Re: LSIDs with vary length masks

[Acee Lindem <[e-mail]>]

Hi Dave,

Thanks for the detailed response. I've added my hastily typed
and grammatically incorrect subject for the archives.

See one response inline.


Dave Katz wrote:

> On Apr 14, 2005, at 4:31 PM, Zhao-Yang Dong wrote:
>
>> In an OSPF link state database, each LSA is supposed to have a unique 
>> Link State ID (LSID). Sometimes this is not true, especially in 
>> multiple vendor devices environment.
>
>
> The LSA ID is qualified by the originating system, so it is entirely 
> reasonable to have multiple LSAs with the same LSA ID but different 
> router IDs.
>
> What is *not* allowed is to have multiple LSAs with the same LSA ID 
> originated by the same system.  This cannot happen according to the 
> rules, and they will not be propagated if they were, also according to 
> the rules.  Whether there are multiple vendors has no impact on this.
>
>> When originating summary and/or AS-external LSAs, how to assign 
>> unique LSID for a network number who has multiple (different length 
>> of) masks is described in RFC 2328 appendix E. However, I did not see 
>> any discussion in RFC 2328 nor this archive how to handle/process 
>> summary and/or AS-external LSAs received from other routers with the 
>> same LSID but different length of masks.
>
>
> A router can do anything it wants to guarantee LSA ID uniqueness on 
> the LSAs it generates; appendix E is one way (though there are ways 
> that will go further before they fail than the one outlined there.)  
> Note that RFC2328 also refers to appendix E for Summary LSAs as well;  
> see appendix A.4.5.
>
> It is not the job of a receiving router to deal with this case, 
> because it cannot happen (according to the rules) and it is 
> indistinguishable from the case where somebody changes the netmask on 
> a redistributed static route.
>
>>
>>   
>>
>> According RFC 2328, a LSA is identified by LS type, LSID and 
>> advertising router. To determine which LSA of two LSA instances is 
>> newer, LS sequence number, checksum and age are compared. Network 
>> mask does not seem play any role while processing the received LSAs.
>
>
> It does not, nor should it.  An LSA is identified by originating 
> router ID, LSA type, and LSA ID.  Period.  Any router that attempts to 
> generate two different LSAs with the same ID is broken.
>
>>
>>   
>>
>> My question is, for example, if I received a LSA with LSID=A.B.C.D 
>> and 24 bits mask and installed in my database, and later I received 
>> the same LSA (i.e. same LS type, LSID and advertising router) but 
>> with 28 bits mask. If the second LSA has the larger sequence number, 
>> should I replace my database copy with the second LSA?
>
>
> That's what the rules say.  This is why a broken system generating 
> multiple LSAs with the same ID will not get far;  only one of them 
> (with the higher sequence number) will get acknowledged, and the other 
> will be retransmitted ad infinitum.
>
>
> The problem is that the Founding Fathers that spec'ed OSPF screwed up 
> and overloaded the LSA ID with routing information because they were 
> afraid of spending another four bytes per LSA.  One could blame it on 
> classful networking, but in a purely classful environment you wouldn't 
> even need masks.  Basically it was yet another mistaken optimization.
>
> It's impossible to guarantee uniqueness;  if you try hard enough you 
> can generate a situation that no algorithm can work around (because 
> the LSA ID space is by definition sparse due to the masking 
> requirement, but the space of possible external routes is dense.)  As 
> a practical matter, however, it's very unlikely that there will be an 
> LSA ID collision if a reasonably good algorithm is chosen.
>
>
> BTW, the cisco IOS implementation appears to attempt to detect this 
> case, though it's not clear why.  I assume it complains if it sees the 
> same LSA ID and a different netmask, but that's a perfectly legal 
> situation.  If anybody from cisco can explain this, I'm quite curious.

A warning message will only be generated when the originating router 
encounters
an LSA advertisement conflict that cannot be resolved using the 
algorithm in
RFC 2328 Appendix E. This is to let the network administrator know that the
conflict exists and not all prefixes are advertised.

Thanks,
Acee

>
> --Dave
>
>
>>
>>  
>>
>> Thanks,
>>
>> Zhaoyang
>>
>>  
>
>