Re: RFC 2370 Update and a Proposed Change to Stub Area Behavior

Acee Lindem <acee@CISCO.COM> Fri, 12 August 2005 15:42 UTC

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Date: Fri, 12 Aug 2005 11:42:05 -0400
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From: Acee Lindem <acee@CISCO.COM>
Subject: Re: RFC 2370 Update and a Proposed Change to Stub Area Behavior
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Hi Mitchell,
Thanks for reponding - I was hoping someone would initiate some 
discussion (the
ADs always ask whether a change was discussed on the mailing list :^).
Anyway, what I'm proposing is no worse that OSPFv2. Consider the 
following points:

- The unknown LSA types in question are link or area scoped. Hence,
this implies that at least one router in the stub or NSSA area 
understands them (at
least one would hope an implementation would not originate an LSA it didn't
understand :^).
- The OSPFv2 analogy is a link or area scoped opaque LSA with unknown type.
There is no such restriction for these LSAs in OSPFv2 stub or NSSA areas.
- The mechanism is topology dependent. As long as there is a spanning 
tree of
OSPFv3 routers in the stub or NSSA understanding the LSA type, it will be
flooded to all routers in the area. If there is a real requirement to 
limit the
flooding domain, customers and vendors will implement filters which will
deterministically limit flooding.
- Experience has shown that the introducion of new OSPFv3 LSA types is
extremely slow. Hence, I don't see the need to limit the flooding to limit
database size (if there a need, see the previous point).


Erblichs wrote:

>	I vote NOT to remove the restriction on the
>flooding of unknown LSAs into stub area. I vote for
>#2 or #3. Sorry, I have not spent any major time looking
>at the pros / cons between the later 2.
>    1) The primary reason is that some of the these LSAs
>       are unknown to a percentage of the routers within
>       the stub area. Even the "attempt" to limit them
>       would follow the reason to limit the database size,
>       memory requirements, sizing of OSPF control packets,
>       etc... This limit is suggested in the 1st paragraph
>       of every OSPF v2 RFC. A copy is in the middle of this
>	email.
>     2) What is an unknown LSA? What LSA type greater than
>	X is an unknown? What help is it by having just 1
>	router understand it? Can they equal in number
>	over time External-LSAs and be totally useless in
>	our env? Where should we put the older routers that
>	we want to isolate from our network?
>     3) Is is possible to have 30% - 90% of LSAs in a router's
>	db be present in a stub area, be unknown LSAs? Shouldn't
>	their be an attempt to limit this percentage?
>	3b) Could we be using / spending a large percentage
>	    of our OSPF control packet time / resources
>	    handling unknown LSAs?
>     4) Backward compatibility.. I would assume that most
>	environments would not like to just start seeing
>	something new in their network just show up.
>	"An area can be configured as a stub when there is a single exit
>        point from the area, or when the choice of exit point need not
>        be made on a per-external-destination basis."
>	Lets look at the third word, can. It would be different
>	if we used the word SHOULD or MUST.
>	Thus, if a area that CAN be configured as a stub wishes
>	to process unknown LSAs, then why not configure the
>	without the STUB area identification? Wouldn't this allow
>	for backward capability? Yes, we then allow AS-external-LSAs
>	in this non named stubby area.
>	Or create a new "stubby area" type that accepts or
>	not accept, xyz type LSAs. This new area type would then be
>	allowed to accept new LSAs as they show up? The diff
>	would be that "unknown LSAs" have no restrictions and
>	could consume the majority of the router's LSDB.
>	Mitchell Erblich
>	-----------------------
>RFC 1247, 1583, 2178, 2328 : OSPFv2.
>3.6 Supporting stub areas
>In some Autonomous Systems, the majority of the topological database may
>consist of external advertisements.  An OSPF external advertisement is
>usually flooded throughout the entire AS.  However, OSPF allows certain
>areas to be configured as "stub areas".  External advertisements are not
>flooded into/throughout stub areas; routing to AS external destinations
>in these areas is based on a (per-area) default only.  This reduces the
>topological database size, and therefore the memory requirements, for a
>stub area's internal routers.
>Acee Lindem wrote:
>>At the 63rd IETF in Paris, I proposed that we remove the restriction on the
>>flooding of unknown LSAs into stub areas. Here is an excerpt from the
>>- Section 2.9 mandates that an OSPFv3 router should NOT advertise an
>>unknown LSA if the U bit is set to “1” – flood as if known.
>>->Should be removed in RFC 2740 respin.
>>->Limits backward compatibility for new LSA types
>>->No corresponding rule for opaque LSAs
>>->Fact that LSA is flooded at all implies one router is stub/NSSA
>>understands it.
>>->Ineffective/non-deterministic database limit
>>->As long as there is an intra-area spanning tree of routers that
>>understand the LSA type - The LSA will be in everyones database
>>Comments? Speak now if you wish to retain the current stub area restriction.
>>My intent is to deprecate it with an appendix documenting it's removal.
>>Acee Lindem wrote:
>>>In the evolution of the OSPFv2 protocol specification
>>>(RFC 1247->RFC 1583 -> RFC 2178 -> RFC 2328) numerous
>>>bugs were fixed and some protocol behaviors were altered. Examples
>>>include the metric cost for area ranges and the selection of the
>>>ASBR for AS external route computation.
>>>In the context of documenting the OSPFv3 NSSA differences I've
>>>looked again at section 2.10 and I really think the idea of not flooding
>>>unknown LSA types with the U-bit set to 1 is broken. I think it breaks
>>>the whole idea of being able to introduce new LSA types in a backward
>>>compatible fashion. Furthermore, it won't stop the leakage of these
>>>unknown LSAs when some routers understand them and others do not -
>>>it all depends on whether you have a spanning tree of routers that
>>>understand them. Since the LSAs in question are area scoped or link
>>>scoped, it implies that at least one router (the originator)
>>>understands the
>>>new type and you will have a mixture. IMHO, this is broken. I've had
>>>some discussions with others who agree. At this juncture,
>>>we have 3 alternatives:
>>>1) Remove the restriction for that unknown LSAs with the U-bit
>>>set to 0 for stub areas.
>>>2) Extend the broken restriction to NSSAs in the update.
>>>3) Limit the damage to stub areas and only restrict AS scoped LSAs
>>>from NSSAs.
>>>Of course, I'd vote for #1 or I wouldn't be sending this E-mail.