[no subject]

[Dave Katz <dkatz at JUNIPER.NET>]

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.

--Dave


>  
>
> Thanks,
>
> Zhaoyang
>
>