It actually has little to do with physical topology (noted exception of NSSA). They
real differentiator is which LSA's flow within the area, and ultimately, what level of
knowledge does the area have about the outside world. The stub concept involves the
trade off of sub optimal routing for less LSA flooding. In a large OSPF domain, a
huge number of Type 5 LSA's can be created which will flood area wide. Sometimes,
this can overwhelm the processing of a smaller area and hence the stub concept makes
sense. The trade off is, without the first hand knowledge via LSA type 4's about
where each ASBR lives, these routers in stub areas may not always take the fastest
path toward a external prefix.
Pete
*********** REPLY SEPARATOR ***********
On 12/11/2000 at 1:08 AM Chuck Larrieu wrote:
>Can someone perhaps help educate me here. I have now seen this on a couple
>of posts, did a bit of reading, and have learned that stub and totally
>stubby areas are not necessarily limited to a single ingress/egress point.
>I.e. can have more than one ABR.
>
>Area0-------Area1 area 1 is a stub area ( or totally stubby, depending
>upon the LSA, summarization, and default route handling )
>
>Area0------Area1_ABR1
> |------Area1_ABR2 still a stub area / totally stubby area
>
>What makes and area NOT a stub area? Is it only the summarization / LSA /
>default route handling?
>
>Yes I understand the NSSA, with its ABSR connecting to an outside AS and
>that information being passed into an OSPF domain in a particular manner.
>
>I guess the question is this: other than the NSSA, is EVERY OSPF area either
>stub or totally stubby?
>
>Area0---------Area1-------|
> | ABR?
> |---------Area2--------|
>
>would the above layout make and area NOT stubby or totally stubby?
>
>Chuck
>
>-----Original Message-----
>From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED]] On Behalf Of
>Peter A van Oene
>Sent: Saturday, November 11, 2000 8:01 PM
>To: [EMAIL PROTECTED]
>Subject: Re: OSPF Totally Stubby Areas and area default-cost
>
> Totally stubby is a cisco nob that takes the concept of a stub area a step
>further. In a stub area, only LSA types 1 (router) 2 (network) and 3
> summary) flow within the area. Hence, no routing information concerning
>prefixes outside of the OSPF domain is injected into the area. In a totally
>stubby area, the flow of normal type 3 LSA's is halted as well. This leaves
>the area with no information about any prefixes outside of the area. In
>order to allow traffic to exit the area, a single type 3 LSA is propagated
>by each ABR which advertises a default route. The default cost nob simply
>allows you to set a cost for the route instead of using the standard OSPF
>metric to the ABR itself.
>
>Hope this helps some
>
>Pete
>
>
>*********** REPLY SEPARATOR ***********
>
>On 11/08/2000 at 4:29 PM Bob Hunter wrote:
>
>>Hi,
>> I'm confused on the subject of totally stubby areas, and the command "area
>>default-cost". From what I'm reading, one of the qualifications of a
>totally
>>stubby area is that if multiple exits (ABRs) exist, routing to outside the
>>area does not have to take an optimal path. Does this mean that each router
>>within the area picks the closest ABR as the gateway to everything outside
>>the area, and that there is no way to control the default route? If so,
>does
>>that imply that the area default-cost is used for incoming routes? Would
>>incoming routes even exits if the area was a totally stubby area?
>>
>> I would very much appreciate it if someone would please set me straight.
>>
>> Thank you.
>>
>>Bob Hunter, CCNA, CNE
>>
>>
>>
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>
>
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