Subject: RE: Few questions related to LDP

From: "Vijayanand C" <[email-masked]>
Date: Fri, 8 Jun 2001 14:55:14 +0530

Hello,

That apart one more issue.

	|domain|----R1--- R2----R3---

If you are talking about the situation where  R1 has only p.q.r.0/24 and R2
has only p.q.0.0/16, then you are not leaking the route p.q.r.0/24 outside
the subnet. Actually in this case p.q.r.0/24 is the address of the
subnetwork which lies to the left of R1. p.q.0.0/16 is the subnet which is
encompassing many such subnets say p.q.r.0/24, p.q.s.0/24 etc.

So at R1 a packet arriving for p.q.r.0/24 ahould be forwarded to the left,
ie, the next hop for this FEC is inside the domain not outside !!

So, R1 will never request a label for p.q.r.0/24 to R2, simply because it is
not the next hop!

One more thing in this case R1 should be having p.q.0.0/16 since it is a
border router for p.q.r.0/24.

YOu have to state clearly your topology/subnert/AS configuration

Regards,
Vijay

-----Original Message-----
From: Vijayanand C [mailto:[email-masked]]
Sent: Friday, June 08, 2001 2:55 PM
To: [email-masked]
Subject: RE: Few questions related to LDP

Hello,

That apart one more issue.

	|domain|----R1--- R2----R3---

If you are talking about the situation where  R1 has only p.q.r.0/24 and R2
has only p.q.0.0/16, then you are not leaking the route p.q.r.0/24 outside
the subnet. Actually in this case p.q.r.0/24 is the address of the
subnetwork which lies to the left of R1. p.q.0.0/16 is the subnet which is
encompassing many such subnets say p.q.r.0/24, p.q.s.0/24 etc.

So at R1 a packet arriving for p.q.r.0/24 ahould be forwarded to the left,
ie, the next hop for this FEC is inside the domain not outside !!

So, R1 will never request a label for p.q.r.0/24 to R2, simply because it is
not the next hop!

One more thing in this case R1 should be having p.q.0.0/16 since it is a
border router for p.q.r.0/24.

YOu have to state clearly your topology/subnert/AS configuration

Regards,
Vijay

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]
Sent: Friday, June 08, 2001 2:18 PM
To: Vijayanand C
Cc: [email-masked]
Subject: RE: Few questions related to LDP

hi!
     What you are saying is correct tat it is highly improbable but it is
not
impossible. The hint for such a topology lies in a scenario where adjoining
routers have MPLS enabled over one of the interfaces connesting them but
nowhere
is it essential that they also have an IGP(say OSPF) also enabled on that
interface. I think it is not mandated anywhere in the specs that an MPLS
enabled
interface should also have IGP enabled on the same interface. Am i wrong?

regards
sumit

"Vijayanand C" <[email-masked]> on 06/08/2001 02:25:43 PM

To:   Sumit Chauhan/HSS@HSS
cc:   [email-masked]

Subject:  RE: Few questions related to LDP

Hello,
I want to add something regarding this.

The routes you are talking are nothing but the routes maintained by the
routing protocol. Now, within a domain, should'nt the routers have their
routes in sync. Route updates are meant precisely for that purpose. I don't
understand how 2 neighbouring routers have 2 routes of the form p.q.r.0/24,
p.q.0.0/16. This will happen only when the  former is the gateway to the
subnet p.q.r.0/24 and the subnet route is not leaked.  In that case , it
automatically becomes the ingress for p.q.r.0/24. Hence, as Eric says it can
issue an explicit NULL and act as ingress for the FEC p.q.r.0/24. It goes
one level down the heirarchy. p.0.0.0/8 is one level up the heirarchy, it is
outside p.q.0.0/16

Regards,
Vijay

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of
[email-masked]
Sent: Friday, June 08, 2001 10:22 AM
To: Abhijit Gadgil
Cc: Eric Gray; Nagendra Singh Tomar; [email-masked]
Subject: Re: Few questions related to LDP

hi abhijit!
     You are correct in your analysis of the problem (the one with R4
involved)
but there is still a question that needs to be answered. for R2 to be able
ro
also accept unlabeled packets from R4 and put them on the LSP R1-R2-R3 there
has
to be a FTN entry created at R2 for say p.q.r.0/24 (which is the LSP
R1-R2-R3)
where as the trigger for creation of an FTN (DoD or DU) would only be for
p.q.0.0/16 as that is the routing update received from R3 which is the next
hop
for p.q.0.0/16.Thus we are talking of R2 to be able to be smart enough to do
traffic merging for two seperate FEC. This can be done but I guess then the
intelligence will have to be explicit in terms of -  to what level we want
to do
it (consider another p.0.0.0/8 entry in r2 routing table from a next hop R5
!).
You are absolutely right that this is a local issue and does not need to be
LSP
wide as Eric pointed out that someone having a route to p.q.0.0/16 will
surely
have a route to p.q.r.0/24 (even if it might not be the best one ;o)

regards
sumit

Abhijit Gadgil <[email-masked]> on 06/08/2001 05:28:08 AM

To:   Eric Gray <[email-masked]>
cc:   Nagendra Singh Tomar <[email-masked]>, [email-masked] (bcc:
Sumit
      Chauhan/HSS)

Subject:  Re: Few questions related to LDP

Eric,

If R2 is distributing bindings unsolicitedly, it will always distribute
bindings for p.q.0.0/16 and won't distribute for p.q.r.0/24 as it does not
have entry for that FEC in its routing table.

