Subject: RE: [Isis-wg] confusion regarding IP summarization

From: Neal Castagnoli <[email-masked]>
Date: Wed, 27 Mar 2002 12:14:59 -0800

RFC 2966 has something to say about this too:

   Another detail that implementors should be aware of is the fact that
   L1L2 routers should only advertise in their L2 LSP those L1 routes
   that they use for forwarding themselves.  They should not
   unconditionally advertise into L2 all prefixes from LSPs in the L1
   database.

Is it OK to discuss on this list about whether or not the whole specified
automatic redistribution thing is good default behavior?

  --Neal

-----Original Message-----
From: Terry Canning [mailto:[email-masked]]
Sent: Wednesday, March 27, 2002 7:50 AM
To: stefano previdi; Rajesh Saluja; Aravind Ravikumar
Cc: [email-masked]
Subject: RE: [Isis-wg] confusion regarding IP summarization

Folks, 

RFC 1195 is quite clear on this matter. Section 3.2 states: 

   The manually configured addresses are included in level 2 LSPs only 
   if they correspond to at least one address which is reachable in the 
   area. For manually configured level 2 addresses, the associated 
   metric values to announce in level 2 LSPs are also manually 
   configured. The configured addresses will supercede reachable address 
   entries from level 1 LSPs based only on the IP address and subnet 
   mask -- metric values are not considered when determining if a given 
   configured address supercedes an address obtained from a level 1 LSP. 
 
   Any address obtained from a level 1 LSP which is not superceded by 
   the manually configured information is included in the level 2 LSPs. 

Regards 
Terry Canning 

-----Original Message----- 
From: stefano previdi [mailto:[email-masked]] 
Sent: 27 March 2002 15:21 
To: Saluja, Rajesh [BL60:432:EXCH]; Aravind Ravikumar 
Cc: [email-masked] 
Subject: Re: [Isis-wg] confusion regarding IP summarization 

On Wednesday 27 March 2002 15:57, Rajesh Saluja wrote: 
> Aravind Ravikumar wrote: 
> 
> > Hi, All 
> > 
> > I am not  clear about, what other information 
> > should be included in IP reachability 
> > field of level 2 LSP by a L1L2 IS, apart from IP addresses 
> > which are directly reachable via its own interfaces. 
> > 
> > Is it required to advertise ALL IP networks that are learned 
> > from other system's L1 LSP's IP reachability entries? 
> > 
> >  or 
> > 
> > is it enough if the IS  advetises(in its level2 LSP) ,only those 
> > networks 
> > which it considers reachable after the level1 SPF? 
> 
> I think you should advertise the configured summary addresses and then 
> go through the networks reachable after the level1 SPF and if the 
> network is not covered by the summary address, you should advertise that 
> network as well. 

I would suggest to advertise summary addresses only if you have 
more specific routes. 

s. 

> 
> Thanks, 
> Rajesh 
> 
> > 
> > 
> > 
> > Thanks in advance 
> > Aravind 
> > 
> > 
> > ----------------------------------------------------------------------- 
> > Do You Yahoo!? 
> > Yahoo! Movies - coverage of the 74th Academy Awards® 
> 
> -- 
> 
> ------------------------------------------------------------- 
> Rajesh Saluja 
> Nortel Networks Inc 600 Technology Park Billerica, MA  01821 
> Ph: (978) 288-7791      Fax: (978) 670-8760 
> -------------------------------------------------------------- 
> 
> 
> 
> _______________________________________________ 
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> 
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---

From: Tony Li <[email-masked]>
Date: Wed, 27 Mar 2002 12:58:50 -0800

| Is it OK to discuss on this list about whether or not the whole specified
 | automatic redistribution thing is good default behavior?

It's fine by me within reason.  Just understand that it's an off-topic
conversation about implementation issues and isn't a WG issue.

Tony

---

From: Jeff Learman <[email-masked]>
Date: Wed, 27 Mar 2002 16:13:55 -0500

Tony, based on an earlier post, it seems to me that 1195 requires
automatic L2 distribution of L1 routes that aren't covered by L2
configured summary routes.  You didn't contradict it, but I think
that the text in 2966 that Neal quoted did.

Sounds to me that there's more than an implementation issue here.

Jeff

---

From: Aravind Ravikumar <[email-masked]>
Date: Thu, 28 Mar 2002 01:27:02 -0800 (PST)

Hi, 

I also feel that its not an implementation issue.

I was trying to find what exactly certain section of RFC 1195 means

The  quoted section of RFC 1195 ( More specifically the following
sentance was the source of my confusion,
"Any address obtained from a level 1 LSP which is not superceded by 
the manually configured information is included in the level 2 LSPs.")

