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Conference kernel::cisco

Title:	CISCO
Notice:	<<< The Menlo Park Routers >>>
Moderator:	KERNEL::SYSTEM

Created:	Wed May 26 1993
Last Modified:	Thu Jun 05 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	515
Total number of notes:	1440

501.0. "OSPF Area 0 and loopback interface?" by WOTVAX::2h0533.olo.dec.com::Tim_Banks (tim.banks@olo.mts.dec.com) Fri Mar 07 1997 11:59

A question about OSPF backbone areas.

Areas in OSPF must communicate via a backbone area 0.  The Area is defined 
as an OSPF router statement, giving the network/subnet a mask and an area 
number.

Is there anything to stop you using the loopback address on the router as 
the Area 0?  If you can then the loopback address never goes down, so you 
never loose your area 0 and therefore your areas can always communicate.

This scenario would have a large router as the hub with remote routers 
spoked into it and in turn being a hub for more down stream routers, each 
spoke being a different OSPF area.  The hub router needs one area 0 
interface, so could it be the loopback interface?

Thanks

	Tim

T.R	Title	User	Personal Name	Date	Lines
501.1	yes, even desirable	MUNICH::SCHALLER	Eva Schaller DSC 895-6146	`Fri Mar 07 1997 16:00`	9
	Tim, if you define a loopback interface and put its ip address into the OSPF definitions, it also becomes the ROUTER ID, which is desireable, especially if you need virtual links. So short answer, YES, define the loopback IP address to be a network for your ospf process. regards eva
501.2	What about Area 0?	WOTVAX::BANKST	Network Mercenary	`Fri Mar 07 1997 19:19`	9
	Hi Eva Thaks for the answer, do you know if the loopback address can be in Area 0, and if it can will it allow different areas to communicate with each other, in the same way that a physical interface in Area 0 would? Thanks Tim
501.3	should work	MUNICH::SCHALLER	Eva Schaller DSC 895-6146	`Tue Mar 11 1997 17:48`	17
	it can be in area 0. I have not explicitly tested it, but I believe, that the following is allowed: int loop 0 ip address 1.1.1.1 255.0.0.0 int ether 0 ip address 2.1.1.1 255.0.0.0 int ether 1 ip address 3.1.1.1 255.0.0.0 router ospf 1 network 1.1.1.1 0.0.0.0 area 0 network 2.0.0.0 0.255.255.255 area 2 network 3.0.0.0 0.255.255.255 area 3 Sorry, I actually have no router to test with.
501.4	Yes, it can be area 0	NETRIX::"khobbie@colbj3.coo.dec.com"	Karsten Hobbie	`Thu Mar 13 1997 18:14`	22
	Hi Tim, Eva definitely the loopback interface can be in area 0 and should be on the hub router. We have tested this during our CCIE preparation lab session. If you have a pure hub and spoke environment, I would recommend at least to think about putting all connection lines to the edge routers as well into area 0 (and the loopback addresses of the edge routers). If you do in this manner, you are reducing the area databases on the hub router to 1. Every edge router will have 2 areas - acting as an area border router, also in a cascaded environment. I think that is a more clear design as you have not to maintain so many areas on the central router and you will be able to support route summarization, if you have a well structured IP address design. Regards, Karsten [Posted by WWW Notes gateway]