And R2 can send Mapping for label request p.q.r.0/24 when it receives
explicitely such request and in that case it MUST send a No Route
Notification.(according to rfc3036 section 3.5.8).. I accept it is bit
harsh on R2's part in doing so, but this is what the RFC says....

The original question was Whether R2 can put packets from some other guy
R4(pls see below), onto the LSP R1-R2-R3. To which someone has pointed out
that no it can't do so. IMO R2 can do so.. whether it does or not is
implementation choice and standards don't prevent that.

 >   --- R1 --- R2 --- R3 ---
                 |
                 R4

btw there  is a syntax error in ur mail in follwing para. "No matching
brackets". :-)

regards,

-abhijit

 >If R1 requests a label from R2 for the FEC p.q.r.0/24, R2 could return the
 >IPv4 explicit NULL label (indicating that it will egress packets destined
 >for addresses matching this FEC.  In any case, R1's requesting a label
 >R2 was not intended to cause R2 to ingress packets for this FEC (so an
 >FTN would not be created for this FEC, but for different reasons than are
 >indicated in Abhijit's reply).  R2's giving R1 an implicit NULL label for
 >FEC p.q.r.0/24 allows R2 to examine unlabled packets for this FEC and
 >- possibly - assign those packets to an LSP corresponding to the FEC
 >p.q.0.0/16.  For this LSP, R2 could have an FTN and ingress packets.
 >
 >--
 >Eric Gray

---

From: Nagendra Singh Tomar <[email-masked]>
Date: Fri, 8 Jun 2001 15:04:06 +0530 (IST)

Hi Vijaya!
	 Following is a more appropriate diagram of the network we were
discussing

           ----R1--- R2----R3---|domain|

Don't ask me how R1 has got a more precise route p.q.r.0/24  than R2
p.q.0.0/16, when apparantly R2 is the nexthop for R1 for the FEC
p.q.r.0/24. The case we are considering might be a transient case.

regards
tomar

-- 

------------------------------------------------------------------------------
 Nagendra Singh Tomar
 
 Residence:
 369 KHB Colony
 5th Block Koramangala
 Bangalore 95
 INDIA
 Phone : 5524839
 email : [email-masked]

"Sometimes quantity becomes quality"                                          
                            --Gary Kasparov,                                  
 When he played against Deep Blue. He said this after looking at a           
 computer which found a forced mate in 249 in an endgame, after calculating   
 all the possible moves with only 5 pieces left on the board.                 
------------------------------------------------------------------------------

---

From: "Vijayanand C" <[email-masked]>
Date: Fri, 8 Jun 2001 15:13:08 +0530

Hello Nagrendra,

Please tell me the situation where R1 has p.q.r.0/24 and R2 has p.q.0.0/16.

What is the domain that you have showed to the right of R3. Is it p.q.r.0/24
? Then R3 and R2 will also have p.q.r.0/24  unless you have a more complex
topology.We need to know what is that topology for R1 which is 2 hops away
from domain p.q.r.0/24 has the more precise route while R2 which is closer
has a less precise route.

Regards,
Vijay

-----Original Message-----
From: Nagendra Singh Tomar [mailto:[email-masked]]
Sent: Friday, June 08, 2001 3:04 PM
To: Vijayanand C
Cc: [email-masked]
Subject: RE: Few questions related to LDP

Hi Vijaya!

regards
tomar

--

----------------------------------------------------------------------------
--
 Nagendra Singh Tomar

 Residence:
 369 KHB Colony
 5th Block Koramangala
 Bangalore 95
 INDIA
 Phone : 5524839

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

---

From: Nagendra Singh Tomar <[email-masked]>
Date: Fri, 8 Jun 2001 15:30:52 +0530 (IST)

Hi Vijaya!
	 One possible transient scenario might be as follows

                           L1 
          R1-----R2-----R3-----|p.q.r.0/24|
                        |
                        |L2
                        |
                   |p.q.s.0/24|

Assuming the above network has reached a quiescent state, so routing
table at R2 will have 2 entries each for p.q.r.0/24 and p.q.s.0/24,
suppose it also has p.q.0.0/16, then if the link L1 goes down and R3
senses that it can no longer reach p.q.r.0/24 thru L1, it will send a
route update to that effect. During the time this update leaves R3 and 
reaches R2, we will have the routing tables as discussed in the previous
few mails.
Am I clear ???

regards
tomar

-- 

 Residence:
 369 KHB Colony
 5th Block Koramangala
 Bangalore 95
 INDIA
 Phone : 5524839

---

From: "Vijayanand C" <[email-masked]>
Date: Fri, 8 Jun 2001 15:54:28 +0530

Nagendra,
You are now talking about a purely transient condition which I think was not
your earlier intention.
Anyway, R1 will also come to know of the failure in the next update. You are
talking about a situation where R1 places a request while the update is on
its way to R1!

