OSPF questions

thedude666

I think I have a good view on how OSPF works.

However I have been a bit confused.

OSPF determines its Router ID by the priority and an IP (unless a router id is manually specified).
The priority:
0 : will never do election
1 : default priority
>1 : is it the higher the value, the higher the priority?

As for the IP:
The IP for the Router id is taken from:
Highest IP on loopback.
If not available highest IP on ALL interfaces(so physical interfaces). Is this correct? Even if an interface is not selected by a network line in the router ospf command?
Does the IP on the interface itself has anything to do with the election? So for example you have 3 interfaces, 2 of them are in OSPF. The last one is not. Will the IP of interface 1 have any impact on election or other stuff, if it is not the highest IP of the switch?
From what I understand the designated router is based on the router id and router id is based on the highest IP.

Thanks in advance for anyone clearing this up for me!

Roguetadhg

The only thing I have been able to fully see from the Odem book is: I'll quote it for keeping the context correct

Rules for electing a Designated Router: (Pg 354)
[Greyed out for anything that didn't pertain to Electing, directly]
- The router sending the Hello with the highes OSPF priority setting becomes the DR.
- If two or more routers tie with the highest priority setting, the router sending the Hello with the highest RID wins.
- It's not always the case, but typically the router with the second-highest priority becomes the BDR.
- A priority setting of 0 means that the router does not participate in the election and can never become the DR or BDR.
- The range of priority values that allow a router to be a candidate are 1 through 255.
- If a new, better candidate come along after the DR and the BDR have been elected, the new candidate does not preempt the existing DR and BDR.

Details of how IOS determines an interface's OSPF cost: (pg 366)
1. If the router-id rid ospf subcommand is configured, this value is used as the RID.
2. If any Loopback interfaces have an IP address configured and the interface has a line and protocol status of up/up, the router picks the highest numeric IP address among the up/up loopback interfaces.
3. The router picks the highest numeric IP address from all other working (up/up) interfaces.

It took a lot of digging to find the exact answer you were looking for as well as myself.

Summary Of the election:
Router Priority > Configured RID (Optional) > Highest Router's Loopback interface COLOR=#008000]up[/COLOR]/[COLOR=#008000]up[/COLOR > Highest Router's interface COLOR=#008000]up[/COLOR]/[COLOR=#008000]up[/COLOR