OSPF Router ID and DR Election

Unanswered Question
Sep 27th, 2007

During DR election, the router OSPF priority is checked first. If this is a tie, then the OSPF RID is used to break the tie. Is OSPF priority changed at the global configuration level or the interface level? Can each interface on a router have a different priority? Neither Odom's book nor Lammle's book has the command, but Bryant's ebook shows it as an interface level command: ospf priority. If I change the ospf priority of a serial interface but also have a loopback address configured, would the OSPF priority be the priority of the loopback? Is this shown with show ip interface lo0 command? Sorry to be so dense, but being a unix admin, I don't have a network to test on. Also, when I loaded Lammle's cd and ran the test, during the simulation, I had to create a connection and search for all the settings for the console connection to the router. Will I have to do that on the test as well, or will I get a console prompt by just clicking on a host (as other simulations do)?

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CSCO10892433 Fri, 09/28/2007 - 01:26

Hi, oren

"ip ospf priority " is a interface configuration mode command. It tells the router to send ospf hello packet out that interface with the priority value you set behind this command. If you set a different priority on another interface, the ospf hello packet leaving out that interface will have different priority value.

For example, RouterA has 2 ethernet interfaces: e0,and e1. e0 connect to LAN1 and has priority set to 110; e1 connects to LAN2 and has priority set to 90. Assume other routers on these two LANs has their priority set to 100. Then RouterA will become a DR on LAN1 because it has the highest priority value on this segment, but it will not become a DR on LAN2 because it has a lower priority value on this segment.

BTW, DR election only takes place on multi-access media such as ethernet. Serial port is a point-to-point media; DR election won't happen on this media by default.



LordFlasheart Fri, 10/12/2007 - 03:48

Router(config-if)#ip ospf priority

It therefore follows that a Router can be the DR in one multiaccess nwk and a DROTHER on another.




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