OSPF Priority

[ciscoskeemz]

Hi again,

I read that if you assign a priority of 255, then you'll automatically have a tie in the election and ties are broken by the highest router ID.
Just want some clarification... if R1 router has a priority of 5 and R2 has a priority of 255, and all others are lower, R1 and R2 will be tied in the election?


thanks,
Mike

[Scott Morris]

In the election process, the dominance is determined by priority + RID. If anyone has a higher priority that will be looked at first. If two devices have the same priority (255) then the RID is looked at next.

But if you leave one router at default (1) and another at (255) the 255 will win.

You can manually influence the RID by use of the "router-id" command. While 223.255.255.255 is the highest IP address you can assign to an interface for the "auto-selection" part, you can manually set your RID to 255.255.255.255 if you'd like.

Note that if you have two devices with the same RID, bad things will happen. Or at least confusing things will happen.


HTH,
Scott

[ciscoskeemz]

thanks Scott!
Exactly how I thought it works....the way this author worded it confused me


Makes perfect sense now!

[Li-Ji]

There is no special meaning for OSPF interface priority numbers from 1 to 255. That is, dunring DR/BDR election, larger priority number wins; as a tie-breaker, when same priority number happens, larger Router ID wins.

There is no tie and no rule exceptions about the number 255!
Only priority number zero (0) has special meaning: it never becomes DR neither BDR. And it takes effect immediately: This is a lab trick for DR/BDR to resign itself!

In you case, R2 is DR and R1 is BDR.

Note: DR/BDR election time is really very short. Once all the roles are selected, even the new comer with higher priority will not change the roles at all, unless DR/BDR resigns by itself!


Cheers!

[raynebc]

I ran into this error in the CCNA Command Quick Reference, by Scott Empson (ISBN: 1-58713-159-5). On page 88, it claims:

"The assigned priority can be between 0 and 255. A priority of 0 guarantees that the router never wins a designated router (DR) election, and 255 guarantees a tie in the election (tie broken by highest router ID). The default priority is 1."

This isn't corrected in the book's errata, so I just emailed them about it.

[Angela]

Well, I think the author mean to say that when both routers have priority of 255, there is a tie because no router can have a higher priority.


Regards

[raynebc]

Probably, but the wording of the book is still wrong. That's why this thread was opened to begin with. The correction should be going into the book's errata soon.