Some of these IP addresses which we get in L1 LSP
may not be added to our forwarding database as a result of
there being no path to the advertising router.
I think that it will be inconsistent if an we advertises about routes
that we ourselves are not using.The question was asked in this context 
The quoted section of rfc 2966 also seems to say the same thing,
So if we go strictly by the words the section of 
rfc 1195 and the  section of rfc 2966 sounds contradictory 

The section next to the quoted section of rfc 2966 says this
   "Not all prefixes need to be advertised up or down the hierarchy.
   Implementations might allow for additional manual filtering or
   summarization to further bring down the number of inter-area prefixes
   they advertise in their LSPs."
RFC 1195 seems to give no such choice and is strictly saying to 
advertise all address obtained from level 1 LSP.(Either advertise 
the Summary which supercedes it or if no such summary is
configured it advertise the netwok itself)
But the following section of rfc 1195 wchich talks about cost computation of advertised 
IP addresses seems to help 
"Any address obtained from a level 1 LSP which is not superceded by the 
manually configured information is included in the level 2 LSPs. In this 
case, the metric value announced in the level 2 LSPs is calculated from 
the sum of the metric value announced in the corresponding level 1 LSP,
plus the distance from the level 2 router to the appropriate level 1 router
. Note: If this sum resultsin a metric value greater than 63 (the maximum 
value that can be reported in level 2 LSPs), thenthe value 63 must be used." 
Does this section imply that if we don't have an entry to the advertising level1
router we should'nt advertise those IP address in level 2 LSP,since we cannot compute the cost

If this interpretation is corrrect
then there is no contradiction. Is it correct?

Aravind

  Jeff Learman <[email-masked]> wrote: 

Jeff

---------------------------------
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---

From: Jeff Learman <[email-masked]>
Date: Thu, 28 Mar 2002 08:24:11 -0500

While it may be safer to include only reachable L1 routes in one's
L2 LSPs, the existence of any unreachable L1 routes indicates that
the area is partitioned.  I don't think it was the intent of 1195
(and it clearly was not the intent of 10589) to deal with area partitions
by changing L2 address advertisements.

Offhand, I can't see what harm would be caused by omitting unreachable
addresses, other than a little more L2 SPF churn for a flapping L1 link
that partitions an area.

Regards,
Jeff

Disclaimer: I do not work on IS-IS at Cisco, and my opinions do not
necessarily reflect those of my employer.

---

From: Tony Li <[email-masked]>
Date: Thu, 28 Mar 2002 10:52:07 -0800

| Offhand, I can't see what harm would be caused by omitting unreachable
 | addresses, other than a little more L2 SPF churn for a flapping L1 link
 | that partitions an area.

If your L1 is partitioned and you have multiple L1L2s for the area, then
you want each L1L2 to only advertise its reachable addresses.

Tony

---

From: Tony Przygienda <[email-masked]>
Date: Fri, 29 Mar 2002 00:03:47 -0800

>you want each L1L2 to only advertise its reachable addresses.
so this is just one example of why e.g. you would _not_ aggregate even 
you have an aggregate configured.
I had a half-written ospf draft 1-2 years ago after discussions with 
John & Rob suggesting that if you
see advertisement from an ABR in your area that is not connected (kind 
of easy to figure out when you
ran intra-area computation), you should _deaggregate_ into area 0. Never 
seemed quite worth it since as
pointed out, aggregation is so implementation-dependent. And guess what, 
changes to ABR behavior
specified recently would influence all that stuff again.
Another problem is e.g. that you may be tempted and go ahead and deploy 
an implementation that per default leaks all
L1s into L2s as 1195 seems to suggest. Any customer with any serious 
multi-level backbone will very quickly
teach you to abandon that practice unless he wants to have hundreds of 
level-1 addresses sloshing around
the network and defeat purpose of level-2 to a good extent.
Another one to make the heads of the let's-standardize-aggregation crowd 
spin: If an aggregate subsumes
another aggregate, which and how many do you advertise? Funny enough, 
there are more than a single
possible correct answer depending on your deployment scenario.
And last juicy morcel: 1195 and OSPF want you to set aggregate metric to 
max. metric of aggregated
prefixes. Go check the largest deployed implementaion and you'll be in 
for a surprise. Yes, it could loop
if you really, really try but there is a method to this madness as well.
We can go on and on ...

We're kind of getting down the drain that I tried to plug so before I 
chime out of this discussion:
Aggregation is very implementation and deployment dependent and I would 
strongly object any push
to standardize it unless interoperability problems can be pointed out. 
Don't forget that this list has
an Occam's razor: if it's not needed for interoperability, don't 
standardize it and especially, don't
standardize for fun and to pass time!

So a draft describing possibilites
and problems with aggregation is fine but at a first MUST or SHOULD I'll 
probably get vocal ;-)

    thanks & out

    -- tony