I think in such a situation R2 can send a NO_ROUTE notification, but by the
time the no route notification reached R1 the bad news of L1 failure would
have got to R1 rending the Label request obslete!! Even if it took any other
action like sending NULL labels it would anyway be meaningless in a
transient situation.

Regards,
Vijay

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Nagendra
Singh Tomar
Sent: Friday, June 08, 2001 3:31 PM
To: Vijayanand C
Cc: [email-masked]; [email-masked]
Subject: RE: Few questions related to LDP

Hi Vijaya!

                           L1

regards
tomar

On Fri, 8 Jun 2001, Vijayanand C wrote:

> > So, R1 will never request a label for p.q.r.0/24 to R2, simply because
it
> is
> > So, R1 will never request a label for p.q.r.0/24 to R2, simply because
it
> is
> > one level down the heirarchy. p.0.0.0/8 is one level up the heirarchy,
it
> is
> > for p.q.0.0/16.Thus we are talking of R2 to be able to be smart enough
to
> do

--

 Residence:
 369 KHB Colony
 5th Block Koramangala
 Bangalore 95
 INDIA
 Phone : 5524839

---

From: Abhijit Gadgil <[email-masked]>
Date: Fri, 8 Jun 2001 16:10:46 +0530 (IST)

>reaches R2, we will have the routing tables as discussed in the previous

Not really... atleast to me :-)

The one reason why entries described so will be there is as follows as I
can see. R2 does "aggregation" (or gets "aggregated" information) and
sends p.q.0.0/16, to R1. But there is a smart administrator who's sitting
at R1 and adds an explicit route to p.q.r.0/24 with R2 as a gateway
(ofcourse for reasons known only to her).  and then R1 requests for
p.q.r.0/24.

In that case one of the following can happen

1. R2 recognizes p.q.r.0/24 and so does R3.. Nice and LSP will be created.
There _can_ also be an _FTN_ entry in R2.

2. R2 recognizes only p.q.0.0/16 but is somewhat _smart_ (read as
generous)  and decides on his own to anyway let R1 create an LSP through
it. It will create so in a manner described in previous mail (ie the two
LSPs)

Is anything still missing?

-abhijit

---

From: "Vijayanand C" <[email-masked]>
Date: Fri, 8 Jun 2001 19:22:14 +0530

Abhijit,

What you are saying according to option 2 is - even if an LSR does'nt have
an exact match for an FEC , if it has a matching address prefix, it should
enable LSP set up. What will be the FEC element that R2/next hop of static
route propagate to its next hop. The matching prefix FEC element or the
original static route FEC element itself? This new semantic(if you use
static route FEC element which is not present in any of the FEC tables of
the LSRs) has to be understood by all the routers upto the LSP termination.
If you use the matching address prefix, then the intention of creating a
static route (which may be to introduce a new FEC for the static route) may
be lost.

If a lot of operators start introducing static routes like this  you also
run into scalability problems, as the purpose of address heirarchy may be
lost and the number of entries in the FEC table (not route table as you
don't want to advertise these routes) may grow.

Correct me if I am wrong

Regards,
Vijay

-----Original Message-----
From: [email-masked] [mailto:[email-masked]]On Behalf Of Abhijit
Gadgil
Sent: Friday, June 08, 2001 4:11 PM
To: Nagendra Singh Tomar
Cc: Vijayanand C; [email-masked]
Subject: RE: Few questions related to LDP

-abhijit

---

From: Abhijit Gadgil <[email-masked]>
Date: Fri, 8 Jun 2001 20:18:35 +0530 (IST)

Vijay,

 >static route (which may be to introduce a new FEC for the static route) may

I think R2 will terminate the LSP, and initiate a new LSP further down the
line acting as ingress for p.q.0.0/16 here. (Note that this is not quite
advisable).

 >don't want to advertise these routes) may grow.

I am certainly not "for" adding static routes like that. Infact they can
cause problems as has been discussed so far. I was just pointing to the
case when entries like that will be present in R1 and R2s routing table,
apart from the transients. I agree that was quite a contrived case. But in
case if such scenario occurs, the best will be for R2 to give a NO_ROUTE
and if required handle those packets in IP, than putting them on LSP.

Is that fine so far?

-abhijit