number: 80067683  Checksum: 0xA7D8   Length: 60  Number of Links: 3  Link connected to: another Router (point-to-point)      (Link ID) Neighboring Router ID: 192.168.2.129      (Link Data) Router Interface address: 192.168.252.13       Number of TOS metrics: 0        TOS 0 Metrics: 66     Link connected to: a Stub Network      (Link ID) Network/subnet number: 192.168.252.12      (Link Data) Network Mask: 255.255.255.252       Number of TOS metrics: 0        TOS 0 Metrics: 66     Link connected to: a Transit Network      (Link ID) Designated Router address: 192.168.253.14      (Link Data) Router Interface address: 192.168.253.14       Number of TOS metrics: 0        TOS 0 Metrics: 1 R1# 

Example 9-276 shows that R2 and R3 have identical router IDs.

Example 9-276 Detecting Duplicate Router IDs

 R2#  show ip ospf  Routing Process "ospf 1" with ID  192.168.1.129  _____________________________________________________________________________________ R3#  show ip ospf  Routing Process "ospf 1" with ID  192.168.1.129  

Solution

To correct this problem, either change the router ID of R3 or change the router ID of R2. Example 9-277 shows how to change the router ID of R3 and gives the output of the show ip ospf command to verify that the router ID has been changed.

Example 9-277 Changing the Router ID of R3

 R3(config)#  interface loopback 0  R3(config-if)#  ip address 192.168.3.129 255.255.255.255  R3(config-if)#  end  R3#  show ip ospf  Routing Process "ospf 1" with ID  192.168.3.129  

Example 9-278 shows that after changing the router ID of R3, the LS age for 192.168.1.129 becomes stable in the OSPF database. The LS age has reached 90 seconds, so the entry is now stable.

Example 9-278 The LS Age for the Problem LSA Is Now Stable

 R1#  show ip ospf database router 192.168.1.129  OSPF Router with ID (192.168.2.129) (Process ID 1)                 Router Link States (Area 0.0.0.0)  LS age: 90  Options: (No TOS-capability, DC)   LS Type: Router Links   Link State ID: 192.168.1.129   Advertising Router: 192.168.1.129  LS Seq Number: 80067686  Checksum: 0xC456   Length: 36    Number of Links: 1     Link connected to: a Transit Network      (Link ID) Designated Router address: 192.168.254.14      (Link Data) Router Interface address: 192.168.254.14       Number of TOS metrics: 0        TOS 0 Metrics: 10