Subject: Question on originating intra-area prefix LSA

From: Mike Fox <[email-masked]>
Date: Thu, 17 Jul 2003 10:54:11 -0400

When a prefix is added to or deleted from a p2p or p2mp link, when is the
prefix change supposed to be advertised to the rest of the area in an
intra-area prefix LSA?  There seems to be a hole in the RFC.

 RFC 2740 says, in 3.4.3:

 o    A new prefix is added to an attached link, or a prefix is deleted
      (both through configuration). This causes the router to
      reoriginate its link-LSA for the link, or, if it is the only
      router attached to the link, causes the router to reoriginate an
      intra-area-prefix-LSA.

Some observations and questions.

Is it really link LSA *or* intra-area prefix LSA?  I.e., never both?

This paragraph also  implies that on p2p and p2mp links with active
neighbors, no intra-area prefix LSA is sent when prefixes change, since the
intra-area prefix LSA is only reoriginated when this is the ONLY router on
the link.   The only other way I can see the prefix being advertised  for
p2p and p2mp is that perhaps the neighbor should advertise the new prefix
in an intra-area prefix LSA when it receives the link LSA.   But the next
paragraph says:

o     A new link-LSA is received, causing the link's collection of
      prefixes to change. If the router is Designated Router for the
      link, it originates a new intra-area-prefix-LSA.

Which implies only the DR for a multiaccess link should ever send a new
intra-area prefix LSA upon receiving a link LSA, nothing about point to
point or point to multipoint is mentioned.  Also, if the set of prefixes is
changed on the designated router, according to strict reading of this
paragraph then the network's intra-area prefix LSA would not get
reoriginated since it's on receiving (not sending) a link LSA that the DR
sends the new set.

To me it boils down the phrase "if it is the only router attached to the
link" in the first quoted paragraph above.  If that phrase were replaced
with "in all cases except when  the link is to a broadcast or multiaccess
network and this router is not the designated router" then it seems like
all the cases would be covered.

What am I missing here?  Am I just reading the text too strictly?

Mike
-----------------------------------------------------------------------
Enterprise Network Solutions
-----------------------------------------------------------------------
Research Triangle Park, NC  USA

---

From: Sina Mirtorabi <[email-masked]>
Date: Thu, 17 Jul 2003 16:34:05 -0700

Mike

The text might be a bit confusing when you try to do a strict reading
;-)
Here is what you would do in general

1) whenever there is a change to a link local prefix or global prefix
you would re-originate a new link-LSA
2) whenever there is a change to a global prefix you would originate a
new intra-area-prefix-lsa if either of the
   the following is true

   2-a) the link type ( I should say interface type connected to the
link) is p2p /p2mp
   2-b) link type is multi-access and there is no neighbor in state FULL
   2-c) link type is multi-access and this router is DR for that link
   2-d) independently of the link type the global prefix has a prefix
options with LA or NU bit set

In other words you would generate a new link-LSA for any prefix change
on a link, and you would originate a new intra-area-prefix-lsa for any
change to the global prefix only when you are supposed to announce this
link's prefix in an intra-area-prefix-lsa ( 2-a or 2-b or 2-c or 2-d)

The above means that on a p2p / p2mp link when there is a change to a
prefix you would both re-originate a new link-lsa and a new
intra-area-prefix-lsa however on a multi-access link ( with at least one
FULL neighbor) a non-DR router would only re-originate its link-lsa.
Upon reception of this new link-lsa the DR will update its
intra-area-prefix-lsa when necessary.

Now there could be an optimization when there is no neighbor on a link
and you could suppress your link-lsa since there is no neighbor on that
link.

Thanks
Sina

->-----Original Message-----
->From: Mailing List [mailto:[email-masked]] On
->Behalf Of Mike Fox
->Sent: Thursday, July 17, 2003 7:54 AM
->To: [email-masked]
->Subject: Question on originating intra-area prefix LSA
->
->
->When a prefix is added to or deleted from a p2p or p2mp link,
->when is the prefix change supposed to be advertised to the
->rest of the area in an intra-area prefix LSA?  There seems to
->be a hole in the RFC.
->
-> RFC 2740 says, in 3.4.3:
->
-> o    A new prefix is added to an attached link, or a prefix
->is deleted
->      (both through configuration). This causes the router to
->      reoriginate its link-LSA for the link, or, if it is the only
->      router attached to the link, causes the router to reoriginate an
->      intra-area-prefix-LSA.
->
->Some observations and questions.
->
->Is it really link LSA *or* intra-area prefix LSA?  I.e., never both?
->
->This paragraph also  implies that on p2p and p2mp links with
->active neighbors, no intra-area prefix LSA is sent when
->prefixes change, since the intra-area prefix LSA is only
->reoriginated when this is the ONLY router on
->the link.   The only other way I can see the prefix being
->advertised  for
->p2p and p2mp is that perhaps the neighbor should advertise
->the new prefix
->in an intra-area prefix LSA when it receives the link LSA.
->But the next
->paragraph says:
->
->o     A new link-LSA is received, causing the link's collection of
->      prefixes to change. If the router is Designated Router for the
->      link, it originates a new intra-area-prefix-LSA.
->
->Which implies only the DR for a multiaccess link should ever
->send a new intra-area prefix LSA upon receiving a link LSA,
->nothing about point to point or point to multipoint is
->mentioned.  Also, if the set of prefixes is changed on the
->designated router, according to strict reading of this
->paragraph then the network's intra-area prefix LSA would not
->get reoriginated since it's on receiving (not sending) a link
->LSA that the DR sends the new set.
->
->To me it boils down the phrase "if it is the only router
->attached to the link" in the first quoted paragraph above.
->If that phrase were replaced with "in all cases except when
->the link is to a broadcast or multiaccess network and this
->router is not the designated router" then it seems like all
->the cases would be covered.
->
->What am I missing here?  Am I just reading the text too strictly?
->
->Mike
->--------------------------------------------------------------
->---------
->Enterprise Network Solutions
->--------------------------------------------------------------
->---------
->Research Triangle Park, NC  USA
->