election of DR and BDR in OSPF

Hi, I am now testing election of BR and BDR on "real physical hardware".

So if you have loopback with smaller IP value than physical, always loopback is choosen for router ID (therfore DR/BDR).

On packet tracer it is according to value of IP not according to physical or loopback.

Also cisco curiculums refers to first option (loopback first, largness IP second)


Have you tested it on PT or on real hardware ?

PS: packet tracer did not recognize command

router# clear ip ospf process

therfore router reload

I've never used PT, I've tested this on real hardware and on dynamips.

So if you have loopback with smaller IP value than physical, always loopback is choosen for router ID (therfore DR/BDR).

Yes. And if you have several loopbacks, the one with the highest IP is choosen.

Also cisco curiculums refers to first option (loopback first, largness IP second)

According to me it is the same...

Now, in real networks, it is a very good and common practice to set the router ID manually. That way, it never change whatever happen to all the router interfaces.

So in conclusion:


TRUE ACCORDING PREVIOUS POSTS:
Priority is the first attribute which is held in election (0 did not elect, 1 default, higher better, maximum 255)

Second attribute (if priorities are equal) is router ID which can be affected by loopback or router id.

If neither loopback or router ID is set then RID is highest IP of physical interface.

If loopback is set (also with smaller IP than real interface) then RID is IP of loopback (if there are more loopback, the highest is choosen)

PT did not elect according loopback/physical but JUST according largeness of IP


AND WHAT IF ? FOR TOTALY CLEAR:
If there are RID set by Router(config-router)# router-id value (which can be smaller than physical highest IP) the RID is the router-id (Not physical IP)

Also If there are RID set by Router(config-router)# router-id value and there is also loopback (both can be smaller ip than physical) then the router-id is set



PS is there any online dynampis lab ? or something like that ? My PC is too slow to run GNS3 simulator.

Thank you

Although PT does not support "clear ip ospf process" command, I did create a loopback int and power cycle the router. After it came up, I checked, router ID was the loopback address was the loopback address.

However, again, PT does not support "router-id" command. But It does support loopback add.

Hi all, I am trying to understand OSPF and I have

As far as I know DR is the router with has highest IP address of loopback, if no loopback is set then the router which has highest (physical interface) IP address is the DR.

.

<i did not bother looking at the topology but>
the router with the highest OSPF priority becomes DR. <ranges from 1 thru 255>. Priority 0 routers never become DR.

The default priority for cisco routers is 1...so next comes router ID

the router with highest router ID becomes DR. The router picks/ prefers the highest configured loopback as its router ID. If no loopbacks r present than the router picks the highest physical interface as its router ID UNLESS 1 IS CONFIGURED with the router-id command.

<the router ID is NOT an ip address...just a 32 bit #>