19-18 ip ospf priority priority

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19-18 ip ospf priority priority

Syntax Description:

  • priority ” The OSPF priority value for the interface. The range of values is 0 to 255. The default value is 1.

Purpose: On a multiaccess network such as Ethernet, the router with the highest priority will be elected the DR. If all interface priorities are equal, then the router with the highest router ID will be elected the DR. The ip ospf priority command is used to influence the selection of the DR on a multiaccess network. If the priority is 0, then the router will not be elected the DR or BDR.

Initial Cisco IOS Software Release: 10.0

Configuration Example: Influencing the Election of the Designated Router

In Figure 19-14, Router B will be elected the DR on the Ethernet network because Router B has a higher router ID than Router A and their OSPF priorities on the Ethernet network are equal.

Figure 19-14. Router B Is the DR on the Ethernet Network Because Its Router ID Is Higher than the Router ID for Router A

graphics/19fig14.gif

 Router A  interface Loopback0   ip address 1.1.1.1 255.255.255.255   !   interface Ethernet0/0   ip address 172.16.1.1 255.255.255.0   !   router ospf 1   network 1.1.1.1 0.0.0.0 area 1   network 172.16.1.0 0.0.0.255 area 0  ______________________________________________________________________________ Router B  interface Ethernet0   ip address 172.16.1.2 255.255.255.0   !   router ospf 1   network 172.16.1.0 0.0.0.255 area 0  

Verify that Routers A and B have formed an OSPF neighbor relationship.

 rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 172.16.1.2        1  FULL/DR  00:00:36    172.16.1.2      Ethernet0/0 ______________________________________________________________________________ rtrB#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 1.1.1.1           1  FULL/BDR  00:00:39    172.16.1.1      Ethernet0 

Router B has been elected the DR because it has a higher router ID and the OSPF priorities for Routers A and B are both 1.

 rtrA#  show ip ospf interface ethernet 0/0  Ethernet0/0 is up, line protocol is up   Internet Address 172.16.1.1/24, Area 0   Process ID 1, Router ID 1.1.1.1, Network Type BROADCAST, Cost: 10   Transmit Delay is 1 sec,  State BDR, Priority 1  Designated Router (ID) 172.16.1.2, Interface address 172.16.1.2   Backup Designated router (ID) 1.1.1.1, Interface address 172.16.1.1   Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5     Hello due in 00:00:09   Index 1/1, flood queue length 0   Next 0x0(0)/0x0(0)   Last flood scan length is 1, maximum is 3   Last flood scan time is 0 msec, maximum is 0 msec   Neighbor Count is 1, Adjacent neighbor count is 1     Adjacent with neighbor 172.16.1.2  (Designated Router)   Suppress hello for 0 neighbor(s) 

Modify the OSPF priority for the Ethernet interface on Router B so the value is 2.

 Router B  interface Ethernet0   ip address 172.16.1.2 255.255.255.0    ip ospf priority 2   
Verification

Verify that the new priority has been configured on Router B.

 rtrB#  show ip ospf interface ethernet 0  Ethernet0 is up, line protocol is up   Internet Address 172.16.1.2/24, Area 0   Process ID 1, Router ID 172.16.1.2, Network Type BROADCAST, Cost: 10   Transmit Delay is 1 sec,  State DR, Priority 2  Designated Router (ID) 1.1.1.1, Interface address 172.16.1.1   Backup Designated router (ID) 172.16.1.2, Interface address 172.16.1.2   Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5     Hello due in 00:00:02   Neighbor Count is 1, Adjacent neighbor count is 1     Adjacent with neighbor 1.1.1.1  (Designated Router)   Suppress hello for 0 neighbor(s) 

Router A is still the BDR. For stability, OSPF will not elect another router as DR unless the current DR goes down. Shut down the Ethernet interface on Router B and wait until the dead time has expired on Router A. After the dead time has expired on Router A, reenable the Ethernet interface on Router B. Verify that Router A is now the DR.

 rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 172.16.1.2        2   FULL/BDR        00:00:39    172.16.1.2      Ethernet0/0 ______________________________________________________________________________ rtrB#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 1.1.1.1           1   FULL/DR         00:00:34    172.16.1.1      Ethernet0 

Modify the priority on Router B so Router B is not eligible to be elected DRR or BDR.

 Router B  interface Ethernet0   ip address 172.16.1.2 255.255.255.0    ip ospf priority 0   

Shut down the Ethernet interface on Router B and wait for the dead time to expire. After the dead time has expired, reenable the Ethernet interface on Router B. Then check the status of the OSPF neighbor relationship.

 rtrB#  show ip ospf interface ethernet 0  Ethernet0 is up, line protocol is up   Internet Address 172.16.1.2/24, Area 0   Process ID 1, Router ID 172.16.1.2, Network Type BROADCAST, Cost: 10   Transmit Delay is 1 sec,  State DROTHER, Priority 0  Designated Router (ID) 1.1.1.1, Interface address 172.16.1.1   No backup designated router on this network   Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5     Hello due in 00:00:09   Neighbor Count is 1, Adjacent neighbor count is 1     Adjacent with neighbor 1.1.1.1  (Designated Router)   Suppress hello for 0 neighbor(s) ______________________________________________________________________________ rtrA#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 172.16.1.2        0  FULL/DROTHER  00:00:31    172.16.1.2      Ethernet0/0 ______________________________________________________________________________ rtrB#  show ip ospf neighbor  Neighbor ID     Pri   State           Dead Time   Address         Interface 1.1.1.1           1  FULL/DR  00:00:32    172.16.1.1      Ethernet0 

Router B now has the status DROTHER because it cannot be the DR or BDR.

Troubleshooting
Step 1. Changing the priority value will not automatically cause OSPF to elect a new DR. If your design calls for a specific router to become the DR on a multiaccess network, configure the interface priority before enabling the OSPF process.

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Cisco[r] OSPF Command and Configuration Handbook
Cisco OSPF Command and Configuration Handbook (paperback)
ISBN: 1587055406
EAN: 2147483647
Year: 2002
Pages: 236

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