OSPF

[The_guroo_2]

Hi!!

Just wanna clear my self one one thing.....i know in LAN if we want to make a perticular router DR we can configure a loopback address......if this is so then or what perpose we use priority.......which is 1-255.......cheers second thing is when we configure loopback address.....and that router is then configured on WAN.....why the loopback ip address is coming in gateway when i use show ip protocol??? thanks in advance

[devang_etcom]

hi...

first of all it will look for the priority, higher the priority router will become DR and BDR... if priority of all ther router are same then it will elect the DR and BDR on the basies of the numarically higer IP address...

and you second point i am not able to understand what you are looking for exactly... so make it more clear...

rate this post if it helps

regards

Devang

[mheusinger]

Hi,

one thing to mention is: DR/BDR election is non preemptive.

This means on a LAN, when DR and BDR are already elected, a new router will not become DR or BDR no matter what settings it has. This is standard conforming and done to achieve stability.

Second, an OSPF router will have an OSPF router ID in the form of an IP address. This ID will be set to the Loopback ip, if one is configured. You can modify this through

router ospf 10

router-id 255.255.255.255 ! or any other UNIQUE 32 Bit value

The example above is a valid OSPF router ID, though not a valid host IP.

The command "show ip protocols" lists OSPF routers by router ID. It has nothing to do with WAN, LAN or network topology nor IP routing decisions.

Hope this helps! Please rate all posts.

Regards, Martin

[klthiyagu]

Hi,

First, I believe Loopback interface has nothing to do with DR/BDR enlection, bcoz DR/BDR election is the property of the interface that connects to a BMA network. Loopback interface are always treated as Stub network in OSPF and no neighborship will be formed in that subnet. In a BMA (Ethernet or FastEthernet network) router interface with highest IP address will become DR and if you want to overcome this you can use "Priority" to make a particular router in that BMA as DR.

Second, In the output of "show ip protocols" gateway will list you the Routing Information Sources i.e the routers that are there in that particular area running OSPF. When you create a Loopback interface and configure OSPF on that router then the Loopback IP will be the RouterID and you can see that in all the router of that area when you give "sho ip protocols"

Hope this clears your doubt.

[mheusinger]

Hi,

from the OSPF Design Guide

http://www.cisco.com/warp/public/104/2.html#10.1

"DR and BDR election is done via the Hello protocol. Hello packets are exchanged via IP multicast packets (Appendix B) on each segment. The router with the highest OSPF priority on a segment will become the DR for that segment. The same process is repeated for the BDR. In case of a tie, the router with the highest RID will win. "

So the router ID (usually Loopback IP) is part of the DR/BDR election process in case the priority is the same (f.e. with default priority settings).

Regards, Martin

[mchoo2005]

Loopback interfaces are only used as Router ID. Priority is what OSPF uses to elect DR/BDR, which will need to be configured on the interface connected to the broadcast network (e.g. Ethernet) where OSPF needs to elect DR/BDR.

show ip protocols use Router IDs to display list of routers. Since you have configured a Loopback address on the router, it'll be used as the Router ID and displayed on the show ip protocols command across the WAN.