Subject: Multiple Router LSAs in OSPFv3

From: Darshan Purohit <[email-masked]>
Date: Wed, 28 Aug 2002 03:37:31 -0400

Hi,

RFC 2740 says that in presence of multiple router lsas from a router,
"The Options field and the router type bits (bits W, V, E and B) should
always be taken from the fragment with the smallest link state id".

So if there is a virtual link configured and it comes up, the router will
always have to reoriginate a new instance of its router LSA with the least
Link state id, with the 'V' bit set, in order for the "TransitCapability"
of the area to be set to TRUE, even if it chooses to generate a separate
router LSA for the virtual link.

Is the above statement true.

Is there a particular reason why the LSA with smallest LSID is chosen and
not , say, the last generated router LSA or the logical OR of the fields of
all router LSAs.

Please correct me if there are any flaws in my observation.

Thanks.
Darshan

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From: Acee Lindem <[email-masked]>
Date: Wed, 28 Aug 2002 13:30:44 -0400

> Please correct me if there are any flaws in my observation.

Hi Darshan,

I think you are basically correct. However, the other methods (OR'ing the
options in all router LSA instances or using the last generated router
LSA instance's option) each have their own inefficiencies. IMHO, the simple
approach of putting the definitive router LSA options in the first or only
router LSA is a good engineering choice.

--
Acee

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From: Yasuhiro Ohara <[email-masked]>
Date: Thu, 29 Aug 2002 02:51:13 +0900

Following.

acee> Hi Darshan,
acee>
acee> I think you are basically correct. However, the other methods
acee> (OR'ing the options in all router LSA instances or using the
acee> last generated router LSA instance's option) each have their own
acee> inefficiencies. IMHO, the simple approach of putting the
acee> definitive router LSA options in the first or only
acee> router LSA is a good engineering choice.

Basically, when Router-LSA changes a OSPF router must re-calculate his
SPF tree from scratch. In this case, a router must find (and look
into) all route-lsa eventually, so I guess we can say there'll be no
extra tasks in either way.

If we can calculate SPF incrementally, OR'ing will be not good because
a calculating router must check all Router-LSAs of a changing router.
Last LSA (i.e. the LSA having the most high Link-State-ID) technique
will be not better in comparison with the first Router-LSA technique,
because we know the first is "0", but we don't know what is the last
(most high) Link-State-ID value.

regards,
yasu

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From: Acee Lindem <[email-masked]>
Date: Wed, 28 Aug 2002 14:16:25 -0400

> (most high) Link-State-ID value.

Yasu,

I was thinking more of the maintenance of the options from the
originating router's standpoint. One could come up
with a scheme of OR'ing the options and assuring a given
option is only set in one router LSA instance but I don't think it is
worth the complexity versus the benefit.

Thanks,
Acee

--
Acee

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From: Amit Srivastava <[email-masked]>
Date: Wed, 28 Aug 2002 23:08:19 -0700

Hi All,
    I was going through the Traffic Engineering
Extentions to OSPF
draft-katz-yeung-ospf-traffic-06.txt, in this draft
section 2.2 shows instance field.
                  I want to know what for this field
is used for. Actually my percepsion is that it is used
for if multiple instance of TE is running over OSPF.
But i am not sure about this. If anyone of you and
could help.
Regards
Amit

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From: Acee Lindem <[email-masked]>
Date: Thu, 29 Aug 2002 09:45:33 -0400

> for if multiple instance of TE is running over OSPF.

Nope. Since a single OSPF router will originate multiple
TE LSAs it is required for a unique LSA ID. OSPF routers
supporting the TE extensions are required to originate
one LSA for each OSPF interface considered in traffic
engineered paths (section 2.4.2).

--
Acee

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From: Amit Srivastava <[email-masked]>
Date: Thu, 29 Aug 2002 21:17:19 -0700

Hi Accee,
      Thanks for the help.
Regards
Amit